River and Stream

How Rivers and Streams Develop

Kinds of Rivers and Streams

The Water Column

Geography of Rivers and Streams

Plant Life

Animal Life

Human Life

The Food Web

Spotlight on Rivers and Streams

For More Information

A river is a natural flow of running water that follows a well-defined, permanent path, usually within a valley. A stream (also called a brook or a creek) is a natural flow of water that follows a more temporary path that is usually not in a valley. The term stream is often used to mean any natural flow of water, including rivers. Although some rivers are larger than some streams, size is not a distinguishing factor.

The origin of a river or stream is called its source. If its source consists of many smaller streams coming from the same region, they are called headwaters. Its channel is the path along which it flows, and its banks are its boundaries, the sloping land along each edge between which the water flows. The point where a stream or river empties into a lake, a larger river, or an ocean, is its mouth. When one stream or river flows into another, usually larger, stream or river, and adds its flow, it is considered a tributary of the larger river. Many tributaries make up a river system. The vast Amazon system in South America, for example, is fed by at least 1,000 tributaries.

The world’s longest river is the Nile in eastern Africa. It begins in Ethiopia and travels 4,000 miles (6,437 kilometers) into Egypt, where it discharges its waters into the Mediterranean Sea. The river system that obtains water from the largest area, 2,722,000 square miles (7,077,200 kilometers), is that of the Amazon. The Amazon River transports the largest volume (20 percent) of all the water in all the rivers of the world.

How Rivers and Streams Develop

Rivers and streams are part of Earth’s hydrologic cycle. The hydrologic cycle describes the manner in which molecules of water evaporate, condense and form clouds, and return to Earth as precipitation (rain, sleet, or snow). Rivers pass through several stages of development.

Formation

Rivers and streams owe their existence to precipitation, lakes, and groundwater, combined with gravity and a sloping terrain.

When rain falls on the land, often the soil cannot absorb it all. Much of rain runs off and travels downhill with the aid of gravity, creating rills (tiny gullies). Many of these rills may meet at some point and their waters run together to form bigger gullies until all this water reaches a valley or gouges out its own large channel. When enough water is available to maintain a steady ongoing flow, a stream or river results. Gravity and the pressure of the flowing water cause the river to travel until it is either blocked, in which case the water backs up and forms a lake, or empties into an existing lake or ocean.

Most of the precipitation that feeds streams and rivers comes from runoff. Precipitation may also be stored as ice in glaciers in arctic regions or on mountaintops. As the glaciers melt, they nourish streams, and the streams feed rivers. The Rhine River in Germany, for example, obtains much of its water from the Rheinwaldhorn Glacier in the Swiss Alps.

A lake can be a source of river water. If the land slopes away from the lake at some point and the water level is high enough for it to overflow, a river or stream may form.

Another source of river water is groundwater. Groundwater is water that has seeped beneath Earth’s surface where it becomes trapped in layers of rock called aquifers. The Ogallala Aquifer, the largest aquifer in North America, under the Great Plains in the United States is an example. When

an aquifer is full, its water escapes to the surface, either by seeping directly into a river or stream bed or by forming a spring (an outpouring of water), which may then become a river’s source. As much as 30 percent of the world’s freshwater comes from groundwater. It is estimated that ground-water supplies about half of the water in the Mississippi River. In contrast, river water may seep into the ground and fill an aquifer, as the Colorado River does as it travels through Arizona, Nevada, and California.

Stages of development

Rivers are said to age not in terms of how long they have existed but in terms of their development as they travel across the land. They are young, mature, or old. Some rivers, such as the Mississippi, may be in all three stages of development at one time.

Young rivers usually occur in highland or mountainous regions and have narrow, rocky channels with many boulders. The water may form waterfalls or foam and gurgle as it rushes over the rocks. Young rivers have few tributaries.

Mature rivers are those that have reached flat land as their tributaries pour more water into them. They often become flooded during periods of heavy rain or snowmelt. Their beds tend to be muddy rather than rocky because of the sediment (particles of sand or soil) carried into them by swift-flowing streams, and the valleys through which they flow are usually fairly broad. Few waterfalls occur on mature rivers, but they may have many bends or loops as they curl across the land looking for the lowest level to follow.

A river is old near its mouth, where layers of sediment build up over time. Here, the land is wide and flat, and the water travels more smoothly than in young and mature rivers.

Kinds of Rivers and Streams

Rivers and streams can be classified according to their degree of permanence, the shape of their channels, and their branching network.

Degree of permanence

Permanent streams and rivers flow all year long. Enough water is available to keep them from drying up completely, even during a long, dry spell. Most large rivers are permanent.

Intermittent streams and rivers are seasonal. They occur only during the rainy season or in the spring after the snow melts. Rivers and streams in desert regions tend to be intermittent, where they are also called wadis or arroyos.

Interrupted streams and rivers flow above ground in some places and then disappear from sight as they dip down under sand and gravel to flow underground in other places. The Santa Fe River in Florida is an example of an interrupted river.

Channel shape

The material over which a stream or river flows, and the force of the water as it travels determine the shape of the channel, which can be straight, braided, or meandering.

Straight streams or rivers flow in a straight line. This type is very rare because flowing water tends to trace a weaving path. Straight streams that do occur tend to have rocky channels.

Braided streams or rivers, such as the Platte River in Nebraska, consist of a network of interconnecting channels broken by islands or ridges of sediment, primarily mud, sand, or gravel. Braided streams often occur in highland regions and have a steep slope.

Meandering streams or rivers wind snakelike through relatively flat countryside. Meandering streams tend to have low slopes and soft channels of silt (soil) or clay. The Menderes River in Turkey meanders, and the term, which means to wander aimlessly, is derived from its name.

Branching network

Rivers do not usually originate from a single spot but are the result of a branching network that resembles the branches on a tree. The smallest streams or branches that do not have tributaries are called first-order streams. If two first-order streams join to form a new stream, the new stream is considered a second-order stream. When two second-order streams join, they form a third-order stream, and so on. This system of ordering was developed by an American engineer, Robert E. Horton (1875–1945) in 1945. A very large river the size of the Mississippi is usually considered a tenth-order stream. The Amazon River is rated a twelfth- or thirteenth-order stream.

Architects of the Underground

In areas where large deposits of limestone and other soft rock are found, underground rivers may create vast networks of caves. The best example is Mammoth Cave in Kentucky, which has large chambers and underground passageways on five levels that wind back and forth for a total length of 365 miles (587 kilometers). Several underground rivers and streams still flow through the cave, including Echo River, on which tour boats were once operated. The beautiful caverns and rock formations carved by the rivers have been given intriguing names, such as King Solomon’s Temple, the Pillars of Hercules, and the Giant’s Chamber. Ancient Native American tribes once used the caves, and their mummified bodies have been found there.

The Water Column

The water column refers to the water in a river or stream, exclusive of its channel (path) or banks. All of the world’s rivers and streams combined contain less than 1 percent of all the water on Earth. Most water is held in the oceans.

Composition

Rivers and streams carry fresh water, but this water may be clear or cloudy, polluted or clean. Its characteristics are determined by where it originates and the nature of its channel.

As the water moves across the land, particles of rock, soil, and decaying plant or animal matter, called sediment, become suspended in it. If a river carries a large quantity of sediment, it will be muddy. The smaller the amount of sediment, the clearer the water.

River water reacts chemically with the rocks in its channel and dissolves some of the minerals they contain, called salts. These dissolved

salts give the water its taste and make it “hard.” Rivers and streams carry large amounts of these salts. The Niagara River, which lies on the border between the state of New York and Ontario, Canada, carries an average of 60 tons (54 metric tons) of dissolved mineral salts per minute as it pours over Niagara Falls, giving the water a green color.

Circulation

Water in a river or stream is almost always in motion. The direction from which it comes is called upstream, and the direction toward which it flows is called downstream. If you paddle upstream, you ascend the river. If you paddle downstream, you descend.

Velocity

Rivers and streams acquire their initial energy from their elevation. Their waters are always falling downhill toward their ultimate goal, sea level (the average level of the surface of the sea). The fastest velocities (speeds) occur at waterfalls. The water flowing over Niagara Falls has been clocked at 68 miles (109 kilometers) per hour. Most rivers and streams seldom exceed 10 miles (1.6 kilometers) per hour, and the average speed is 2 to 4 miles (3.2 to 6.4 kilometers) per hour.

Although gravity helps get things started, gushing mountain streams actually move more slowly than large, broad, mature rivers. This happens because friction (resistance to motion when one object rubs against another) caused by water molecules rubbing against the channel and banks decreases downstream where the channel is usually smoother and less rocky. Where the river is wider, deeper, and the volume of water is greater, a smaller percentage of water molecules is exposed to friction.

The water at different locations or levels in the same section of river may move at varying speeds. The velocity is fastest in the middle and just below the surface. Movement slows down with depth and along the banks because friction increases. Along the very bottom of the channel is the boundary layer, a layer of water only 0.1 inch (0.25 centimeter) deep, where friction has stopped the flow completely.

Mark Twain

Samuel Clemens (1835–1910), a famous American author, wrote under the name Mark Twain. The word twain means “two fathoms deep” (a fathom is about 6 feet [2 meters]). It was one of the terms shouted by crewmen on Mississippi river boats who were measuring the river’s depth to help the captain know if the boat could pass through that part of the channel. Clemens himself worked for a time as a steamboat pilot, and many of his stories have to do with the river. His river adventures are chronicled in Life on the Mississippi and The Adventures of Huckleberry Finn.

Wave and current

Waves are rhythmic rising and falling movements in the water. Surface waves are caused by wind blowing across a river’s surface. Winds tend to follow a regular pattern. They occur

in the same place and blow in the same direction, and the movement of waves follows this pattern.

The current is the steady flow of the water in a particular direction, usually from upstream to downstream. River currents can be influenced by the slope and composition of the channel and the position of landmasses. When a current travels down a very steep slope, it may gain speed and force. When it meets a landmass, such as an island, it may be deflected and turn in a new direction.

An eddy is a current that moves against the regular current, usually in a circular, or whirlpool, motion. A riffle is a stretch of rapid, shallow, or choppy water that alternates with a quiet pool. A riffle is usually caused as the current flows over stones or gravel. A quiet pool is a deep, still area of water.

Discharge

A river’s discharge is the amount of water that flows out of it over a given period of time into another river, a lake, or the ocean. All of the world’s rivers and streams combined discharge a total of 28,000,000,000 gallons (106,000,000,000 liters) of water every day. If all this water were to flow out over the land, it would cover the land to a depth of 9 inches (25 centimeters).

Discharge is usually measured by multiplying the river’s width at the surface by its average depth times its velocity. The Amazon, which empties into the Atlantic Ocean, discharges more water than any other river—more than 52,834,410 gallons per second (199,999,999 liters per second).

Tidal bores

Tidal bores are surges of ocean water caused when ridges of sand direct the ocean’s flow back into a narrow river channel, sometimes as a single wave. Most tidal bores are harmless. The Tsientang River in China, however, has bore speeds of 18 to 25 miles (29 to 40 kilometers) per hour.

Floods

Floods are caused when more water enters the river than its channel can hold. The result is increased discharge and high water levels. Most rivers overflow their banks every two to three years, but the amount of overflow is usually moderate. Depending on the river’s origins and location, flooding is caused by melting snow, melting glaciers, or heavy rainfall, and sometimes by all three. Rivers supplied by snow or glaciers ordinarily produce only moderate flooding because snow and glaciers melt slowly and the extra water does not enter the river all at once. When heavy rainfall is added the discharge may be enormous. Rain in a location upstream may cause a flood downstream in a region that received no rain at all because rivers are so long.

