RIVERS

RIVERS . Among the Native American Yurok people, who live along the Yurok River in northern California, orientation in the world was not provided by the four cardinal directions, but by the river itself: upstream and downstream. To these salmon fishermen, dependent upon the river for livelihood, the river alone was the primary axis of orientation.

In ancient times too, there were great civilizations whose life was so oriented toward one major river that they have come to be called river civilizations: Mesopotamia along the Tigris and Euphrates, Egypt along the Nile, the Indus Valley civilization along the Indus. In all these it is not surprising that the river itself should function as a fundamental means of world orientation and become associated with yearly inundation, fertility, and with life in its fullest sense.

Ancient Mesopotamian civilization made a distinction between Tiamat, the great mother of the salt waters of chaos and creation, and Apsu, the lord who ruled the "sweet waters" under the earth that fill the rivers and the springs. Ea was a descendent of Apsu, and Ea's offspring included Marduk, who was "born within the holy Apsu," associated with rivers, and called in one hymn the creator of the Tigris and Euphrates. Among his other creative tasks, Marduk "has opened the fountains [and] has apportioned waters in abundance" (Heidel, 1942, p. 56). Tammuz, too, is called a "son of the deep" and is the corn spirit who comes to life each year with the fertilizing waters of the rivers.

Ancient Egyptian civilization also saw fresh waters as springing from the abyss beneath the earth. There were said to be two rivers called the Nile, however, one that flowed on earth and one that flowed across the sky in heaven. This vision of the heavenly river, identified with the Milky Way, is also part of the mythology of the river Ganges in India. In ancient Egypt the Nile was so central that many of the great gods and goddesses are associated with the river in some way. The river itself is often depicted as the male god Hapi, with two full breasts, from which the northern and southern branches of the Nile spring; he holds two vases, which represent in another fashion the northern and southern Nile. The goddesses Anuket and Isis are both identified with the Nile as she inundates the land and fertilizes the fields. Khnemu (Khnum), the water deity with four rams' heads, is seen to represent the four sources of the Nile. Osiris, the dying and rising god, is also identified with the Nile as it sinks and rises again.

Although there is no systematic mythology available from the Indus Valley civilization to clarify how the inhabitants regarded the river itself, there is surely evidence in the large bathing pool of Mohenjo-Daro and in the elaborate drainage systems of both Harappa and Mohenjo-Daro that the inhabitants cared greatly for the cleansing properties of running water. Later Indian civilization, preserving this emphasis on running water and purification, has developed a full range of mythological and ritual traditions concerning sacred rivers.

During the Vedic period, sacred rivers are mentioned, often numbering seven: the five rivers of the Punjab, plus the Indus and the mysterious Sarasvati. Later on, with the movement of the center of Aryan civilization into the Ganges Valley, the river Ganges (Skt., Gaṅgā) becomes preeminent among rivers. As a female divinity in the form of a heavenly river, Gaṅgā agreed in her mercy to flow upon the earth, falling first upon the head of Śiva, who broke the force of her cascade from heaven. It is said in the Hindu epics and Purāṇas, which tell the tale of Gaṅgā's descent and which contain descriptions of the world's mythic geography, that Gaṅgā actually split into four streams when she fell. From Mount Meru, the cosmic mountain in the form of a lotus flower at the center of the world, Gaṅgā flowed north, south, east, and west—watering the whole world with the waters of life. The southern branch became the Ganges of India.

The importance of the Ganges as the paradigmatic river—holy, cleansing, and life-giving—is further seen in its widespread duplication in other rivers. Today, India counts seven sacred rivers, often called the Seven Gaṅgās, that are thought to supply the whole of India with sacred waters. In addition to the Ganges, there are the Indus, also called Sindhu; the Sarasvatī, said to have disappeared from earth and to flow underground; the Yamuna, which flows from the Himalayas, through North India, past Kṛṣṇa's birthplace at Mathura, and on to its confluence with the Ganges at Prayāga, the modern Allahabad; the Narmadā, which flows west across central India from its source in Amarakanṭaka to the Arabian Sea; the Godāvārī, which flows eastward from its sacred source above the temple of Tryambaka in Maharashtra; and the Kāverī, which flows eastward across southern India from its source at Talai Kāverī in Coorg country.

The ritual treatment of such rivers in India confirms their sanctity. The Narmadā, for instance, is circumambulated in a long pilgrimage that takes several years to complete. The confluence of rivers, like that at Prayāga where the Ganges, Yamunā, and Sarasvatī are said to meet, is an especially holy place and often becomes the site of special pilgrimage observances like the great Kumbha Melā held every twelve years. Along the banks of the Ganges, or at the source of the Narmadā or Kāverī, one may see the tall multiwicked lamps of the evening āratī, a ritual prayer performed for the very waters of the river itself.

River Gods and Deities

For Hindus, the Ganges is not only a sacred river but a liquid form of the divine. She is called "liquid śakti," or female energy, and is said to be Śakti, the female counterpart of the great lord Śiva, in the form of a river. Gaṅgā as a goddess is depicted as utterly auspicious, holding a lotus and a water pot while riding a crocodile. She is often addressed as Gaṅgā Mātā, Mother Ganges; the other seven rivers are similarly depicted as goddesses and addressed as "Mother."

Among many African peoples, rivers and streams are considered the homes of water spirits. The feminine names of rivers often signify a direct connection between flowing waters, fecundity, and the female. Some rivers are themselves seen as goddesses. The goddess Yemọja of the Yoruba, for instance, is said to have turned into the river Ogun and is symbolized by river-worn stones through which offerings are made to Yemọja. Yemọja's son was Ṣango, whose many wives were rivers. The most important among them was the faithful Ọya, who became the river Niger.

The personification of rivers and their identification with spirits was also prominent in ancient Syria, where the baalim had seats on the banks of streams and springs, as well as in ancient Greece, where Homer speaks of altars built upon river banks and of bulls sacrificed to the river. Native Americans have also identified rivers as spirits. In the Southwest, the Colorado River was traditionally thought of as female, and the San Juan River as male. The confluence of the two near Navajo Mountain in Utah was traditionally called the "nuptial bed," where numerous "water children" of springs, clouds, and rains were born.

Living Waters

It has been noted that Hindu cosmology views the heavenly river Gaṅgā as flowing in four, or sometimes seven, streams into the four quarters of the earth. The waters of the Ganges, identified with the milk of mother cows, are truly life-giving waters, and are called "mother" as they are sipped by devout Hindus. The vision of Eden presented briefly in Genesis 2 also evokes a river issuing forth from the garden and splitting into four streams. Of the four, the Tigris and Euphrates are named and well known, but concerning the Pishon and the Gihon there is disagreement, although some speculation identified the Pishon with the Indus and the Gihon with the Nile. Josephus Flavius in the first century ce, Eusebius in the third century, and others after them identified the Pishon with the sacred Ganges, which by that time had become well known in the ancient world. The notion that such divine waters issue forth from Paradise is also present in the Sumerian myth of the land of Dilmun, where the living waters are associated with Tammuz. The Egyptian Nile, with its four sources, has also been identified with Osiris and Isis, both of whom are associated with the notion of the river as "living waters."

In one of the prophet Ezekiel's visions (Ez. 47), he sees a stream of water issuing from beneath the main door of the Temple in Jerusalem, flowing from the Holy of Holies itself. At first it is ankle-deep. Gradually it becomes a great river, deep enough to swim in. Its waters are the waters of life; even the salt waters become sweet and living waters once this sacred river flows into them. Along the banks of the river, on both sides, are trees of all kinds, bearing fresh fruit and healing leaves.

This vision is repeated in the Revelation to John in the New Testament (Rv. 22). An angel shows John the heavenly Jerusalem: There is no temple, but the Christ, the Light, the Lord alone, sits enthroned at the center. From "the throne of God and of the Lamb" flows the river of the waters of life. "Bright as crystal," it flows through the city and produces on either side the tree of life, bearing twelve kinds of fruits and yielding leaves for "the healing of the nations."

Purification and Rebirth

Living waters are purifying waters. The running water of rivers is often used ritually for purification, or where it is not available, the pouring of water may accomplish the same aim. The Hindu ritual tradition makes it clear that water used in purification must not be standing water, but flowing, living water. Lustrations with such water prepare one ritually for worship, or for eating, and remove the impurity associated with childbirth or with death. Bathing in the Ganges is said to purify not only the sins of this birth but also those of many previous births.

Such use of running water, which is homologized to river water, is common elsewhere as well. Greek ritual prescribed bathing in a river or spring after an expiatory sacrifice. As recorded by Fray Bernardino de Sahagún, the Aztec prayer over a newborn child asks, "May this water purify and whiten thy heart: may it wash away all that is evil." Similar rites of baptism, in the Isis tradition as well as in the Christian tradition, use the symbolic power of living water to wash away the sins of the past.

Rites of healing are a form of rites of purification. The rivers of biblical Syria, Abana, and Pharpar were famous for healing. So it was with indignation that Naaman, the commander of the army of the king of Syria, received word from the prophet Elisha that he should bathe in the river Jordan seven times in order to be cleansed of his leprosy (2 Kgs. 5). To the present day the Jordan retains its reputation for healing, but especially so among Christians. The source of the Euphrates River was also famous for healing, and a bath there in the springtime was said to keep one free of disease all year long. The healing properties of the river Ganges are also well known, and pilgrims bring small sealed bottles of Ganges water home for medicinal use. Among the Hindus of Bali, the springs of Tampak Siring are filled with healing waters.

Rivers of Death

Crossing the river at the time of death, as part of the journey to another world, is a common part of the symbolic passage that people have seen as part of one's journey after death. In the Epic of Gilgamesh, the hero encounters a boatman who ferries him across the waters of death as he seeks the secret of immortality. The river Styx of Greek mythology is well known as the chief river of Hades, said to flow nine times around its borders. Styx is married to the Titan Pallas and according to Hesiod counts as her children Rivalry, Victory, Power, and Force. The power of the Styx is evidenced in the fact that Achilles gained his invulnerability by being dipped in the river as a baby held by his heel, the only part of his body thereafter vulnerable to mortal wounds. In addition, the most inviolable oath of the gods is sworn with a jug of water from the Styx, poured out while the oath is being uttered.

In Hindu mythology, the river Vaitaraṇī marks the boundary between the living and the dead; in the Aztec journey, the river Mictlan must be crossed on the way to the underworld; in Japan, rivers are part of certain landscapes designated as realms of the dead in both the Shintō and Buddhist traditions. The Sanzunokawa, for example, is said to divide the realms of the living and the dead. The dry riverbed of Sainokawara is said to be the destination of dead children.

The far shore of the river of life and death, or birth and death, thus becomes an important symbol for the destination of one's spiritual journey in many religious traditions. In the Buddhist tradition, nirvāṇa is referred to as the "far shore." In the Hindu tradition, holy places are called tīrtha s ("fords") because they enable one to make that crossing safely. Riverbank tīrtha s, such as Banaras and Prayaga, are thought to be especially good places to die. In the Christian tradition, crossing over the Jordan has come to have a similar symbolism. On the far shore is not only the promised land, but the spiritual promised land of heaven. Home is on the far shore. As the African American spiritual puts it:

I look'd over Jordan and what did I see?
Comin' for to carry me home.
A band of angels comin' after me.
Comin' for to carry me home.

See Also

Baptism; Boats; Ganges River; Water.

Bibliography

Darian, Steven G. The Ganges in Myth and History. Honolulu, 1978. A study of mythology, symbolism, sculpture, and history of the Ganges River.

Eck, Diana. "Gaṅgā: The Goddess in Hindu Sacred Geography." In The Divine Consort: Rādhā and the Goddesses of India, edited by John Stratton Hawley and Donna M. Wulff. Berkeley, 1982. A study of the mythology, ritual, and theology associated with the river Gaṅgā in the Hindu tradition.

Eliade, Mircea. Patterns in Comparative Religion. New York, 1958. An investigation of the nature of religion through the classification of hierophanies. See especially chapter 5, "The Waters and Water Symbolism."

Glueck, Nelson. The River Jordan. New York, 1968. An exploration of the geography, the archaeology, and the history of the valley of the Jordan.

Heidel, Alexander. The Babylonian Genesis. Chicago, 1942. A translation of the published cuneiform tablets of various Babylonian creation myths.