Yukon Gold Rush

Between 1860 and 1910, people stampeded to the Yukon Territory of Canada in search of gold. They found it not only in the ground, but also in the sediments of rivers, especially the Klondike, the Lewes, the Stewart, and the lower Yukon. These sediments were scooped up into a flat pan and sloshed with water in an effort to separate bits of rock and other debris from the shining gold particles. It is estimated that about $100 million in gold was taken from the Yukon Territory during that time.

The seriousness of a flood is measured by comparing the river’s average annual discharge rate to the rate during the flood. The Huang He (Yellow) River in China, for example, has a discharge rate of 88,286 to 158,916 cubic feet (2,500 to 4,500 cubic meters) per second. During a flood in 1958, its rate increased to 812,237 cubic feet (23,000 cubic meters). The flood covered 34,000 square miles (88,060 square kilometers) and killed 1 million people. The worst flood to occur in the United States was in 1993,

Amazon River Name Origin

The first European to explore the Amazon and its basin was Francisco de Orellana of Spain (c. 1511–1546). In 1541, he made a seventeen-month trip from the foot of the Andes Mountains in the west to the mouth of the Amazon River in the east. The journey was difficult and involved many battles with the native peoples. At times food was scarce and he and his men had to eat toads and snakes. This lack of food may partly explain why de Orellana thought they had been attacked by a band of female warriors resembling the famous Amazons of Greek legends. Other explorers failed to encounter these hostile females, but de Orellana had named the river in their honor, and the name stuck.

along the middle and lower portions of the Mississippi River. Killing 487 people, the flood caused $15 billion in property damage.

In very dry regions, a river or stream may dry up completely for several weeks or months. During a sudden rainstorm, the channel may not be large enough to contain the amount of water, and a flash flood may occur. Flash floods are dangerous because they happen very suddenly. This wall of water brings down everything it reaches.

Effect on climate and atmosphere

The climate of a stream or river depends upon its location as it travels over land. In general, if it passes through a desert area, such as Northern Africa, the climate will be hot and dry. If it begins on top of a mountain in the Northern Hemisphere, the climate will be cold. Rivers and streams in temperate (moderate) climates are often affected by seasonal changes. In these areas, small streams or rivers may dry up in summer or develop a layer of ice in winter.

The presence of a large river can create some climatic differences. In summer, evaporation may create more moisture in the air. When the water temperature is cooler than the air temperature, winds off the river can help cool the nearby region. The water in rivers and streams is partly responsible for the precipitation that falls on land. The water evaporates in the heat of the sun, forms clouds, and falls elsewhere.

Geography of Rivers and Streams

Rivers and streams are strong forces in shaping the landscape through which they flow. As the current moves against the channel and banks, water and the particles of sediment the river carries wear away the surface with a cutting action called erosion (ee-ROH-zuhn).

The faster a river flows, the faster it wears the land away and the more sediment it bears. Some of the eroded chunks and particles may sink to the bottom. Others are carried along and, as the river slows down, are dropped farther downstream. Erosion (wearing away) and deposition (dep-oh-ZIH-shun; layering) make many changes over time.

The drainage basin

A river or stream obtains runoff from an area of land called its drainage basin, or watershed. The drainage basin can be extremely large. The Mississippi-Missouri river system, for example, has as its drainage basin the entire area between the Appalachian Mountains in the east and the Rocky Mountains in the west. The world’s largest drainage basin is that of the Amazon, which measures 2,700,000 square miles (7,000,000 square kilometers).

Channel and banks

The channel of a river or stream slopes down to the center where it is deepest. The bottom of the channel is called its bed. Channel beds may be rocky or covered with gravel, pebbles, or mud. Some river bed sediments are formed from the waste products and dead tissues of animals and plants. Others usually consist of clay, stone, and other minerals.

The general shape of the banks is partly determined by the general shape of the surrounding land. If the river runs through a plain, then its banks will be broad and level. If it occurs in the mountains, then its banks may be steep and rocky.

As water rushes through the channel of a young river, erosion tends to scour it out and make it deeper. Young river valleys are often narrow and steep sided. As the river matures and velocity and volume increase, width tends to increase more than depth. A mature valley tends to have a gentle slope, and an old valley may be almost flat.

Moderate periodic flooding is a major cause of changes in bank shape. Although large floods cause more erosion, they do not occur often enough to make drastic or permanent changes.

Landforms

Landforms created by rivers and streams include floodplains and levees; wetlands; canyons and gorges; rapids and waterfalls; spits, bars, and shoals; and deltas and estuaries.

Floodplains and levees

When a stream carrying a large quantity of sediment flows into another stream or out over a plain, it slows down and its sediments settle to the bottom. These sediments (called alluvium [a-LOO-VEE-um]) often spread out in a fan shape, creating an alluvial fan. When this occurs frequently, the result may be a floodplain. A floodplain is a flat region with soil enriched by river sediments and usually makes good farmland.

Very silty rivers may create high banks of sediments called levees. The levees of the Mississippi River are several yards (meters) high in places.

Wetlands

Sometimes a river may change its path during a flood. The current may seek out a new path or, as the flood waters recede, sediments prevent the water in a loop or bend from rejoining the rest of the river. Here, a wetland, such as a marsh or swamp, or an oxbow lake may form. Oxbow lakes have a curved shape, which gives a clue to the origin of their name. (An oxbow is a U-shaped piece that loops around an ox’s neck, and is part of the harness.)

Canyons and gorges

Many riverbanks consist of both hard and soft rock. Moving water erodes the soft rock first, sometimes sculpturing strange shapes. In regions where much of the rock is soft, rivers and streams may cut deep canyons (long, narrow valleys between high cliffs) and gorges (deep, narrow passes). The Gooseneck Canyon in Utah was cut into the soft limestone of the region by the San Juan River over millions of years. The distance from the top of the canyon to the river is now 1,500 feet (456 meters).

Caves may be gradually carved into the sides of cliffs by erosion. In large rivers, headlands may be created. A headland is an arm of land made of hard rock that juts out from the bank into the water after softer rock has been eroded away.

Rapids and waterfalls

When fast-moving water erodes softer rock downstream, gradually cutting away the bed in some places and creating many short drops, rapids may form. Rapids are often called “white water” because of the foam created when the rushing water hits the exposed bands of rock. The Salmon River in Idaho flows through steep canyons and has many rapids. Its rapids are so dangerous to travelers that it has been nicknamed the “River of No Return.”

When a river or stream falls over a cliff or erodes the channel to such an extent that a steep drop occurs, it creates a waterfall. The world’s highest waterfall is Angel Falls on the Rio Churun in southeastern Venezuela. Its greatest drop is 3,212 feet (979 meters). Victoria Falls, on the Zambezi River between Zimbabwe and Zambia, is considered one of the Seven Natural Wonders of the World. It is the world’s largest falls at nearly 4,400 feet (5,577 meters) wide. Africans all it Mosi-oa-tunya, the “smoke that thunders.”

Where a waterfall strikes the valley below, it gouges out a deep basin called the plunge pool. Eventually, it erodes the lip of rock over which it flows. As erosion continues over time, the waterfall slowly moves upstream. Niagara Falls, for example, is being cut back at a rate of 3 feet (1 kilometer) a year.

Spits, bars, and shoals

A spit is a long, narrow point of deposited sand, mud, or gravel that extends into the water. A bar is an underwater

ridge of sand or gravel, formed by currents, that extends across a channel or the inside bank of a curve. Shoals are areas where enough sediments have accumulated that the water is very shallow and dangerous for navigation.

Deltas and estuaries

Where rivers meet a lake or ocean, huge amounts of silt can be deposited along the shoreline. Large rivers can dump so much silt that islands of mud build up, eventually forming a triangle-shaped area called a delta. The finer debris that does not settle as quickly may drift around, making the water cloudy. The Nile River in Egypt and the Ganges (GANN-jeez) in India both have large deltas. The Ganges Delta is 220 miles (350 kilometers) wide and covers 25 percent of India’s territory, making it the largest delta in the world.

When a river traveling through lowlands meets the ocean in a semi-enclosed channel or bay, the area is called an estuary. The water in an estuary is brackish—a mixture of fresh and salt water. In these gently sloping areas, river sediments collect and muddy shores form. Chesapeake

Bay, along the Atlantic Coast of Maryland, is the largest estuary in the United States. Estuaries are often sites for harbors because they can serve both river and ocean travel.

Elevation

Rivers and streams occur at all altitudes. Their altitude changes over their length because they usually begin in highlands or mountains and move to lower elevations. The headwaters of the Ganges River in India, for example, start from an ice cave in the Himalaya Mountains at an elevation of 10,300 feet (16,480 meters). A thousand miles (1,600 kilometers) from its mouth, the Ganges is only about 600 feet (180 meters) above sea level, and 0 feet (0 meters) at its mouth where it enters the Bay of Bengal.

Plant Life

Few plants can root and grow in running water. Most plants that live in rivers and streams are found along the banks or in quiet pools where the environment is similar to that of a lake or pond. Plants with floating leaves do not do well in fast-moving streams because the current pulls the leaves under. When rooted plants gain a foothold and manage to reproduce, they can grow so numerous that they slow the flow of water and can even cause flooding.

River and stream plants may be classified as submergent, floating aquatic (water), or emergent according to their relationship with the water.

A submergent plant grows beneath the water; even its leaves lie below the surface. Submergents include the New Zealand pygmyweed, tape grass, and water violet.

Floating aquatics float on the water’s surface. Some, such as duckweed, have no roots. Others, such as the water lily, have leaves that float on the surface, stems that are underwater, and roots that are anchored to the bottom.

An emergent plant grows partly in and partly out of the water. The roots are usually underwater, but the stems and leaves are at least partially exposed to air. They have narrow, broad leaves, and some even produce flowers. Emergents include reeds, rushes, grasses, cattails, and sedges.

Plants that live in rivers and streams can be divided into four main groups: bacteria; algae (AL-jee); fungi (FUHN-ji) and lichens (LY-kens); and green plants.

Algae

Most submergent plants are algae. (It is generally recognized that not all algae fit neatly into the plant category.) Some forms of algae are so tiny they cannot be seen without the help of a microscope. These, like spirogyra, sometimes float on the water in slow-moving rivers or coat the surface of rocks, other plants, and river debris. Other species, such as blanket weed, are larger and remain anchored to the riverbed in quiet pools.

Most algae have the ability to make their own food by photosynthesis (foh-toh-SIN-thih-sihs), the process by which plants use energy from sunlight to change water and carbon dioxide from the air into the sugars and starches they use for food. A by-product of photosynthesis is oxygen, which combines with the water and enables aquatic animals, such as fish, to breathe. Algae require other nutrients that must be found in the water, such as nitrogen, phosphorus, and silicon. Algal growth increases when nitrogen and phosphorus are added to a stream by sewage or by runoff from fertilized farmland.

Common river and stream algae

Phytoplankton float on the surface of the water. Two types of phytoplankton, diatoms and dinoflagellates (dee-noh-FLAJ-uh-lates), are the most common. Diatoms have simple, geometric shapes and hard, glasslike cell walls. They live in colder regions and even within arctic ice. Dinoflagellates have two whiplike attachments that make a swirling motion. They live in tropical regions (regions around the equator).

Macrophytic algae are not as common in fast-moving water, but some filamentous species can be found growing on stable surfaces.

Growing season

Algae contain chlorophyll, a green pigment used during photosynthesis. As long as light is available, algae can grow. Growth is often seasonal. In some areas, such as the Northern Hemisphere, most growth occurs during the summer months when the sun is more directly overhead. In temperate zones, growth peaks in the spring but continues throughout the summer. In regions near the equator, no growth peaks occur since it is warm year round. As a result, growth is steady throughout the year.

Reproduction

Algae may reproduce in one of three ways. Some split into two or more parts, each part becoming a new, separate plant. Others form spores (single cells that have the ability to grow into a new organism), and a few reproduce sexually, during which cells from two different plants unite to create a new plant.