Hopkins, E. Washburn. "The Sacred Rivers of India." In Studies in the History of Religions Presented to Crawford Howell Toy, edited by D. G. Lyon and George Foot Moore, pp. 213–229. New York, 1912. An overview in short compass of India's sacred rivers.

Zahan, Dominique. The Religion, Spirituality, and Thought of Traditional Africa. Translated by Kate Ezra Martin and Lawrence M. Martin. Chicago, 1979. See especially the chapter "The Elementary 'Cathedrals,' Worship and Sacrifice," which discusses natural manifestations of divinity in Africa, including the places associated with water.

New Sources

Feldhaus, Anne. Water and Womanhood. New York, 1995.

Fineman, Mark. "A Scheme to Harness India's Sacred Waters Brings Tempers to a Boil." Smithsonian 21 (1990): 118 ff.

Lai, Whalen W. "Looking for Mr Ho Po: Unmasking the River God of Ancient China." History of Religions 29 (May 1990): 335–350.

Mason, John. Olóòkun: Owner of Rivers and Seas. Brooklyn, N.Y., 1996.

Sauer, James A. "The River Runs Dry: Creation Story Preserves Historical Memory." Biblical Archaeology Review 22 (July–August 1996): 52–57, 64.

Sinclair, Bryan T. "Merging Streams: The Importance of the River in the Slaves' Religious World." Journal of Religious Thought nos. 53–54 (Winter–Fall 1997): 1–19.

Wrigley, Christopher. "The River-God and the Historians: Myth in the Shire Valley and Elsewhere." Journal of African History 29, no. 3 (1988): 367–383.

Diana L. Eck (1987)

Revised Bibliography

Rivers

Formation of rivers

River systems

Climactic influences

Hydrological cycle

River floods

Human control of rivers

Resources

A river is a natural stream of freshwater that carries a larger volume of water than streams and the smaller tributaries that can empty into it. Rivers can range in size from only tens of yards wide to water-courses like the Mississippi River and the Nile, whose maximum width can approach a mile.

Rivers are normally the main channels or largest tributaries of drainage systems. Typical rivers begin with a flow from headwater areas made up of small tributaries, such as springs. They then travel in meandering paths at various speeds before ultimately emptying into basins, lakes, or oceans.

Sixteen of the world’s largest rivers account for close to half of the world’s river flow. By far, the largest river is the Amazon River, running 3,900 miles (6,275 km) long. Discharging an average of four million cubic feet (112,000 m³) of water each second, the Amazon River alone accounts for 20% of the water discharged each year by all the rivers on the planet.

Formation of rivers

Precipitation in the form of rain or snow is the source of the water flowing in rivers. Rainwater can return to the oceans as run-off, or it can be evaporated directly from the surface from which it falls, or it can be passed into the soil and mantle rock. Water can reappear by evaporation from Earth’s surface, by transpiration directly from vegetation, and by emerging from underground sources.

When a heavy rain falls on ground that is steeply sloped or is already saturated with water, water runoff trickles down Earth’s surface, rather than being absorbed. Initially, the water runs in an evenly distributed, paper-thin sheet, called surface run-off. After it

travels a short distance, the water begins to run in parallel formations called rills and, at the same time, gathers turbulence. As these rills pass over fine soil or silt, they begin to dig shallow channels, called runnels. This is the first stage of erosion.

These parallel rills last only a few yards, before uniting to form a small watercourse (typically a stream). As a stream acquries more water and grows larger, it represents a brook. As a brook gains sufficient volume from groundwater supplies, the volume of water it carries becomes more constant. Once the volume of water carried reaches a certain level, the brook becomes a river.

River systems

Rivers can have different origins and, as they travel, often merge with other bodies of water. Thus, the complete river system consists of not only the river itself but also of all the converging tributaries. Every river has a beginning. Because gravity plays a key role in the direction that rivers take, rivers almost always follow a down hill gradient. The point of origin for rivers tends to be the highest point in the watercourse. Some rivers start from springs, which are the most common type of river source in humid climates. Springs occur as groundwater rises to Earth’s surface and flows away. Other rivers are initiated by run-off from melting glaciers located high in the mountains. Often, rivers having their origins in huge glaciers are quite large by the time they emerge from openings in the ice.

Lakes and marshes are the sources for other rivers. As river sources, lakes can be classified in three ways. They can be true sources for rivers; they can be an accumulation of water from small feeder streams; or they can hide a spring that is actually the true source of the river. The Great Lakes are prime examples of source lakes. Although there are a few springs that feed them, the majority of the water coming into the lakes arises from precipitation falling onto their surfaces. Therefore, they, not their tributaries, are the source of surrounding rivers.

As rivers make the trip from their source to their eventual destination, the larger ones tend to meet and merge with other rivers. Resembling the trunk and branches of a tree, the water flowing in the main stream often meets the water from its tributaries at sharp angles, combining to form the river system. As long as there are no major areas of seepage and as long as the evaporation level remains reasonable, the volume of water carried by rivers increases from its source to its mouth with every tributary.

When two bodies of water converge, it is clearly evident as their shorelines merge. However, the water from the two bodies often continues to flow separately, like two streams flowing in a common river bed. This occurrence is especially clear when two rivers meet that contain different amounts and types of suspended sediment. For example, when the Ohio and the Mississippi rivers meet, a clear difference in the color of water in the Mississippi river can be seen by a strip of clear water one quarter of a mile wide on the river’s eastern side that runs for miles. To the west of this strip, however, the water color is a cloudy yellow.

Along its path, a single river obtains water from surface run-off from different sections of land. The area from which a particular section of a river obtains its water is defined as a catchment area (sometimes called a drainage area). The lines that divide different catchment areas are called watersheds. A watershed is usually the line that joins the highest point around a particular river basin. Therefore, at every point along the line of a watershed, there is a downward slope going into the middle of the catchment area.

Climactic influences

Rivers are highly influenced by the climate conditions, which determine the amount of precipitation, its seasonality, and its form as rainwater or as ice. Because of the climate and subsequent rainfall patterns, three general types of rivers exist. The first are the perennial or permanent rivers. Normally, these rivers are located in more humid climates where rainfall exceeds evaporation rates. Thus, although these rivers may experience seasonal fluctuations in their levels of water, they have constant streamflow throughout the year. With few exceptions, streamflow in these rivers increases downstream, and these rivers empty into larger bodies of water, such as oceans. In fact, 68% of rivers drain into oceans. All of the world’s major rivers are perennial rivers.