Fungi and lichens

As with algae, it is generally recognized that fungi and lichens do not fit neatly into the plant category. Fungi are plantlike organisms that cannot make their own food by means of photosynthesis. Instead, they grow on decaying organic (material derived from living organisms) matter or live as parasites (organisms that depend upon other organisms for food or other needs) on a host. Fungi grow best in a damp environment and are often found along riverbanks. Common fungi include mushrooms, rusts, and puffballs.

Lichens are combinations of algae and fungi that tend to grow on rocks and other smooth surfaces. The alga produces the food for both itself and the fungus by means of photosynthesis. The fungi may provide moisture for the algae. One of the most common lichens found near streams is called reindeer moss, which prefers the solid surfaces of rocks.

Green plants

Most green plants need several basic things to grow: light, air, water, warmth, and nutrients. Near a stream, light and water are in plentiful supply. Nutrients, primarily nitrogen, phosphorus, and potassium, are usually obtained from the soil. Some soils are lower in these nutrients. They may also be low in oxygen. These deficiencies may help limit the kinds of plants that can grow in an area.

The true green plants found growing in the water along the banks of rivers and streams are similar to those that grow on dry land. Unlike algae, they have roots and some, like the eelgrass, may even bloom underwater. Large beds of green plants slow the movement of water and help prevent erosion. Some water animals use green plants for food and for hiding places. Plants that occupy the emergent zone between the water-covered area and dry land, such as sallows (European species of willows) and sedges, need moist, but not saturated, soil.

Common river and stream green plants

Common plants found in and around rivers and streams include willow moss, water lily, pondweed, duckweed, aquatic buttercup, great pond sedge, water plantain, water lettuce, mosquito fern, water soldier, bur reed, marsh horsetail, reed mace, mare’s tail, greater spearwort, flowering rush, water forget-me-not, St. John’s wort, alder, and weeping willow.

Growing season

Climate and the amount of precipitation both affect the length of the growing season. Warmer temperatures and moisture usually signify the beginning of growth. In regions that are colder or receive little rainfall, the growing season is short. Growing conditions are also affected by the amount of moisture in the soil, which can range from saturated, during flooding or a rainy season, to dry.

Reproduction

Green plants reproduce by several methods. One is pollination, in which the pollen from the male reproductive part (called a stamen in flowering plants) of a plant is carried by wind or insects to the female reproductive part (called a pistil in flowering plants). Water lilies, for example, are closed in the morning and evening, but open during midday when the weather is warmer and insects are more active and more likely to visit. Some shoreline plants, like sweet flag, reproduce by sending out rhizomes (RY-zohms), which are stems that spread out under water or soil and form new plants. Reed mace and common reeds reproduce by this means.

Endangered species

Changes in the habitat, such as pollution and the presence of dams, endanger river and stream plants. Plants are also endangered by people who collect them, and by the use of fertilizers and herbicides (poisons used to control weeds), which enter the water as runoff from farms.

The papyrus plant in Egypt, for example, is endangered because of the Aswan High Dam. The dam traps Nile water in its reservoir upstream, and the swamps and ponds that form the habitat for papyrus are disappearing.

Animal Life

In addition to the land animals that visit for food and water, rivers and streams support many species of aquatic animals. Some swim freely in the water, while others live along the muddy bottom. Some prefer life in midstream or on rocks beneath a fast-moving flow; others seek the shallows or quiet pools. Shallows are often warm and exposed to sunlight, but animals that prefer cool shady spots can find them along the banks beneath overhanging trees. Many river and stream animals are adapted to life under high-speed conditions. Salmon and trout, for example, are torpedo-shaped and can swim against the current more easily than other fish. Some, like torrent beetles and stonefly larvae, have low-slung or flattened bodies that enable them to cling to rocks without being washed away.

Water quality often determines which species will be supported in a particular river or stream. Temperature, velocity, oxygen content, mineral content, and muddiness are all factors. Cold-water trout, for example, prefer cool, shady streams, while snails require calcium-rich waters in order to build their shells. Estuaries are a combination of both salt- and freshwater environments. They are home to species especially adapted to those conditions.

Microorganisms

Microorganisms cannot be seen by the human eye without the use of a microscope. Those found in rivers and streams include the transparent stentor. The larvae of animals whose adult forms may be larger, such as dragonflies, worms, and frogs, may spend some time as microorganisms living in the water.

Bacteria

Bacteria are microorganisms found throughout rivers and streams and make up much of the dissolved matter in the water column. They provide food for smaller animals and help decompose (break down) the dead bodies of larger organisms. For those reasons, their numbers increase along the banks where the most life is found.

Invertebrates

Invertebrates are animals without a backbone. They range from simple flatworms to more complex animals such as spiders and snails.

Many species of insects live in rivers and streams. Some, like the diving beetle, spend their entire lives in the water. Others, like black flies, live in the water while young but leave it when they become adults.

River Blindness

In tropical Africa, South America, and Central America, a round worm causes a disease in humans called river blindness. The worm is introduced by the black fly, which thrives around fast-moving rivers. The fly carries the larvae of the worm and spreads them to humans in its bite. The larvae burrow under the skin and spread throughout the body, eventually reaching the eyes. In some African villages, 15 percent of the people are infected.

Mollusks, such as freshwater mussels, are invertebrates with a hard outer shell that often inhabit rivers.

Common river and stream invertebrates

Common invertebrates found in rivers and streams include diving beetles, mayflies, water fleas, torrent beetles, stoneflies, dobsonflies, water boatmen, water striders, whirligig beetles, black flies, fisher spiders, water scorpions, water sticks, leeches, flatworms, snails, freshwater clams, freshwater mussels, raft spiders, caddisflies, and freshwater crayfish.

Food

Insects may feed underwater as well as on the surface. They may be herbivores (plant eaters), carnivores (meat eaters), or scavengers that eat decaying matter.

The diets of invertebrates other than insects varies. Some snails are plant feeders that eat algae, while freshwater crabs are often omnivorous, eating both plants and animals.

Reproduction

Most invertebrates are insects, which have a four-part life cycle. The first stage is spent as an egg. The second stage is the larva. It may be divided into several stages between which there is a shedding of the outer skin casing. A caterpillar is an example of an invertebrate in the larval stage. The third stage is the pupal stage, during which the insect lives in yet another protective casing, like a cocoon. Finally, the adult breaks through the casing and emerges.

For some insects, their entire life cycle is centered on breeding. Mayflies, for example, live for an entire year in streams as larvae. After they shed the larval casing, they breed, lay eggs, and die; all within a day or two. If enough mayflies survive the larval stage, the numbers of dead adults can be enormous. On one occasion, a snowplow had to be used to remove a layer of mayflies several feet thick from a bridge across the Mississippi.

Raft spiders live along the edges of slow-moving rivers where they are able to run across the water’s surface in search of insect prey. Bristles on their feet help distribute their weight so they do not sink. Raft spiders can be identified by two yellow stripes running the length of their bodies.

Adult caddisflies are creatures that come out only at night, rarely feed, and live for only a couple of days. The larvae of caddisflies use stones, sand, shells, and other items to build a protective shell around themselves. The only opening is a hole at the front through which the larva’s head emerges to feed. Some larvae eat algae, and others build nets to catch food carried by the current.

The 540 species of freshwater crayfish are found in rivers on all continental landmasses except Africa. The crayfish is related to the lobster and needs mineral-rich waters to build its protective shell. Species range from 1 inch to 16 inches (2.5 to 40 centimeters) in length. They seek shelter under stones in the stream bed or may even burrow into it, sometimes as far as 20 feet (6 meters) down. They catch prey, such as small fish, with their large pincer claws. Some blind, albino (colorless) species live in underground rivers.

Amphibians

Amphibians, including frogs, toads, newts, and salamanders, are vertebrates, which means they have a backbone. Amphibians live at least part of their lives in water. They must usually remain close to a water source because they breathe through their skin, and only moist skin can absorb oxygen. If they are dry for too long, they will die.

Amphibians are cold-blooded animals, which means their body temperatures are about the same temperature as their environment. As temperatures grow cooler, they slow down and seek shelter in order to be comfortable. In cold or temperate regions, some amphibians hibernate (remain inactive), digging themselves into the mud. When the weather gets too hot, many go through another similar period of inactivity called estivation.

Common river and stream amphibians

The most common amphibians found in rivers and streams all over the world are salamanders, frogs, toads, and newts.

The palmate newt lives on land for part of the year where it hunts at night for worms and other small animals. In spring it returns to the water where it breeds and lays eggs. Most newts lay eggs one at a time and attach them to water plants. Some species even wrap each egg in a leaf for protection.

Walking on Water

The Jesus Christ lizard, or basilisk, of Central America got its name from its ability to walk on water. Scales and a flap of skin on its hind toes increase the surface area of its feet and enable it to scamper on the water’s surface. Being lightweight also helps.

Another water-walker, or one that appears to be, is the jacarana, or lily trotter, bird of the tropics. Jacaranas have extremely long toes that distribute their weight over the surface, but they are actually walking on the floating leaves of water plants.

Food

In their larval form, amphibians are usually herbivorous, while the adults are usually carnivorous, feeding on insects, slugs, and worms. Those that live part of their lives on land have long, sticky tongues with which they capture their food.

Reproduction

Most amphibians lay jelly-like eggs in the water. Frogs, depending on the species, can lay up to as many as 50,000 eggs, which float beneath the water’s surface. After being fertilized by the sperm of male frogs, the eggs hatch into tadpoles (larvae) and require a water habitat as they swim and breathe through gills. When the tadpole turns into a frog, it develops lungs and can live on land.

Spotted newts hatch in the water and live there as larvae, even developing gills. Later, they lose their gills and live for a time on dry land. Two or three years later, they return to the water where they live the remainder of their lives.

Some amphibians, like the common frog, prefer quiet water for breeding. Others, like the large hellbender salamander, seek rushing streams.

Reptiles

Reptiles are cold-blooded vertebrates that depend upon the environment for warmth. They are more active when the weather and water temperature become warmer. Many species of reptiles, including snakes, lizards, turtles, alligators, and crocodiles, live in temperate and tropical rivers and streams. All are air-breathing animals, with lungs instead of gills, which have adapted to life in the water. The Southeast Asian fishing snake, for example, can close its nostrils while swimming submerged.

Many reptiles go through a period of hibernation in cold weather because they are so sensitive to the environment. Turtles, for example, bury themselves in the mud. They barely breathe and their energy comes from stored body fat. At the other extreme, when the weather becomes very hot and dry, some reptiles go through estivation.

Common river and stream reptiles

Common reptiles found in rivers and streams include the water moccasin, the viperine water snake, the eastern water dragon, the soft-shelled turtle, the matamata turtle, the yellow-bellied terrapin, the snapping turtle, the anaconda, and the crocodile.

The anaconda of South American rivers is the world’s largest snake, some individuals attain lengths of more than 33 feet (10 meters). Although they are excellent swimmers and prefer to hunt along the edges of rivers and swamps, they can also climb trees. Anacondas are constrictors, killing their prey by coiling their body around it and squeezing it. Their young are born alive in the water.

Crocodiles hunt at twilight in tropical rivers. During the day they bask in the sun at the river’s edge. As the light dims, they seek their prey. Some, such as the New Guinea crocodile, eat just fish. Most other species eat a variety of foods. They swallow smaller prey whole, but larger animals are dragged underwater where they drown and are torn apart and eaten.

Food

All snakes are carnivores. Water snakes eat frogs, small fish, and crayfish. Some species of turtles are omnivores.

Reproduction

The eggs of lizards and turtles have either hard or rubbery shells and do not dry out easily. Most are buried in warm ground, which helps them hatch. The eggs of a few species of lizards and snakes are held inside the female’s body, and the babies are born alive.