The second type of river is the periodic river. These rivers are characterized with predictably intermittent streamflow. Usually appearing in arid climates where evaporation is greater than precipitation, these rivers run dry on occasion, but there are regular intervals of streamflow. Typically, these rivers have a decrease in streamflow as they travel due largely to high levels of evaporation. Often, they do not reach the sea, but instead run into an inland drainage basin.

The third type of river is the episodic river. These rivers are actually the run-off channels of very dry regions. In these regions of the world, there are only slight amounts of rainfall and it evaporates quickly. This type of streamflow occurs rarely.

Interestingly, some rivers span two types of climactic regions. These rivers, known as exotic rivers, begin in humid or polar regions and flow into dry areas. The largest of these rivers have enough water at their sources to enable them to reach the sea. The Nile River, for example, gets sufficient water at its humid source to travel over the Nubian and Arabian deserts. While it receives a substantial amount of water from the Blue Nile at Kartoum, it then must travel 1,676 miles (2,700 km) before it reaches the Mediterranean Sea.

Hydrological cycle

The hydrologic cycle is very important to the existence of rivers, as it is to all life on Earth. Without this cycle, every stream and watercourse would dry up. The hydrological cycle is the continuous alternation between evaporation of surface water, precipitation, and streamflow. It is a cycle in which water evaporates from the oceans into the atmosphere and then falls as rain or snow on land. The water, then, is absorbed by the land and, after some period of time, makes its way back to the oceans to begin the cycle again.

The water content of the atmosphere is estimated to be no greater than 0.001% of the total volume of water on the globe. Despite its seemingly insignificant amount, atmospheric water is essential in the hydro-logical cycle. As water falls as rain, three things can happen. First, usually some of the rain falls directly into rivers. Second, some of it is soaked up by ground, where it is either stored as moisture for the soil or where it seeps into ground water aquifers. Third, rainfall can freeze and become either ice or snow. Interestingly, water is sometimes stored outside the hydrological cycle for years in cavities as fossil ground water in continental glaciers. The next event, evaporation, is the most critical link in the cycle of water circulation. If rain water evaporates too rapidly, rivers cannot form. For example, in hot deserts, heavy downpours sometimes occur, but the water evaporates completely in a short period of time. However, as long as the evaporation is slower than the typical amount of rainfall, viable rivers can exist.

Rivers, like precipitation and evaporation, are a vital part of the hydrological cycle. Of all of the forms of water in nature, watercourses (rivers and streams) make up the smallest total amount of water on Earth, about 0.0001% of the total volume. However, when combined with the precipitation falling on the ocean and the run-off from melting ice in Antarctica and Greenland, rivers replace about the same amount of water as is evaporated by the oceans. In addition to this, because they carry water away from saturated soil, they prevent marshes and bogs from forming in many low-lying areas.

Although the hydrologic cycle is a constant phenomenon, it is not always evident in the same place, year after year. If it occurred consistently in all locations, floods and droughts would not exist. Thus, each year some places on Earth experience more than average rainfall, while other places endure droughts. It is not surprising, then, that people living near rivers often endure floods at some time or other.

River floods

River levels have a direct influence on the activities and well-being of human beings. While low flowing rivers interfere with transport, trade, and navigation, high water threatens human life and property. Basically, floods are a result of a river’s discharge behavior and the climate within which it is located. The most common cause of flooding is when it rains extremely hard or for an unusually long period of time. Additionally, areas that experience a great deal of snow in the wintertime are prone to springtime flooding when the snow and ice melt, especially if the thaw is relatively sudden. Furthermore, rainfall and snowmelt can sometimes combine to cause floods, such as when rain falls on an area covered with melting snow.

Under normal conditions, rivers move fairly slowly as they transport silt and other debris produced by rain and snow. During floods, however, this transport is achieved much more rapidly, sometimes with beneficial side effects and sometimes with disastrous ones. One example of beneficial flooding is where the high water transports new top soil to local crops. Furthermore, floods can provide local crops badly needed moisture. The negative aspects of flooding are fairly obvious; often people drown and their property is destroyed.

Rivers in more humid regions are less likely to experience significant flooding than those located in more arid climates. In fact, floods in humid areas occur an average of about one time per year. Although on rare occasions these rivers experience larger floods, the water is normally no more than twice the size of a normal flood. While rivers in arid regions experience small flooding on an annual basis as well, when they experience rare, large floods, it can be devastating.

Human control of rivers

For centuries, rivers have been very important to human society. Aside from soil, no other feature on Earth is as closely bound to the advancement of human civilization. Trying to control river flow has been a key part of civil engineering. This is especially true because of the need to avoid natural flooding and the desire to take advantage of the benefits that flood plains offer agriculture. Furthermore, managing rivers can also satisfy human needs to store water for times of drought.

While the techniques of river management are fairly well understood, true river management is not commonly put into practice because of the expense and the size of the projects involved. In fact, few of the major rivers in the world is controlled or even managed in a way that modern engineering and biological techniques would allow. One river that has been dammed is China’s Yangtze River. The Three Gorges Dam, which was completed in 2006, is creating a lake that will displace almost two million people. Management of smaller watercourses is more typical. For example, the San Joaquin in California has been

KEY TERMS

Catchment area— The area from which a particular section of a river obtains its water; also known as a drainage area.

Erosion— Movement of material caused by the flow of ice, water, or air, and the modification of the surface of the earth (by forming or deepening valleys, for example) produced by such transport.

Exudation— The oozing of water out of the ground.

Hydrologic cycle— The continuous, interlinked environmental circulation of water.

Transpiration— The movement of water from plants into the atmosphere.

Tributary— A stream or other body of water that flows into a larger one.

Watershed— The land from which water flows into a wetland, waterbody, or stream.

completely developed to take advantage of the irrigation opportunities that the stream offers.

Resources

BOOKS

Hessler, Peter. River Town: Two Years on the Yangtze. New York: Harper Perennial, 2006.

Palmer, Tim. Rivers of America. New York: Harry N. Abrams, Inc., 2006.

Pearce, Fred. When the Rivers Run Dry: Water–The Defining Crisis of the Twenty-First Century. Boston: Beacon Press, 2006.

Kathryn D. Snavely

Rivers

A river is a natural stream of freshwater with significant volume when compared to the volume of its smaller tributaries. Conveying surface water run-off on land, rivers are normally the main channels or largest tributaries of drainage systems. Typical rivers begin with a flow from headwater areas made up of small tributaries, such as springs. They then travel in meandering paths at various speeds; finally, they discharge into desert basins, into major lakes, or most likely, into oceans.

Sixteen of the world's largest rivers account for close to half of the world's river flow. By far, the largest river is the Amazon River, running 3,900 mi (6,275 km) long. Discharging an average of four million cubic feet (112,000 cu m) of water each second, the Amazon River alone accounts for 20% of the water discharged each year by Earth's rivers.

Formation of rivers

Precipitation , such as rainwater or snow, is the source of the water flowing in rivers. Rainwater can either return to the oceans as run-off, it can be evaporated directly from the surface from which it falls, or it can be passed into the soil and mantle rock. Water can reappear in three ways: (l) by evaporation from Earth's surface; (2) by transpiration from vegetation; (3) by exudation out of the earth , thereby forming a stream. The third way, by exudation, is of primary importance to the formation of rivers.

When a heavy rain falls on ground that is steeply sloped or is already saturated with water, water run-off trickles down Earth's surface, rather than being absorbed. Initially, the water runs in an evenly distributed, paper-thin sheet, called surface run-off. After it travels a short distance, the water begins to run in parallel rills and, at the same time, gathers turbulence . As these rills pass over fine soil or silt, they begin to dig shallow channels, called runnels. This is the first stage of erosion .

These parallel rills do not last very long, perhaps only a few yards. Fairly soon, the rills unite with one another, until enough of them merge to form a stream. After a number of rills converge, the resulting stream is a significant, continuously flowing body of water, called a brook. The brook now flows through what is termed a valley. As a brook gains sufficient volume from groundwater supplies, the volume of water it carries becomes more constant. Once the volume of water carried reaches a certain level, the brook becomes a river.

River systems

Rivers can have different origins and, as they travel, often merge with other bodies of water. Thus, the complete river system consists of not only the river itself but also of all the converging tributaries. Every river has a point of origin. Because gravity plays a key role in the direction that rivers take, rivers almost always follow a down hill gradient. Thus, the point of origin for rivers tends to be the highest point in the watercourse. Some rivers start from springs, which are the most common type of river source in humid climates. Springs occur as groundwater rises to the earth's surface and flows away. Other rivers are initiated by run-off from melting glaciers located high in the mountains . Often, rivers having their origins in huge glaciers are quite large by the time they emerge from openings in the ice .

Lakes and marshes are the sources for other rivers. As river sources, lakes can be classified in three ways. They can be true sources for rivers; they can be an accumulation of water from small feeder streams; or they can hide a spring that is actually the true source of the river. The Great Lakes are prime examples of source lakes. Although there are a few springs that feed them, the majority of the water coming into the lakes arises from precipitation falling onto their surfaces. Therefore, they, not their tributaries, are the source of surrounding rivers.

As rivers make the trip from their source to their eventual destination, the larger ones tend to meet and merge with other rivers. Resembling the trunk and branches of a tree , the water flowing in the main stream often meets the water from its tributaries at sharp angles, combining to form the river system. As long as there are no major areas of seepage and as long as the evaporation level remains reasonable, the volume of water carried by rivers increases from its source to its mouth with every tributary.

When two bodies of water converge, it is clearly evident as their shorelines merge. However, the water from the two bodies often continues to flow separately, like two streams flowing in a common river bed. This occurrence is especially clear when two rivers meet that contain different amounts and types of suspended sediment. For example, when the Ohio and the Mississippi rivers meet, a clear difference in the color of water in the Mississippi river can be seen. Specifically, there is a strip of clear water one quarter of a mile wide on the river's eastern side that runs for miles. To the west of this strip, however, the water color is a cloudy yellow.

Along its path, a single river obtains water from surface run-off from different sections of land. The area from which a particular section of a river obtains its water is defined as a catchment area (sometimes called a drainage area). The lines that divide different catchment areas are called watersheds. A watershed is usually the line that joins the highest point around a particular river basin . Therefore, at every point along the line of a watershed, there is a downward slope going into the middle of the catchment area.

Climactic influences

Rivers are highly influenced by the prevailing climate conditions. The climate determines the amount of precipitation, its seasonality, and its form as rainwater or as ice. Because of the climate and subsequent rainfall patterns, three general types of rivers exist. The first are the perennial or permanent rivers. Normally, these rivers are located in more humid climates where rainfall exceeds evaporation rates. Thus, although these rivers may experience seasonal fluctuations in their levels of water, they have constant streamflow throughout the year. With few exceptions, streamflow in these rivers increases downstream, and these rivers empty into larger bodies of water, such as oceans. In fact, 68% of rivers drain into oceans. All of the world's major rivers are perennial rivers.

The second type of river is the periodic river. These rivers are characterized with predictably intermittent streamflow. Usually appearing in arid climates where evaporation is greater than precipitation, these rivers run dry on occasion, but there are regular intervals of stream-flow. Typically, these rivers have a decrease in stream-flow as they travel due largely to high levels of evaporation. Often, they do not reach the sea, but instead run into an inland drainage basin.

The third type of river is the episodic river. These rivers are actually the run-off channels of very dry regions. In these regions of the world, there are only slight amounts of rainfall and it evaporates quickly. This type of streamflow occurs rarely.

Interestingly, some rivers span two types of climactic regions. These rivers, known as exotic rivers, begin in humid or polar regions and flow into dry areas. The largest of these rivers have enough water at their sources to enable them to reach the sea. The Nile River, for example, gets sufficient water at its humid source to travel over the Nubian and Arabian deserts. While it receives a substantial amount of water from the Blue Nile at Kartoum, it then must travel 1,676 mi (2,700 km) before it reaches the Mediterranean Sea.