Crocodiles keep their eggs warm in nests that can be simple holes in the ground or constructions above the ground made from leaves and branches.

Fish

Like amphibians and reptiles, fish are cold-blooded vertebrates. There are two types of freshwater fish. The first type, the parasites, which include some species of lampreys, attach themselves with suckers to other animals and suck their blood for food. The second type, which eat plant foods or catch prey, are the most numerous. They use fins for swimming and breathe with gills.

Many species of fish, such as archer fish, swim in schools. A school is a group of fish that swim together in a coordinated manner to discourage predators.

Food

Some fish, such as the grass carp, eat plants while others, such as bluegills, depend upon insects, worms, and shellfish. Larger fish, such as the sturgeon, often eat smaller fish, and a few species feed

on carrion (dead animals). Most fish specialize in what they eat and where they find their food. Some feed on the surface, and others seek food in deep water. Some prefer rushing streams and others calm pools. The arawana of Southeast Asian rivers has a large, upward-pointing mouth that helps it feed on the surface.

Common river and stream fish

Typical fish found in rivers and streams include the roach, rudd, tench, bream, perch, gudgeon, carp, bitterling, African knifefish, elephant fish, hatchetfish, shovel-nosed sturgeon, white sturgeon, paddlefish, freshwater shark, arapaima, stickleback, piranha, catfish, salmon, and trout.

Piranhas (purr-AN-ahs) are predatory fish of South America, known for their several rows of sharp, triangular teeth. They usually feed on other fish, or fruits and seeds that fall into the water. They become particularly dangerous during the dry season when the water level drops and fish gather together in schools. A school of piranhas can attack and devour large animals, including humans.

Many species of catfish inhabit rivers worldwide. Some tropical varieties can climb out of the water and walk overland from one river or stream to another. They range in size from the small species that live in mountain streams, to those that live in the Danube River in Europe and grow to a length of 13 feet (4 meters) and a weight of 400 pounds (180 kilograms).

Shocking River Residents

There are lots of good reasons to not swim in tropical rivers, and one is to avoid an electric shock. The African catfish and the Amazonian eel are two tropical river residents that produce their own electricity and use it as a weapon.

The body of an electric eel functions like a chemical battery. Thousands of electricity-producing cells are stacked in columns and oriented in the same direction. When the eel sights something tasty, a nerve impulse from the eel’s brain triggers the flow of electric current from its tail, through the water, toward its head. When the eel touches its prey, the circuit is interrupted and the current flows into the body of the victim, stunning it. An adult eel can produce 600 volts of electricity, which is enough to knock out an animal the size of a horse or a human.

Catfish got their name from the long feelers around the mouth that resemble cats’ whiskers. They have poisonous spines along the fins that can cause painful injuries. Catfish adapted to life in fast-moving streams have fins with suckers or large, fleshy lips that help them cling to rocks and even climb. Bullheads, a species of catfish, have flattened bodies, which enables them to squeeze under stones and resist the current. Some freshwater catfish communicate with each other through grunting and clicking sounds.

Salmon live in both fresh and salt water. The young hatch in fast-moving rivers and streams, where they live for about the first three years of their lives. They then migrate to the sea where they live for up to four years, feeding on small fish and shellfish. When it comes time to breed, they return to the river of their birth, swimming upstream. Salmon are called anadromous fish, which is Greek for “running upward.” After breeding, they die; their decaying bodies provide an injection of nutrients to their birth area.

In general, trout are found in oxygen-rich, cool, fast-moving streams with beds of gravel. They are carnivorous, feeding on insects, freshwater shrimp, and clams. Some species also prey on other fish. Rainbow trout, which originated in North America, have a silver belly and an iridescent stripe along the side. The Aurora trout, a subspecies of the brook trout, is a spectacularly colored fish. They are native to two small lakes in Ontario, Canada. In the 1960s, the Aurora trout was almost driven to extinction due to acid rain. The lakes were treated with lime to decrease the acidity of their waters and the trout was reintroduced.

Reproduction

Most fish lay eggs, and many species abandon the eggs once they are laid. Others build nests and care for the new offspring until they hatch. The male stickleback lures several females to his nest where they lay their eggs. He then fertilizes the eggs and guards them until they hatch. Still other species, such as certain catfish, carry the eggs with them until they hatch, usually in a special body cavity or even in their mouths.

The Fish that can Climb Ladders

In order to breed, Atlantic salmon return from the sea to rivers along the Atlantic and Pacific coasts where they first hatched. Most of these rivers have been dammed in places, preventing the fish from continuing on their journey upstream. To aid them, some states have built “fish ladders,” water-covered steps alongside the dams that allow the fish to jump from one level to the next as they would over a natural waterfall. In spite of this technological aid, the numbers of salmon continue to decline, as do the numbers of animals who depend upon the salmon for food.

Birds

Birds are vertebrates, and many different species live near rivers and streams. These include many varieties of wading birds, waterfowl, and shore birds. Most visit in search of food, and certain species use riverbanks as nesting places.

Most aquatic birds prefer quiet waters, but a few are specially adapted for the speed and turbulence of life upstream. One example is the torrent duck of the Andes Mountains in South America. Built like an ordinary duck in most respects, it has a longer tail to help it steer when swimming in fast currents, and claws on its webbed feet that allow it to cling to rocks.

Common river and stream birds

Birds found around rivers and streams include mergansers, warbles, reed buntings, ospreys, sunbitterns, finfoots, egrets, and wood ducks. Kingfishers, herons, fish eagles, ibises, grebes, and cranes are the few bird species that spend their entire lives around freshwater habitats. They fall into four basic categories: wading birds, shorebirds, waterfowl, and birds of prey.

Wading birds, such as herons, have long legs and wide feet for wading through shallow water. They have long necks and bills necessary for nabbing food. There are approximately 100 species of heron.

Shorebirds, such as the sandpiper, feed and nest along the banks and prefer shallow water. Some, like the godwit, have long, slender, upturned bills that help them sift through mud in search of food. Other birds, like the ruddy turnstone, have bills that are curved to one side, which helps them to overturn pebbles. The red knot is an example of a shorebird that has specialized bill cells that are pressure sensitive, allowing them to detect shellfish buried beneath the sand.

Waterfowl are birds that spend most of their time on water, such as ducks, geese, and swans. Their flattened bills are designed for grabbing vegetation, such as sedges and grasses, on which they feed. Waterfowl are strong fliers and swimmers. Of the 150 species, most prefer freshwater environments. Migrating waterfowl play a major role in dispersing small aquatic organisms that attach to their feet or feathers.

Osprey and fish eagles are birds of prey that only hunt in freshwater. Other birds of prey include gulls, terns, and stilts.

Food

Nearly all birds must visit a source of fresh water each day to drink, and many feed on aquatic vegetation or the animals that live in the water. The dipper can actually walk underwater in search of small animal prey.

One of the adaptations of birds to aquatic life is their beak, or bill. Some are shaped like daggers for stabbing prey such as frogs and fish; others are designed to root through mud in search of food. Water-dwelling birds spend much time preening their feathers because they depend on their feathers to keep them dry.

Reproduction

All birds reproduce by laying eggs. Male birds are often brightly colored to attract the attention of females. After mating, female birds lay their eggs in a variety of places and in nests made out of many different materials. One parent usually sits on the nest to keep the eggs warm while the other searches for food.

Mammals

Mammals are warm-blooded vertebrates covered with at least some hair, that bear live young and nurse their young with milk. Aquatic mammals, such as the muskrat, often have fur that is waterproof. They may have webbed toes for better swimming. The toes of the duckbilled platypus of Australia, for example, are webbed.

Food

Certain aquatic mammals are carnivores. The water shrew, for example, eats insects, worms, and frogs. Others, like muskrats, are omnivores, while beavers are herbivores.

What Am I?

The duck-billed platypus, which is found in Australian rivers, appears to be a cross between a reptile, a duck, a beaver, and an otter. It has a ducklike beak, webbed feet, and lays eggs. It also has dense fur and a flat, beaverlike tail, and suckles its young with milk. The platypus is a good swimmer and dines on the river’s insect population. Officially, the platypus is a mammal and cousin to the anteater. Some scientists believe that the species originated during the evolution of reptiles into mammals.

Reproduction

Mammals give birth to live young that have developed inside the mother’s body. Some mammals, such as otters, are helpless at birth, while others, like dolphins, are able to walk or swim immediately. Many are born with fur and with their eyes and ears open. Others, like baby muskrats, are born hairless and blind. Mammals feed their young with milk produced by the mother.

Common river and stream mammals

Mammals found in and around rivers and streams include manatees (sea cows), capybaras (large rodents), tapirs, minks, water shrews, beavers, dolphins, otters, and hippopotami.

Of the five recognized species of freshwater dolphins, four live in rivers. River dolphins are found in the Ganges, Indes, Amazon, and Yangtze rivers. They are almost blind and find their way through muddy waters by sending out sound waves that bounce off obstacles to warn them of what’s ahead and help them find the fish on which they feed. Like their saltwater cousins, freshwater dolphins are considered intelligent and some fishermen use them to herd schools of fish into waters where they are more easily caught. They differ from their marine cousins in that they are slower swimmers and have larger heads.

Dolphins produce a single calf that is capable of swimming and breathing within the first minutes of its life. Some mothers have been observed guiding the calf to the surface, as if to help it reach air. River dolphins are threatened by hunting, entanglement in fishing nets, and pollution.

Of the thirteen species of otters, twelve are found in freshwater. River otters have webbed feet and strong tails that make them good swimmers. Their thick, water repellent fur coasts keep them warm and dry. They feed on frogs, birds, fish, and small mammals. Their eyes, ears, and nostrils are placed high on their heads, which enables them to look sharply around while swimming. They dig dens into the riverbank where they care for their young until the young are able to live on their own. Otters are playful animals and spend much of their time at acrobatic games. Weasels, skunks, and badgers are relatives of the otter that are found in rivers and streams.

The name hippopotamus means “river horse” in Greek, and, although hippos look ungainly, they are fast both in the water and on land. They graze on vegetation on land at night and spend their days dozing in the muddy waters of African rivers. Adult males may reach a length of 16 feet (5 meters) and weigh as much as 4 tons (3.6 metric tons). Their skin is nearly hairless, and the eyes and nostrils protrude so that they are above the water even when the rest of their body is submerged. There are two species of hippo. The larger, common species is widespread in Central Africa. The scarcer, pigmy hippo is restricted to Western Africa.

Endangered species

Seven types of salmon and two species of trout are on the endangered list in the northwestern United States. The Topeka shiner and the Arkansas River shiner are threatened in the central United States and the snail darter in Tennessee. As rivers and streams become polluted, many species of birds, such as the fish eagle, have become threatened. Birds cannot use the polluted waters for food or as nesting areas, which causes their numbers to decline.

At one time hippos were found in almost all African rivers. As the human population has grown, their habitat has shrunk and they are now found in only a few rivers. The Amazonian manatee, the only species of freshwater manatee, is now protected against hunters, who kill it for its meat.

River of Death

In Greek mythology, the River Styx traveled through the underworld, the land of the dead. The water of that river was supposedly so poisonous that it would dissolve any vessel used to carry it except one made from a horse’s hoof. An ancient account claims that Alexander the Great (356–323 BC), king of Macedon, was poisoned by Styx water. There may be a bit of truth in the claim; some experts believe he died of a bacterial infection caught by drinking water from the Euphrates River, which carried raw sewage out of the ancient city of Babylon in what is now southern Iraq.

Human Life

Rivers have always been intimately connected to human life, and river valleys have been the birthplaces of the world’s great civilizations. The Nile River is the scene of the oldest and perhaps the most remarkable of ancient civilizations, Egypt. The Nile’s yearly floods brought rich sediments to the soil, and the people learned to divert the floodwaters for agriculture and drain the swamps. Chinese civilization got its start in the lower valley of the Huang He, where many floods and extreme weather forced people to develop the technology necessary for life to continue successfully there. In the valley of the Tigris and Euphrates rivers, Sumerian civilization developed as people fought to clear jungle swamps and manage floodwaters. The same was true of the Indus civilization, which arose in the valleys of the Ganges and Indus rivers in India and Pakistan. While rivers offered opportunities for water and transportation, they also presented challenges that inspired people to excel in order to survive.

Impact of rivers and streams on human life

People use rivers and streams for water and food; boundaries, transportation, and trade; recreation and building sites; and creating hydroelectric power.

Water and food

Many rivers and streams are sources of drinking water and for such things as bathing and laundry. In developed countries, about half the freshwater supply is used by industry. In less wealthy countries, up to 90 percent is used for irrigation (watering) of crops. The earliest irrigation systems were built on the banks of the Nile River about 6,500 years ago. Pumping systems and waterwheels soon followed. About 642,473,992 acres (260,000,000 hectares) of land is now irrigated worldwide.

The fish and plants in rivers and streams are often a source of food for humans. Although most fish used for food come from the ocean, commercially important freshwater fish include trout and salmon. Farmers often use aquatic plants, such as marsh grass, reeds, and sedges, for feeding livestock.

Boundaries, transportation, and trade

Throughout history, rivers have acted as boundaries. If they were too big or too fast to cross easily, they limited overland travel. During exploration of the American West, for example, wide rivers like the Missouri meant that travelers had to build a boat or raft or find a spot where the channel was narrow and the water shallower, so that horses could swim across.

Rivers can act as boundaries between two different territories. Many countries use rivers to help define their borders. The Rio Grande separates Texas from Mexico, and the Congo in Africa divides the Democratic Republic of the Congo and Zaire.

Rivers have always provided a means of transportation and trade. Cities built at the edge of rivers could import or export goods upstream or downstream or even to sea more easily and less expensively than cities reachable only by land routes. An example is the Mackenzie River of Canada’s Northwest Territories, which flows into the Arctic Ocean. Early European settlements, such as Fort Good Hope, were built along its shores as part of the fur trade. It is now used to transport petroleum and uranium ore.

Many rivers are still important routes for bulk cargo, such as coal. The Chang Jiang River in China, for example, is navigable by ocean-going freighters for 680 miles (1,090 kilometers) inland and the St. Lawrence Seaway between the United States and Canada for about 2,300 miles (3,800 kilometers).

Recreation and building sites

Streams and even some rivers are popular for sport fishing and boating, but pollution makes most of them undesirable for swimming. In urban regions, the areas along riverbanks have become popular sites on which to build homes. Some rivers and streams may be diverted to create artificial lakes or ponds in order to add beauty and wildlife to a residential area.

Swan Song for the Thames

Even before 1800, the Thames (TEMS) River in England was polluted by chemicals and other materials used in manufacturing. With the invention of flush toilets, things got much worse. Where did the sewers empty out? In the Thames. The fish suffocated to death and swans and other waterbirds left for a healthier part of town. Until the 1950s, the Thames was considered one of the most polluted rivers on the planet.

Public protest finally grew so loud that things began to change. Strict regulations were created to limit what could be dumped into the water. Sewage-treatment plants were improved, and special equipment was installed in some locations to mix the water with oxygen. Before long, the river began to recover. Within thirty years, pollution had been reduced by 90 percent, and oxygen content rose to healthy levels. The fish returned, and so did aquatic plants. Even the swans moved back.

Hydroelectric power and other resources

When river waters rush over the blades of a turbine (energy-producing engine), the blades turn

Saving the Salmon

Saving threatened species of animals usually requires sacrifices on the part of humans who live in the region. When seven species of salmon that breed in the Columbia and Willamette Rivers in the northwestern United States were put on the endangered list in 1999, people living in Seattle, Washington, Portland, Oregon, and other communities agreed to:

• limit logging and move that activity back from riverbanks to decrease erosion
• recycle water used for golf courses and limit the use of fertilizers on golf greens
• redesign some hydroelectric dams and remove others to allow the salmon to reach their breeding grounds
• keep domestic cattle away from rivers, reduce the cattle population, and treat animal waste before it is used as a fertilizer
• limit washing of cars
• limit use of pesticides and herbicides
• build environmentally friendly sewage treatment plants
• limit salmon fishing
• enact new pollution controls for industry

rapidly. The rapid turning can be used to produce electricity. This form of electricity, called hydroelectric power, can provide heat and light for many people. Worldwide, about 15 percent of electricity is produced by hydroelectric power plants, such as Hoover Dam on the Colorado River in Nevada and Grand Coulee Dam on the Columbia River in Washington.

Plant materials found along rivers and streams can be used for building. Reeds are used for huts in Egypt and stilt houses in Indonesia. River sediments, such as clay and mud, are used to produce bricks.

Fish are used for more than just food. They yield such products as fish oils, fishmeal, fertilizers, and glue. Other animals may be introduced into rivers to help control problems. The Amazonian manatee, for example, was placed in reservoirs because they eat the river plants that were clogging the turbines of hydroelectric plants.

The water in rivers and streams may be used to cool power stations and for other industrial purposes. Unfortunately, some industries have used them as dumpsites for industrial chemicals.

Impact of human life on rivers and streams

Initially, the effect of human life on rivers and streams was small. As the human population has grown, abuses have increased, and many rivers and streams resemble open sewers.

Each year a conservation group called American Rivers, publishes a list of the most endangered American rivers. Those endangered in 2007 were:

• Santa Fe River in New Mexico
• San Mateo Creek in California
• Iowa River in Iowa
• Upper Delaware River in New York
• White Salmon River in Washington
• Neches River in Texas
• Kinnickinnic River in Wisconsin
• Neuse River in North Carolina
• Lee Creek in Arkansas and Oklahoma
• Chuitna River in Alaska

Water supply

Although all water on Earth is in constant circulation, some regions may have a limited supply. As populations grow, the supply diminishes even more. Forests, for example, are cut down and replaced by farms. Because trees help conserve underground water, much of this water is being lost. Rivers and streams are often linked to groundwater, so they gradually disappear as well.

Irrigation practices can cause damage, especially in desert regions, because there is not enough precipitation to replace the water that is used from the rivers and streams.

Use of plants and animals

The distribution of many popular species of fish has been affected by changes made to rivers and streams in Europe and America. The giant catfish, the Atlantic salmon, the Danube salmon, the sturgeon, the beluga sturgeon, and many others are now found in only a few rivers.

Overdevelopment

People may wish to live near a stream or river for its beauty and a sense of being close to nature, but overdevelopment can result in erosion, loss of wildlife, and the scenic value. Removal of trees during housing development can cause ground water to disappear and deprive the river or stream of its source. Large tracts of homes and areas covered by concrete can prevent rain from soaking into the ground. Instead, the runoff enters rivers and streams, causing flooding.

Quality of the environment

Pollution by communities, industries, shipping, and poor farming practices has led to poisoning of water and changes in its temperature, as well as loss of animal habitats.

The Rock Breaker

Henry Morton Stanley (1841–1904) was given the nickname “rock breaker” for his ability to overcome difficulties. An illegitimate child, Stanley spent part of his young life in a workhouse in Wales, the country of his birth, where he was routinely mistreated. At the age of fifteen he ran away to America where a kindly merchant adopted him and turned his life in a new direction.

Stanley served as a soldier in the American Civil War, as a seaman in merchant ships, and most importantly as a journalist. In 1869, he was commissioned by the New York Herald to travel to Africa to find David Livingstone (1813–1873), a British explorer who was missing. Stanley succeeded, and together he and Livingstone explored Lake Tanganyika in eastern Africa. After Livingstone died, Stanley carried on his explorations, eventually tracing the route of the Congo River and helping to found the Congo Free State. He described his dangerous journey in the book titled Through the Dark Continent, which was published in 1878.

The most common form of river pollution is domestic sewage. If there is enough oxygen in the water, bacteria can break the sewage down

The African Queen

A mixture of comedy, adventure, and romance, The African Queen is a classic film about two people during World War I (1914–18) who take an ancient, ramshackle boat up a treacherous river in Africa in hopes of finding and destroying an enemy ship. They encounter rapids, leeches, hippos, and other river dangers during the wild ride to their destination. The film starred Katharine Hepburn and Humphrey Bogart and won the Academy Award in 1951. It was based on the book of the same name by British author C. S. Forester (1899–1966).

quickly. Problems occur when the volume of sewage is so large and bacteria use up so much oxygen that plants and animals cannot survive. Additionally, this process results in the formation of ammonia, which is poisonous to most animals. Sewage fungus, a combination of bacteria and molds, may grow on the surface of the water.

Another source of river pollution is industrial chemicals and heavy metals, which may dissolve in the water or remain as solids. Enormous quantities of these materials poison animals, clog gills in fish, and bury the riverbed in sediments. Industries are responsible for dumping heated water (thermal pollution) into rivers, raising their temperature and killing many species of plants and animals that require a cooler environment.

Shipping can cause pollution. Ships discharge waste into the water or develop leaks and spills that threaten the water’s purity and destroy wildlife.

Eutrophication (yoo-troh-fih-KAY-shun) occurs when fertilizers used in farming get into rivers and streams, spurring a greatly increased growth of algae in slow-moving waters. These plants form a thick mat on the surface and block the sunlight, causing submerged plants to die. As the dead plants decay, oxygen in the water is used up, killing fish and other plant life.

Flood control

Flooding nourishes the soil in floodplains, creates natural wetlands, and supports wildlife. Floods can also be destructive and dangerous to human life. For this reason, efforts have long been made to control floods using dams and artificial levees. Many of these changes have not only harmed wildlife, they have sometimes caused more problems than they have helped. The Aswan High Dam in Egypt is an example. Completed in 1970, the dam is 2.5 miles (4 kilometers) in length and rises 364 feet (111 meters) above the Nile riverbed. The dam was built in order to irrigate desert regions. However, dissolved salts have built up in the irrigated areas and increased the saltiness of the Nile itself. The Nile used to carried vast amounts of sediments to the Mediterranean. With their reduction, the sardine fishery in the eastern Mediterranean has been destroyed. The dam has increased erosion in the Nile waterway and in the delta region.

Giants of the River

The last part of the twentieth century has seen the building of colossal dams designed to bring hydroelectric power and flood control to undeveloped countries.

As of 1999, the world’s largest was the Itaipu Dam on the Parana River between Brazil and Paraguay in South America. Completed in 1991 at a cost of over $20 billion, it has eighteen huge turbines that generate as much as 12,600 megawatts of electricity. (By comparison, the largest dam in the United States, the Grand Coulee, outputs only 9,700 megawatts.) Over 30,000 workers took sixteen years to create the 4.8-mile- (7.6-kilometer-) long, 500-foot-(152-meter-) high structure, using enough concrete to build eight medium-sized cities. Its reservoir covers 870 square miles (2,262 square kilometers), and thousands of farm families had to be relocated. The presence of the reservoir has caused some cooling of the local climate, which endangers wheat and other commercial crops, but the dam produces clean, renewable energy and creates thousands of jobs.

Soon to overtake the Itaipu in scope and power production is the Three Gorges Dam on the Chang Jiang (Yangtze) River of China. With its twenty-six turbines, the Three Gorges is expected to output 18,200 megawatts of electricity—as much as eighteen nuclear plants—and help bring prosperity to central China. At a projected cost of $75 billion, the dam is scheduled for completion in 2010 and will be 610 feet (187 meters) high and 6,864 feet (2,092 meters) wide. Its reservoir will be 370 miles long (592 kilometers), filling the towering limestone canyons along the river’s route. As many as fourteen towns will be drowned and nearly two million people relocated. There is opposition to the dam because it will kill many aquatic animals and destroy ecosystems and valuable archaeological sites. On the other hand, it is hoped that the dam will end the disastrous flooding common to the Chang Jiang, which has killed as many as 300,000 people during the past century alone.