Hydrological cycle

The hydrologic cycle is very important to the existence of rivers, indeed, to all life on Earth. Without it, every stream and watercourse would dry up. The hydrological cycle is the continuous alternation between evaporation of surface water, precipitation, and streamflow. It is a cycle in which water evaporates from the oceans into the atmosphere and then falls as rain or snow on land. The water, then, is absorbed by the land and, after some period of time, makes its way back to the oceans to begin the cycle again. Scientists have found that the total amount of water on the earth has not changed in three billion years. Therefore, this cycle is said to be constant throughout time.

The water content of the atmosphere is estimated to be no greater than 0.001% of the total volume of water on the planet . Despite its seemingly insignificant amount, atmospheric water is essential in the hydrological cycle. As water falls as rain, three things can happen. First, usually some of the rain falls directly into rivers. Second, some of it is soaked up by ground, where it is either stored as moisture for the soil or where it seeps into ground water aquifers. Third, rainfall can freeze and become either ice or snow. Interestingly, water is sometimes stored outside the hydrological cycle for years in cavities as fossil ground water in continental glaciers. The next event, evaporation, is the most critical link in the cycle of water circulation. If rain water evaporates too rapidly, rivers cannot form. For example, in hot deserts, heavy downpours sometimes occur, but the water evaporates completely in a short period of time. However, as long as the evaporation is slower than the typical amount of rainfall, viable rivers can exist.

Rivers, like precipitation and evaporation, are a vital part of the hydrological cycle. Somewhat surprisingly, of all of the forms of water in nature, watercourses-rivers and streams-make up the smallest total amount of water on Earth, about 0.0001% of the total volume. However, when combined with the precipitation falling on the ocean and the run-off from melting ice in Antarctica and Greenland, rivers replace about the same amount of water as is evaporated by the oceans. In addition to this, because they carry water away from saturated soil, they prevent marshes and bogs from forming in many low-lying areas.

Although the hydrologic cycle is a constant phenomenon, it is not always evident in the same place, year after year. If it occurred consistently in all locations, floods and droughts would not exist. Thus, each year some places on Earth experience more than average rainfall, while other places endure droughts. It is not surprising, then, that people living near rivers often endure floods at some time or other.

River floods

River levels have a direct influence on the activities and well-being of human beings. While low flowing rivers interfere with transport, trade, and navigation, high water threatens human life and property. Basically, floods are a result of a river's discharge behavior and the climate within which it is located. The most common cause of flooding is when it rains extremely hard or for an unusually long period of time. Additionally, areas that experience a great deal of snow in the wintertime are prone to springtime flooding when the snow and ice melt, especially if the thaw is relatively sudden. Furthermore, rainfall and snowmelt can sometimes combine to cause floods, such as when rain falls on an area covered with melting snow.

Under normal conditions, rivers move fairly slowly as they transport silt and other debris produced by rain and snow. During floods, however, this transport is achieved much more rapidly, sometimes with beneficial side effects and sometimes with disastrous ones. One example of beneficial flooding is where the high water transports new top soil to local crops . Furthermore, floods can provide local crops badly needed moisture. The negative aspects of flooding are fairly obvious; often people drown and their property is destroyed.

Rivers in more humid regions are less likely to experience significant flooding than those located in more arid climates. In fact, floods in humid areas occur an average of about one time per year. Although on rare occasions these rivers experience larger floods, the water is normally no more than twice the size of a normal flood. While rivers in arid regions experience small flooding on an annual basis as well, when they experience rare, large floods, it can be devastating.

Human control of rivers

For centuries, rivers have been very important to human society. Aside from soil, no other feature on Earth is as closely bound to the advancement of human civilization. Trying to control river flow has been a key part of civil engineering . This is especially true because of the need to avoid natural flooding and the desire to take advantage of the benefits that flood plains offer agriculture. Furthermore, managing rivers can also satisfy human needs to store water for times of drought . Thus, civil engineers have a number of goals. They try to conserve water flow for release at times when human need is greatest. They try to keep water quality above acceptable levels. And they try to confine flood flows to designated channels or to planned flood storage areas.

While the techniques of river management are fairly well understood, true river management is not commonly put into practice because of the expense and the size of the projects involved. In fact, none of the major rivers in the world is controlled or even managed in a way that modern engineering and biological techniques would allow. So far, only medium-sized streams have been successfully managed. For example, the San Joaquin in California has been completely developed to take advantage of the irrigation opportunities that the stream offers.

See also Dams; Lake; Water conservation.

Resources

books

Crickmay, C.H. The Work of the River. New York: American Elsevier Publishing Company, Inc., 1974.

Czaya, Eberhard. Rivers of the World. New York: Van Nostrand Reinhold Company, 1981.

Parker, Sybil P., and Robert A. Corbitt, eds. McGraw-Hill Encyclopedia of Environmental Science and Engineering. 3rd ed. New York: McGraw-Hill, Inc., 1992.

Parker, Sybil P., ed. McGraw-Hill Encyclopedia of Oceans, andAtmospheric Sciences. New York: McGraw-Hill, Inc., 1980.

periodicals

Bandler, Hans. "River Symposium 2002: The Scarcity Of Water. " Water International 27, no. 3 (2002): 452.

Brismar, Anna. "River Systems As Providers Of Goods And Services: A Basis For Communication." Environmental Management 29, no. 5 (2002): 598-609.

Kathryn D. Snavely

KEY TERMS

Brook

—A significant, continuously flowing body of water formed by the convergence of a number of rills.

Catchment area

—The area from which a particular section of a river obtains its water; also known as a drainage area.

Erosion

—Movement of material caused by the flow of ice, water, or air, and the modification of the surface of the earth (by forming or deepening valleys, for example) produced by such transport.

Exudation

—The process of water oozing out of the ground.

Hydrologic cycle

—The continuous, interlinked circulation of water among its various compartments in the environment.

Perennial rivers

—Located in more humid climates where rainfall exceeds evaporation rates. Although these rivers may experience seasonal fluctuations in their levels of water, they have constant stream flow throughout the year.

Periodic rivers

—Characterized with predictably intermittent streamflow. Usually appearing in arid climates where evaporation is greater than precipitation, these rivers run dry on occasion, but there are regular intervals of streamflow.

Rill

—A small channel of water that forms from surface run-off; a small brook.

Runnels

—Eroded channels in the ground in which rills of water pass over fine soil.

Transpiration

—The process of water being emitted into the atmosphere through vegetation.

Tributary

—A stream or other body of water that flows into a larger one.

Valley

—The area in which a brook flows.

Watershed

—The expanse of terrain from which water flows into a wetland, waterbody, or stream.

Rivers

Rivers are bodies of flowing surface water driven by gravity. Hydrologists, scientists who study the flow of water, refer to all bodies of flowing water as streams. In common language, it is accepted to refer to rivers as larger than streams. Water flowing in rivers is only a very small portion of Earth's fresh water. The oceans contain about 96% of the water on Earth, and most fresh water is bound up in glacial ice near the North and South Poles. Rivers shape the landscape and are integral to the hydrologic cycle (circulation of water on and around Earth) on the continents.

Rivers shape the lands as they erode (wear away) and deposit sediment (particles of gravel, sand, and silt) along their courses. Running river water acts to level the continents. When geologic forces slowly raise (uplift) mountain ranges, rivers wear them away. The streams that form the Ganges River of India (headwater streams), for example, are presently tearing down the Himalayas almost as quickly as they are uplifted by the movements of Earth's crustal plates (plate tectonics). When geologic forces create depressions or low areas on the continents, rivers act to fill them. River sediment replenishes floodplain (Flat land next to rivers that are subject to flooding) soils and coastal sands. Earth's major rivers, including the Nile, Amazon, Yangtze, and Mississippi, drain the waters of vast continental areas and set down (deposit) huge deposits of sediments at the ends of rivers that flow into the ocean (for example, in deltas at the end of many rivers) Rivers host vibrant communities of plants and animals, and refill groundwater reservoirs and wetlands that support biological life far beyond their banks.

Rivers are a main focus of human interaction with the natural environment. Human agriculture, industry, and biology require fresh, accessible water from rivers. Ancient human civilizations first arose in the fertile valleys of the world's great rivers: the Yangtze and Yellow Rivers in China, the Tigris and Euphrates Rivers in the Middle East, and the Nile River in Egypt. The distribution of Earth's rivers and systems of rivers has influenced human population patterns, commerce, and conquest since ancient times. Rivers flow through the great cities of the world, and the imagery of rivers is deeply embedded in our language, culture, and history. Today, billions of people depend directly and indirectly on rivers for food and water, transportation and recreation, and spiritual and religious inspiration.

Almost all major rivers are today confined by man-made dams and levees (walls along the banks) that provide people with the means to generate electricity and protection from floods. These alterations to rivers have come at an environmental cost. When flood-waters are contained by levees or other flood-control dams, they no longer supply nutrients and sediment to floodplain soils that support agriculture. Furthermore, dams and levees that upset a river's natural path and profile (side view) cause changes to the patterns of erosion and deposition (depositing sediments) throughout the entire river system. Dams have contributed to beach erosion on many coastlines because dams trap sediment in reservoirs. Agricultural and urban development along riverbanks has threatened many species of plants and animals that live in riverside wetlands.

Also, the very dams and levees that prevent frequent small floods create an increased risk of infrequent, disastrous flooding. The city of New Orleans, for example, lies at a lower elevation than the bed of the Mississippi River that runs through the center of the city in an artificial channel behind massive levees. If the levees failed, a flash flood would engulf the city and potentially threaten the lives of its residents.

Major rivers

Earth's largest river systems define the natural and human environment within their watersheds. A watershed is the land area that drains water into a river or other body of water. A list of the world's major rivers is also a list of the major natural and cultural geographic regions on six continents. (The continent Antarctica is too cold for liquid water. Its fresh water is bound up in large masses of moving ice called glaciers.)

• Africa: The Nile is, by most measurements, the world's longest river. (River lengths are difficult to measure because rivers constantly shift their courses and change length. There is also disagreement about which branches of water (tributaries) are considered part of the main river. By some measurements, the Amazon River in South America is actually slightly longer than the Nile.) The Nile has sustained life in the inhospitable Sahara desert of eastern Africa for thousands of years. Its headwater (uphill end) streams flow from lakes in Ethiopia and Uganda and feed two branches, the White Nile and the Blue Nile, which meet in the Sudanese city of Khartoum. From there, the Nile cuts a green-bordered lifeline through the Egyptian desert. It flows through Cairo, the bustling capital of modern-day Egypt, past the pyramids of Giza and the ancient Egyptian capital of Thebes, to its outlet in the Mediterranean Sea. The Congo River (called the Zaire River from 1971 to 1997) makes a long loop through the equatorial rainforests and war-torn nations of central western Africa. The Congo is the main trade and travel route into the African interior, and it is the setting for Joseph Conrad's famous novel Heart of Darkness. The Limpopo, Okavango, Ubangi, and Zambezi are other major African rivers.
• Asia: Huge rivers drain water from the massive Asian continent into the Pacific, Indian, and Arctic oceans. In China, the Yangtze (Chang Jiang), Yellow (Huang He) and Pearl Rivers carry flowing waters (runoff) from the northern slope of the Himalayan Mountains and western China to the East China Sea. Hundreds of millions of Chinese people depend on these rivers for their electricity, food, and livelihoods. Water moving south from the Himalayas flows into the rivers of India and South Asia, including the Ganges-Bramaputra system and the Mekong River. The Ganges River of northern India is sacred in the Hindu religion. Hindus travel to its banks to meditate and wash away their sins. Upon death, cremated remains are placed into the Ganges in hopes of improving the deceased's fortunes in the afterlife. The Ob, Ikysh, Amur and Lena Rivers run across the northern forests and wind-swept tundra (treeless arctic plains) of Siberia (the Asian portion of Russia) into the icy Arctic Ocean. In the Middle East, rivers play an important role in the history and mythology of western civilization. The ancient civilizations of Sumeria and Mesopotamia arose in the "fertile crescent" between the Tigris and Euphrates Rivers (Shat-al-Arab) in what is today Iraq. Along with the Jordan River, they play major roles in Jewish, Christian, and Islamic history.
• Australia: The island continent of Australia has only a few major rivers, and its central desert, the outback, is extremely dry. The Murray River and its major tributary (major branch), the Darling, make up Australia's largest river system. The Murray drains water from the southeastern states of Victoria, New South Wales and southern Queensland and its floodplains are Australia's most productive farmlands.
• Europe: Rivers are intertwined in the history, culture, and geography of Europe. The capital cities of Europe are synonymous with their rivers (London and Thames, Paris and Seine, Vienna, Budapest and Danube). By their very names, the Rhone (France), Rhine (Germany), Volga (Russia), Oder and Elbe (Germany, Poland, Czech Republic), Po and Tiber (Italy), and Ebro (Spain) conjure images of great art and fine wine, desperate battles and bloody conquests, grand castles and ancient hamlets.
• North America: The Mississippi and its major tributaries, the Missouri, Ohio, and Arkansas Rivers, collect water from a huge drainage basin that spans the central plains of North America between the Rocky Mountains and the Appalachians. Canada's Mackenzie and Churchill Rivers empty into the Arctic Ocean, and the St. Lawrence River empties the Great Lakes into the Atlantic Ocean. The mighty Yukon River of northern Canada and Alaska carried prospectors to mines and mills during the Alaskan gold