Other giant dams in progress include the Ataturk Dam on the Euphrates River in Turkey and the Sardar Sarovar in India, part of a proposed thirty-dam project on the Narmada River.

In Quebec, Canada, many people are opposing the James Bay Project, a proposed hydroelectric dam that will alter the region. People are beginning to wonder if it may be wiser to allow rivers to flow unobstructed.

Wild and Scenic Rivers Act

In 1968, the U.S. Congress passed the Wild and Scenic Rivers Act, which established a program to study and protect free-flowing rivers. The act prohibits building hydroelectric or other water development projects on certain rivers. In order to qualify for protection, a river must be undammed and have at least one outstanding resource, such as a wildlife habitat or historic feature. As of 2007, 130 rivers fall under the Act’s protection, including the Klamath River in California and the Rio Grande in Texas.

North American River Explorers

During the great period of world exploration, from about 1400 to 1900, most inland travel was accomplished by means of rivers. The journeys of Europeans into the North American wilderness are no exception. Jacques Cartier (1491–1557), a Frenchman, was the first. Beginning in 1534, he traveled down the St. Lawrence River to the present location of Montreal, Quebec. In 1673, French explorers Louis Jolliet (1645–1700) and Jacques Marquette (1637–1675) traveled down the Mississippi River to where it joins the Arkansas River, then returned via the Illinois River to Lake Michigan. Alexander Mackenzie (1764–1820) from Scotland, descended the Mackenzie River from Great Slave Lake in the Northwest Territories of Canada to the Arctic Ocean in 1789.

Native peoples

Rivers have always attracted human settlements, and most rivers and streams have probably played a part in the lives of people native to the region. In the United States, for example, the Great Plains between the Rocky Mountains and the Mississippi River was the home of many Native American tribes, including the Arapaho, Blackfoot, Cheyenne, Crow, Iowa, Pawnee, and Sioux. Bison (buffalo) were the primary source of meat, but the rivers and streams provided fish and other edibles, as well as water, and villages were often located alongside them. Rafts and dugout canoes made from logs, and more sophisticated craft covered with skins or birchbark, were used for river travel.

Since 1940, native peoples who still live a traditional lifestyle are usually found in undeveloped countries, and most of those have been touched by the modern world in some way. The Jivaro of Ecuador and Peru are an example. They live in the tropical forests and depend upon the rivers of that region—the Tigre, Pastaza, Morona, Alto Moranon, and Santiago—for fish and a means of travel.

The Food Web

The transfer of energy from organism to organism forms a series called a food chain. All of the possible feeding relationships that exist in a biome make up its food web. In rivers and streams, as elsewhere, the food web consists of producers, consumers, and decomposers. These three types of organisms transfer energy within the biome.

Algae are primary producers in rivers and streams. They produce organic materials from inorganic chemicals and outside sources of energy, primarily the sun.

Animals are consumers. Those that eat only plants, such as snails, are primary consumers in the river or stream food web. Secondary consumers, such as carp, eat the plant-eaters. Tertiary consumers are predators, like otters and anacondas, that eat second-order consumers. Humans are omnivores, meaning they eat both plants and animals.

Decomposers, which feed on dead organic matter, include some fly larvae. Bacteria play an important part in decomposition.

Extremely damaging to the river or stream food web is the concentration of pollutants and dangerous organisms that become trapped in sediments where life forms feed on them. These life forms are fed upon by other life forms and, at each step in the chain, the pollutant becomes more concentrated. Finally, when humans eat fish, ducks, or other river wildlife contaminated with the pollutants, they are in danger of serious illness. The same is true of diseases such as cholera, hepatitis, and typhoid, which can survive and accumulate in certain aquatic animals. These diseases can then be passed on to people who eat the animals.

Spotlight on Rivers and Streams

The Amazon

Of all the rivers in the world, the Amazon carries the greatest volume of water (one-fifth of all river water) and has the largest drainage basin (5 percent of the world’s total land area). In 1994, researchers claimed to have discovered its true source as the Ucayli River, and, if they are right, that may make the Amazon the world’s longest river, surpassing even the Nile. Its discharge—up to 7,000,000 cubic feet (196,000 cubic meters) per second—is so strong that it carries fresh water 200 miles (320 kilometers) into the Atlantic Ocean.

Amazon River

Location: Brazil and Peru in South America

Length: 3,915 miles (6,437 kilometers)

Discharge Basin: 2,722,000 square miles (7,077,200 square kilometers)

More than 1,000 streams make up the Amazon’s tributaries, at least 200 of which are major rivers in their own right. They include the Tapajos, the Purus, the Jurua, the Javari, the Rio Negro, the Madeira, the Japura, and the Tigre. The main stream originates in glacier-fed lakes in the Andes Mountains of Peru, only 100 miles (160 kilometers) from the Pacific Ocean, and travels across the continent to empty into the Atlantic.

Average velocity of the Amazon is about 1.5 miles (2.4 kilometers) per hour, although velocity increases greatly at flood times. During a flood, the height of the river may increase as much as 50 feet (15 meters). At its mouth, a 12-foot (3.6-meter) high tidal bore occasionally travels upstream at about 15 miles (24 kilometers) an hour with a force that can uproot trees growing along the river’s banks.

The Amazon receives up to 8 feet (2.4 meters) of annual rainfall, which causes it to overflow its banks for up to 30 miles (48 kilometers). This vast floodplain is underwater for several months each year, and many channels and oxbow lakes form. In Obidos, Brazil, its depth is about 300 feet (91 meters). The maximum width of its permanent bed is 10 miles (16 kilometers). The width of its mouth is 360 miles (580 kilometers). No delta has formed because the 3,000,000 tons (2,721,554 metric tons) of sediment discharged daily are carried northward by ocean currents and deposited along the coast of French Guiana.

The Amazon basin supports the largest area of rain forest in the world and the greatest plant and animal diversity, with about one million species. Plants, such as cassava, tonka beans, guava, and calabash, are found here. Trees include palms, myrtles, laurels, acacias, and rosewoods.

The Amazon region hosts more than two million species of insects. The number of Amazon fish species has been estimated at 1,500, more than 18 percent of the world’s known species. A few, such as the piaraucu, are commercially important. Most streams are home to piranhas, sting rays, and giant catfish.

Turtles and giant constrictor snakes represent reptiles. Birds live among the trees, and some, such as the fish eagle, hunt food in the river. Mammals include the capybara. The giant otter, which can grow to more than 6 feet (1.8 meters), is heavily hunted and almost extinct. It is only found in the Amazon.

Most human inhabitants of the Amazon basin have been Indians, such as the Jivaro and the Yanomamo, but the area has never been heavily populated. The soil is poor and not really suitable for farming, although huge tracts of forest are being destroyed for agricultural purposes. The river itself has been used for transportation and trading since the earliest times, and ocean-going vessels can travel as far as 1,000 miles (1,600 kilometers) upstream to Manaus, a city in northern Brazil.

The Nile

The Nile is the longest river in the world, at a length of 4,160 miles (6,695 kilometers). Its main tributaries are the White Nile, which has its headwaters in Burundi, and the Blue Nile, which rises in the highlands of western Ethiopia. Lake Albert, Lake Victoria, and Lake Tana all contribute water. The Blue Nile and the White Nile join together at Khartoum in Sudan and continue north into southern Egypt. Six stretches of rapids and waterfalls break its flow, ending at Aswan. Between Aswan and the Mediterranean Sea, the river creates a wide, rich flood-plain that has been cultivated for more than 3,000 years. The Nile delta lies north of Cairo, a 100-mile- (160-kilometer-) long stretch of rich silt

Nile River

Location: Egypt, Burundi, Rwanda, Uganda, Ethiopia, Kenya, Tanzania, and Sudan in Africa

Length: 4,160 miles (6,695 kilometers)

Discharge Basin: 1,100,000 square miles (2,860,000 square kilometers)

deposits cut by many streams. This area provides almost all of Egypt’s food crops, and cotton, one of the country’s major exports.

Precipitation varies along the Nile’s length, but most of its basin receives no rain from November to March. At Khartoum, 5 inches (12.7 centimeters) of rain falls annually, but Cairo receives only 1 inch (2.5 centimeters). The far south receives 79 inches (200 centimeters) of rain each year. Before the Aswan High Dam was completed in 1970, heavy rains in the south caused floods. The floods were vital to agriculture in the region, but they often caused serious damage. The dam prevents flooding, but holds back the silt, so the land has to be fertilized by other means. The dam has also led to erosion of the delta.

Grasslands and tropical forests occur where the river passes through Ethiopia and eastern Africa. Papyrus, water lettuce, sedges, and water hyacinth grow in and around the river. Farther north, drier conditions prevail and support only acacia and scrub. Beyond that, the Nile passes through true desert. In Egypt, desert land along the Nile is irrigated and cultivated.

The river supports fish, snakes, turtles, crocodiles, and lizards. In its southern waters, hippopotami can be found.

About 3100 BC, the ancient Egyptian civilization began on the banks of the Nile and was totally dependent upon the river both for water and for the sediments that enriched the soil. Lower Egypt, which was centered around the delta, was fertile and green. Upper Egypt was hot and dry, and the band of land suited for farming was only a few miles wide. Crops of all kinds were grown, and cattle, goats, pigs, and sheep were pastured. The river provided fish and waterfowl, and papyrus and other reeds and grasses were turned into fibers for baskets, boats, and paper. River mud was used to make bricks and, over time, the Egyptians developed great architectural skill, creating the magnificent pyramids that still stand.

Almost 97 percent of Egypt’s population still lives in the Nile valley and delta. Fishers and farmers live in the irrigated north and in southern regions where rainfall is adequate. In northern Sudan, nomads raise cattle and camels. More arid (dry) regions are extremely dependent upon the river for their economy. The Aswan High Dam produces electricity and has increased irrigated land by 30 percent.

The Huang He (Yellow River)

The Huang He (wang HO) or Hwang Ho, which is Chinese for Yellow River, is the second longest river in China and flows eastward from the Kunlun Mountains through the

Huang He (Yellow) River

Location: China

Length: 2,901 miles (4,641 kilometers)

Discharge Basin: 486,000 square miles (1,263,600 square kilometers)

central plains to the Yellow Sea. Its waters are clear in the upper regions, but as it passes through Shanxi and Shanxi provinces, it picks up large quantities of sediments—1,600,000,000 tons (1,440,000 metric tons) annually—that give it its yellow color and, therefore, its name. These sediments made the Huang He’s delta the fastest growing in the world. By the 1990s, dam construction, particularly Sanmen Gorge in the Henan Province, is causing the delta to erode.

Average precipitation in the Huang He basin is about 16 inches (40 centimeters) a year, and average annual discharge is 11 cubic miles (49 cubic kilometers). Rainfall varies greatly. In winter and early summer, the river is at its low point. In spring and late summer, water levels are high, sometimes as much as 70 feet (21 meters) above its banks, and severe flooding is common. The Huang He has overflowed its dikes (retaining walls) hundreds of times in the past 3,000 years, causing so much damage and so many deaths that it has been referred to as “China’s Sorrow.” In an attempt to control flooding, artificial embankments have been created along 1,120 miles (1,800 kilometers) of the river’s length.

Much of the Huang He basin is grassland. Cattail marshes are home to many geese, ducks, and swans. Carp and catfish live in the river itself.

China’s great civilization, dating from at least as early as 2000 BC, probably began in the Huang He basin. The basin supports a population of more than 120 million people. In many places, the current of the Huang He is so swift and the channel so shallow that it is of little use for navigation, but the floodplain is developed for agriculture.

The Chang Jiang (Yangtze)

The Chang Jiang (zhang jee-ANG), also spelled Yangtze, is the longest river in China at 3,915 miles (6,300 kilometers). It crosses the central region from east to west, where it flows into the East China Sea near Shanghai. Its headwaters are in the Tibetan highlands at elevations of about 18,000 feet (5,480 meters), and its course makes many bends. Below Wan-hsien it travels about 200 miles (320 kilometers) through spectacular gorges and deep canyons where rapids form. In the region of the gorges, the river is 500 feet (152 meters) deep, making it the world’s deepest. During summer floods, the water in this region can rise 200 feet (61 meters) and travel at velocities faster than 13 miles (21 kilometers) per hour. Boats are often towed through the gorges by large gangs of men on shore using 0.5-mile-long (0.8-kilometer-long) ropes made from bamboo.

Chang Jiang (Yangtze) River

Location: China

Length: 3,915 miles (6,300 kilometers)

Discharge Basin: 756,498 square miles (1,966,895 square kilometers)

Until 1957, there was only one bridge across the Chang Jiang, near Wuhan. Here, the river forms a vast floodplain, and flooding often occurs in summer. Since that time sixteen other bridges have been built. In 1998, the worst flood in forty-four years caused 3,004 deaths and twenty billion dollars (U.S.) in damages. More than fourteen million people were left homeless.

Precipitation averages 24 inches (60 centimeters) annually. Average discharge into the sea is 1,010,000 cubic feet (28,600 cubic meters) per second, and 250,000,000 tons (336,796,185 metric tons) of sediment is deposited annually.

Deciduous forests cover those areas of the basin that are not cultivated for farming. About 70 percent of China’s rice crop is grown in the Chang Jiang basin, and about 300 million people live in the region.

The Chang Jiang River has been home to the giant paddlefish, which may now be extinct. The rare Chang Jiang river dolphin was declared extinct in 2006, due to pollution, dam construction, hunting, and depletion of its food supply. Animal life varies along its length and includes thrushes, pheasants, and antelopes.

The Chang Jiang is important commercially. The volume of transportation is greater than all the other waterways of China combined. About 25,000 miles (40,000 kilometers) of main river and tributaries can be navigated by small craft, and ocean-going ships can reach Wuhan, a city in central China. Five of China’s largest cities—Shanghai, Wuhan, Chongqing, Chengdu, and Nanjin—are on or near the Chang Jiang system.

The river is considered ideal for generating hydroelectric power, and the gigantic Three Gorges Dam, which began construction in 1994, is predicted to provide at least 15 percent of China’s electricity needs. The dam will also help control flooding, but many scenic areas will be permanently covered by its reservoir. Thousands of people are being forced to relocate, and some scientists believe the dam will have undesirable effects on the environment after it is completed around 2010. Once completed, the dam will be the world’s largest at 6,864 feet (2,092 meters) wide and 610 feet (186 meters) high.

The Ganges (Ganga)

The Ganges (GANN-jeez) crosses northern India and then flows south through the provinces of Uttar Pradesh, Bihar, West Bengal, and finally Bangladesh. Its headwaters are the Alaknanda and Bagirathi Rivers, which begin in the Himalaya Mountains. Its delta

Ganges (Ganga) River

Location: India and Bangladesh

Length: 1,557 miles (2,491 kilometers)

Discharge Basin: 188,800 square miles (490,880 square kilometers)

begins 200 miles (320 kilometers) from the sea, where it joins with the Jumna River to form the Padma and then flows into the Bay of Bengal, where it discharges more sediment into the sea than any other river system.

The flow of water in the Ganges varies with the seasons, but the changes are seldom violent. It travels through dry plains, drawing water from several tributaries such as the Jumna, Gogra, Gandak, Kasi, and Brahmaputra. In the last 1,000 miles (1,600 kilometers), flow is sluggish. Floods are common, especially in the delta region where tropical storms and tidal waves (giant waves associated with earthquakes) occur. During one such storm in 1970, about one million people were killed.

Forests grow on the Gangetic plain. Where rainfall is heavy, evergreens predominate. Carp are found in the river, as are geese and ducks. Mammals include tigers, deer, and wild dogs, but reptiles are more numerous, especially crocodiles, lizards, and snakes. The endangered Bengal tiger makes its home in the Ganges Delta, as does the Indian python and the Asian elephant.

Changing water levels limit the use of the Ganges for large vessels. In the delta region, many places are accessible only by small watercraft. Soils here are rich and fertile, but in the north the land is too high for irrigation unless power equipment is used. Crops include rice, wheat, sugar, cotton, and oil seed, which support 500 million people.

To those of the Hindu faith, the Ganges is holy, and many places along its shore, such as Varanasi (Benares), Allahabad, and Hardwar, have religious significance. Many people go on pilgrimages (religious journeys) to visit these places. Hindus believe that bathing in the Ganges is religiously purifying, and thousands flock to its banks each day. The river is polluted by human sewage, fertilizer runoff, and industrial wastes. In 1985, efforts were begun to clean up the river and are ongoing.

One of the world’s earliest civilizations, the Indus, began in the neighboring Indus River valley and spread into the Ganges valley. One of the most well-known archaeological sites is Mohenjo-Daro, beside the Indus River.

The Indus

The Indus (INN-duhs) River begins in Tibet, travels through part of northern India, and completes its course in Pakistan. Its head-waters are formed by meltwater in the Himalaya Mountains and the Karakoram Range, where its flow is turbulent, unnavigable, and prone to flooding, especially in the summer months. When it leaves the mountains

Indus River

Location: Tibet, India, and Pakistan

Length: 1,800 miles (2,880 kilometers)

Discharge Basin: 372,000 square miles (967,200 square kilometers)

and enters the dry Punjab plains of Pakistan, it broadens and picks up sediments.

The major tributaries of the Indus are the five rivers of the Punjab: the Sutlej, Chenab, Jhelum, Ravi, and Beas. Its delta begins at Thatta and enters the Arabian Sea 70 miles (110 kilometers) farther south, where the river splits into several channels.

Rainfall in the Indus Valley ranges from 5 to 20 inches (13 to 51 centimeters) annually. The Indus is used for irrigation in the drier regions.

Desert vegetation predominates and includes thorn forests and many shrubs. Grasses do not thrive here. Indus baril, Indus gurua, Rita catfish, and snakehead fish are common fish. Sarus cranes and bearded vultures frequent the area, as do crocodiles and cobras. Mammals include deer and wolves.

The Punjab plain is Pakistan’s richest farming region, where wheat, corn, rice, millet, dates, and other fruits are grown. Large dams have been built on the river to provide hydroelectric power and water for irrigation. Use of the Indus has been disputed by India and Pakistan as they have tried to determine their borders.

The cities of the great Indus civilization, Harappa and Mohenjo-Daro, were built in the Indus valley around 2500 BC. The civilization appears to have declined gradually. It is believed that barbarian nomads of the Eurasian high plains invaded the region and destroyed what was left of this civilization.

The Colorado

The Colorado River is created by rain and melting snow in the mountains of central Colorado, Wyoming, and Utah. Its main tributaries are the Gunnison, the Dolores, the Green, the San Juan, the Little Colorado, the Gila, and the Virgin. As it travels south and southwest through Utah, Arizona, and Nevada, it creates the magnificent Grand Canyon, cutting deeper into the rock each year. The canyon is 218 miles (349 kilometers) long and, in one spot, 18 miles (29 kilometers) wide and 5,249 feet (1,600 meters) deep. At its mouth, the Colorado empties into the Gulf of California on the Pacific coast.

Colorado River

Location: Colorado, Arizona, Utah, Nevada, and California

Length: 1,440 miles (2,304 kilometers)

Discharge Basin: 244,000 square miles (634,400 square kilometers)

Discharge rates vary from 3,000 cubic feet (84 cubic meters) per second when water levels are low to 125,000 cubic feet (3,540 cubic meters) per second at flood times.

Much of the region is treeless and desertlike, and plants include those that tolerate semiarid conditions. Desert birds and waterfowl are common. Mammals include deer and bears.

The Colorado is one of the most valuable rivers in the world in terms of irrigation and power. The river is prone to flooding, but Hoover Dam and its reservoir (storage area), Lake Mead, have helped prevent disasters. Hoover Dam is the first multipurpose dam, providing both flood control and power generation. The Los Angeles-San Diego region receives much of its water from the Colorado, and arguments have ensued among neighboring states as to how the water should be divided. Environmental concerns have slowed further development of river resources.

Until the Spanish arrived in 1540 and gave the Colorado its present name, which means “red color,” the Colorado basin was the home of several Native American tribes, including the Pueblo and the Navajo.

The Congo (Zaire)

The Congo, or Zaire (zy-EER), River is the second longest in Africa. During the fifteenth century, Europeans called it the Zaire, a corruption of a Bantu word meaning “river.” Later, the name was changed to Congo after the great African kingdom of Kongo located near its mouth. It is now known by both names.

Congo (Zaire) River

Location: Zambia, Zaire, Congo, and Angola

Length: 2,716 miles (4,346 kilometers)

Discharge Basin: 1,425,000 square miles (3,705,000 square kilometers)

The Congo’s headwaters are in Kabalo, in the Democratic Republic of the Congo (DRC). It travels north across the country of Zaire, turns west, and then angles southwest, forming the border between Zaire and Congo. Its mouth, where it empties into the Atlantic Ocean, is in Angola.

The Congo’s discharge basin is the second largest in the world. It draws water from tributaries in the Central African Republic, Cameroon, Congo, Angola, Zaire, and northern Zambia. Some of its tributaries include the Chambezi, Lualaba, Luvua, Lukuga, Chenal, Ubangi, Sangha, and Kwa. Along its course it forms many lakes, such as Bangweulu, Mweru, and Malebo Pool. Rapids, waterfalls, gorges, and a narrow channel, are found along its length.

The river straddles the equator, passing through the great rain forest of Zaire, which keeps it supplied with rainfall all year long. The Congo basin has uniform average annual rainfall of 90 inches (229 centimeters).

Annual volume of discharge of the Congo River is second only to that of the Amazon. Its force is so great that brownish-colored Congo waters can still be distinguished for more than 300 miles (480 kilometers) out to sea. During prehistoric times its flow cut a submarine canyon as deep as 4,000 feet (1,220 meters) into what is now the ocean floor.

In the northern and southern parts of its basin it passes through grasslands (savannas) where trees are scattered. In Zaire and along the Zaire-Congo border, dense rain forest occupies the basin. The Congo River basin contains the second largest rain forest in the world, second to the Amazon rain forest.

Vegetation in the central basin is abundant and diverse. Huge rain forest trees draped with clinging vines line its banks. In the southern regions, acacias are common. In the highlands, bamboo, heather, and giant senecio are found. Cultivation and burning have altered the vegetation along much of the Congo’s length, and many non-native plants, such as cassava, citrus, and maize have been introduced.

At least 686 species of fish have been identified in Congo waters, 80 percent of which are found nowhere else in the world. Among the best known is the lungfish, which can live in oxygen-depleted water. Mammals typical of grasslands, such as lions and zebras, are found in the northern and southern regions, while gorillas and rhinoceroses are found in the rain forests. Elephants and leopards are common throughout.

The first people to inhabit the Congo were the Pygmy. Most native peoples who live along the Congo catch its fish. Some are also farmers. Many peoples, such as the Bobangi and the Teke, use the river for trade. The river is important as a source of hydroelectric power but the area’s economy has not yet been able to take full advantage of its potential.

In the late nineteenth century, many European explorers, such as David Livingstone (1813–1873) and Henry Morton Stanley (1841–1904), penetrated the African interior by traveling on the Congo.

The Tigris and Euphrates

The Tigris (TY-grihs) river flows though southeastern Turkey and Iraq. Above Basra in southern Iraq, it joins the Euphrates (yoo-FRAY-teez) to form the Shatt-al-Arab, which empties into the Persian Gulf.

Tigris and Euphrates River

Location: Turkey, Syria, and Iraq

Length: Tigris 1,181 miles (1,890 kilometers); Euphrates 1,740 miles (2,784 kilometers)

Discharge Basin: Tigris 145,000 square miles (377,000 square kilometers); Euphrates 295,000 square miles (767,000 square kilometers)

The Tigris-Euphrates basin is a hot, dry plain with daytime temperatures as high as 120°F (49°C). Average annual rainfall is only 8 inches (20 centimeters), and it falls seasonally, from November to March. The Euphrates floods twice during the year, the largest occurring in April and May, and creates marshy regions along its banks.

Desert vegetation, such as thorn and scrub, predominate. Bats, jackels, and wildcats are common mammals, but reptiles are more numerous, particularly snakes and lizards. Many fish live in the river, including the Tigris salmon.

Both rivers are used for irrigation, and two major reservoirs are located on the Euphrates where farmers raise cereal grains and dates and the nomadic tribes raise livestock. The Tigris provides irrigation waters for the cultivation of wheat, barley, millet, and rice.

The Tigris-Euphrates basin was the birthplace of the great Sumerian civilization, which dates back to at least 3000 BC in what was then Mesopotamia. The Sumerians dug canals along the Tigris, and Nineveh, the capital of ancient Assyria, was located there.

The Mississippi

The Mississippi is the second longest river in the United States, and its name in the Algonquian language means “father of waters.” Its headwaters are in Minnesota, and its drainage basin includes at least part of thirty-one states and covers about 40 percent of the country. As it flows south, it forms the boundary between Minnesota, Iowa, Missouri, Arkansas, and Louisiana on the west and Wisconsin, Illinois, Kentucky, Tennessee, and Mississippi on the east.

Mississippi River

Location: Minnesota, Iowa, Missouri, Arkansas, Louisiana, Wisconsin, Illinois, Kentucky, Tennessee, and Mississippi

Length: 2,348 miles (3,757 kilometers)

Discharge Basin: 1,243,700 square miles (3,233,620 square kilometers)

Fed by Lake Itasca, a glacial lake, the upper Mississippi forms rapids where it joins the Minnesota River. Further south it is lined by high bluffs, and below Cairo, Illinois, it becomes an old stream, meandering through flat floodplains between natural levees.

Its most important tributaries are the Illinois, the Chippewa, the Black, the Wisconsin, the Saint Croix, the Iowa, the Des Moines, the Ohio, the Arkansas, the Red, and the Missouri Rivers. The mouth of the Mississippi is at the Gulf of Mexico where its delta covers a region 9,872 square miles (25,568 square kilometers) in area, and about 79,935,000 tons (72,515,812 metric tons) of sediments are deposited each year.

The river travels the length of the United States and precipitation varies from 40 to 60 inches (102 to 152 centimeters). The southern section of the river is prone to severe flooding, and in 1927 Cairo, Illinois, remained flooded for 153 consecutive days. Artificial levees, meander cutoffs, flood outlets, and upstream reservoirs have been built since that time to try to contain floodwaters. The river flooded again in 1993, covering 14,000,000 acres (5,500,000 hectares) and causing fifty deaths.

Willow, oak, and pine trees are common, as are many species of grasses and aquatic plants. Fish include catfish and paddlefish. Cranes, ducks, and other waterfowl frequent the river, as do turtles, muskrats, and deer.

Ocean-going ships can travel upstream to Baton Rouge, Louisiana. Smaller ships can travel through 15,000 miles (24,150 kilometers) of the entire river system, and use of the river for transportation is increasing. Cargo consists mainly of midwestern grain and petroleum products from the Gulf of Mexico. More than 460,000,000 tons (420,000,000 metric tons) of freight are transported on the Mississippi each year.

Native peoples who once lived along the river were the Ojibwa, the Winnebago, the Fox, the Sauk, the Choctaw, the Chickasaw, the Natchez, and the Alabama. The first European to reach the river inland was Hernando de Soto (c. 1496–1542) in 1541. In 1673, Jacques Marquette (1637–1675) and Louis Jolliet (1645–1700) explored the northern regions. The river system enabled Europeans to settle the central United States, and it was an important transportation route during the Civil War.

The Missouri

The Missouri is the longest river in the United States and the largest tributary of the Mississippi. Its headwaters are the Jefferson, Gallatin, and Madison Rivers in Montana, and it travels through North and South Dakota to finally join the Mississippi in St. Louis, Missouri. Its own tributaries include the Little Missouri, the Cheyenne, the White, the Niobrara, the James, the Big Sioux, the Platte, and the Kansas rivers. Its drainage basin includes parts of Canada.

Missouri River

Location: Montana, North Dakota, South Dakota, Nebraska, Iowa, and Missouri

Length: 2,466 miles (3,946 kilometers)

Discharge Basin: 529,000 square miles (1,375,400 square kilometers)

In Montana and North and South Dakota, it passes through high plains where the soil is poor. Farther south it enters the humid grain belt and the drier grasslands where cattle graze. The soil is rich along is southern stretches, although erosion is heavy. The river carries so much sediment that is nicknamed is “Big Muddy.”

Precipation ranges from 20 to 40 inches (51 to 102 centimeters) annually. The river’s average discharge is about 64,000 cubic feet (1,810 cubic meters) per second. Flood season is from April to June, and the federal government has built dams and reservoirs to help control flooding, provide irrigation, and make the river navigable.

Natural vegetation in the Missouri basin consists primarily of grasses. Fish include bass and trout. Mammals such as deer and coyotes can be found in this region.

Native peoples who lived in the region include the Cheyenne, the Crow, the Mandan, the Pawnee, and the Sioux. French explores Jacques Marquette (1637–1675) and Louis Jolliet (1645–1700) explored the region by 1673. By 1843 farmers had begun to settle in the Missouri valley. Today, the Missouri is important to traffic in bulk freight such as grain, coal, steel, petroleum, and cement.

The Volga

The Volga is the longest river in Europe. Its headwaters are in the hills north of Moscow, Russia, and it empties into the Caspian Sea, which lies on Russia’s southern border. It has more than 200 tributaries, the most important of which is the Kama, Oka, Vetluga, and Sura rivers The Volga delta covers more than 34,000 square miles (86,000 square kilometers).

Volga River

Location: Russia

Length: 2,292 miles (3,667 kilometers)

Discharge Basin: 532,800 square miles (1,385,280 square kilometers)

Melting snow and summer rains are the Volga’s main source of water. Annual precipitation averages 20 inches (50 centimeters) in the north and 12 inches (30 centimeters) in the south. Spring flooding is controlled by several dams and reservoirs.

The Volga passes through forested land and forms several lakes in the north. Below that it enters a flat, swampy basin bordered by low hills. Its southern section curves around mountains and finally enters a broad floodplain.

Willow, pine, and birch are common trees. River fish include sturgeon, perch, pike, and carp. Mammals include deer, beavers, and foxes. The Volga delta, with a length of 99 miles (160 kilometers), is Europe’s largest estuary and the only place in Russia where pelicans, flamingo, and lotuses may be found.

Although almost all of the Volga is navigable, ice covers most of the length of the river for three months. The river carries about 50 percent of Russia’s river freight, including timber, petroleum, coal, salt, farm equipment, construction materials, fish, and fertilizers.

For More Information

BOOKS

Angelier, Eugene, and James Munnick. Ecology of Streams and Rivers. Enfield, New Hampshire: Science Publishers, 2003.

Day, Trevor. Biomes of the Earth: Lakes and Rivers. New York: Chelsea House, 2006.

Gleick, Peter H., with Nicholas L. Cain, et al. The World’s Water 2004-2005: The Biennial Report on Freshwater Sources. Washington DC: Island Press, 2004.

Gloss, Gerry, Barbara Downes, and Andrew Boulton. Freshwater Ecology: A Scientific Introduction. Malden, MA: Blackwell Publishing, 2004.

Hauer, Richard, and Gary A. Lamberti. Methods in Stream Ecology, 2nd Edition. San Diego: Academic Press/Elsevier, 2007.

Postel, Sandra, and Brian Richter Rivers for Life: Managing Water For People And Nature. Washington DC: Island Press, 2003.

Voshell, J. Reese, Jr. A Guide to Common Freshwater Invertebrates of North America. Blacksburg, VA: McDonald and Woodward Publishing Company, 2002.

Woodward, Susan L. Biomes of Earth: Terrestrial, Aquatic, and Human Dominated. Westport, CT: Greenwood Press, 2003.

Worldwatch Institute, ed. Vital Signs 2003: The Trends That Are Shaping Our Future. New York: W.W. Norton & Co., 2003.

PERIODICALS

Carwardine, Mark. “So Long, and Thanks for All the Fish: If the Yangtze River Dolphin isn’t Quite Extinct Yet, It Soon Will Be.” New Scientist. 195. 2621 September 15, 2007: 50.

Petty, Megan E. “The Colorado River’s Dry Past.” Weatherwise. 60. 4 (July-August 2007): 11.

Sterling, Eleanor J, and Merry D. Camhi. “Sold Down the River: Dried Up, Dammed, Polluted, Overfished—Freshwater Habitats Around the World are Becoming Less and Less Hospitable to Wildlife.” Natural History. 116. 9 November 2007: 40.

ORGANIZATIONS

American Rivers, 1101 14th Street NW, Suite 1400, Washington, DC 20005, Phone: 202-347-7550; Fax: 202-347-9240, Internet: http://www.americanrivers.org.

Environmental Defense Fund, 257 Park Ave. South, New York, NY 10010, Phone: 212-505-2100; Fax: 212-505-2375; Internet: http://www.edf.org.

Envirolink, P.O. Box 8102, Pittsburgh, PA 15217; Internet: http://www.envirolink.org.

Environmental Protection Agency, 401 M Street, SW, Washington, DC 20460, Phone: 202-260-2090; Internet: http://www.epa.gov.

The Freshwater Society, 2500 Shadywood Rd., Navarre, MN 55331, Phone: 952-471-9773; Fax: 952-471-7685; Internet: http://www.freshwater.org.

Friends of the Earth, 1717 Massachusetts Ave. NW, 300, Washington, DC 20036-2002, Phone: 877-843-8687; Fax: 202-783-0444; Internet: http://www.foe.org.

Greenpeace USA, 702 H Street NW, Washington, DC 20001, Phone: 202-462-1177; Internet: http://www.greenpeace.org.

Global Rivers Environmental Education Network (GREEN), 2120 W. 33rd Avenue, Denver, CO 80211; Internet: http://www.earthforce.org/green.

Izaak Walton League of America, 707 Conservation Lane, Gathersburg, MD 20878, Phone: 301-548-0150; Internet: http://www.iwla.org/.

Project Wet, 1001 West Oak, Suite 210, Bozeman, MT 59717, Phone: 866-337-5486; Fax: 406-522-0394; Internet: http://projectwet.org.

Sierra Club, 85 2^nd Street, 2^nd Fl., San Francisco, CA 94105, Phone: 415-977-5500; Fax: 415-977-5799, Internet: http://www.sierraclub.org.

World Wildlife Fund, 1250 24th Street NW, Washington, DC 20090-7180, Phone: 202-293-4800; Internet: http://www.wwf.org.

WEB SITES

National Geographic Magazine: http://www.nationalgeographic.com (accessed September 12, 2007).

National Park Service: http://www.nps.gov

Nature Conservancy: http://www.tnc.org

Scientific American Magazine: http://www.sciam.com

U.S. Fish and Wildlife Services: http://www.fws.gov/