  The Amazon River

  The Amazon River system is Earth's largest body of fresh water. Its basin, in the vast lowlands of equatorial Brazil, contains about one-fifth of Earth's fresh liquid water. So much water flows from the mouth of the Amazon that where it meets the ocean, the ocean water contains mostly freshwater far out to sea. Ships' captains have reported drawing drinkable (potable) water from the sea while still out of sight of land. A dense network of over 1,100 tributary streams and rivers feed the Amazon along its course. Heavy tropical rains regularly overfill the Amazon and its tributaries, and for much of the year the Amazon system more resembles a marshy lake than a system of rivers.

  The Amazon River snakes thousands of miles (kilometers) across the widest part of South America, from the slopes of the Andes in Peru, across the tropical rainforest of Brazil to the Atlantic Ocean. Its trunk is miles (kilometers) wide far upstream (even in the dry season), and deep enough for large ships to travel hundreds of miles (kilometers) into the Brazilian rainforest. Native American tribes of equatorial South America each had their own name for their particular stretch of the river or one of its tributaries, and the river was unnamed when Spanish and Portuguese explorers arrived. Spanish explorer Vicente Yañez Pinzon was the first European to sail into the Amazon in 1500, and in 1540, Francisco de Orellana was the first to travel its full length. Orellana named the river "Amazonas" after battling with fierce tribes whose women fought alongside the men. (The Amazons were a tribe of warrior women in ancient Asia and Africa that Greek historian Herodotus described in his writings.

  The Amazon basin was then, and is now, a wilderness inhabited by millions of species of plants and animals that thrive in the wet, lush tropical Brazilian rainforest. Scientist estimate that half of Earth's biological species live in the Amazon basin. Swarms of flesh-eating fish called paranhas and 400-pound (181-kilogram) catfish swim beneath towering trees. Jaguars and 30-foot (9-meter) long snakes called anacondas drape across branches. Birds with bright plumage and butterflies by the millions fly in the air.

  Today, human agriculture, industry, and land development are encroaching on the Amazonian rainforest. Deforestation (clearing of the forest) and flood control have led to many negative environmental affects, including the rapid extinction of many plant and animal species (biodiversity loss) and addition of greenhouse gases to the atmosphere. Economic development in the Amazon has also negatively impacted the human population of South America, especially the Native American peoples whose cultures and livelihoods are intertwined with the natural cycles of the rain-forest. The soils beneath the rainforest are poor, and once the plants and animals are removed, nothing grows well there. South American and international environmental groups, scientists, and some governments and industries are working toward human industry and agriculture in the Amazon that can be sutained over many generations.

  rush (1898–99). Many of the great ports of the Atlantic seaboard and Gulf of Mexico lie near river mouths (the end of a river where the river empties into a larger body of water): New York (Hudson), Philadelphia and Washington, D.C. (Potomac, Susquahana), Norfolk (Delaware), New Orleans (Mississippi), and Houston (Brazos). Rivers, including the Mississippi, Missouri, Colorado, Rio Grande, and Columbia, played central roles in European exploration and settlement of the American West. Today, the rivers that carried explorers Meriwether Lewis, William Clark, John Wesley Powell, and other legendary frontiersmen across the continent are used for agricultural irrigation, drinking water, recreation, and power generation. Their water is a valuable and heavily-sought resource.
• South America: The Amazon is the largest river in the world. It flows from the Andes Mountains of Peru, across the Brazil and empties into Atlantic Ocean on the northeast coast of Brazil. The Amazon has more than 1,100 tributaries, 17 of which are longer than 1,000 miles (1,609 kilometers) long. The main river runs from west to east just a few degrees south of the equator, and its massive watershed lies entirely within the warm, wet tropical zone. The central Amazon contains Earth's lushest, wettest, most biologically diverse rainforest. The Orinoco (Venezuela), Sao Francisco (Brazil), Parana (Argentina, Paraguay) and Uruguay (Uruguay, Brazil) rivers are other major waterways of South America.

Laurie Duncan, Ph.D.

For More Information

Books

Maclean, Norman. A River Runs Through It. Washington, DC: Island Press, 2003.

Postel, Sandra, and Brian Richter. Rivers for Life: ManagingWater for People and Nature. Washington, DC: Island Press, 2003.

Websites

"River." Campusprogram.com, Wikipedia.http://www.campusprogram.com/reference/en/wikipedia/r/ri/river.html (accessed on August 16, 2004).

Rivers

Introduction

Climate change affects the world's rivers in various ways, depending on the nature of regional climate changes. Reductions in rainfall or snowpack reduce river flow; accelerated glacial melting temporarily increases flow in glacier-fed rivers; shortened winters change the times at which ice forms and breaks up on rivers; and extreme weather events, such as intense rain storms, can cause rivers to flood more frequently or at higher levels than were likely previously.

Rivers are not only affected by climate change, but can contribute to it. Increased delivery of freshwater to the Arctic Ocean by rivers has, in the past (for example, about 13,000 years ago, during the Younger Dryas cold interval), slowed the overturning ocean circulation in the Atlantic Ocean sometimes known as the Atlantic conveyor belt. Recent global warming has increased river discharge to the Arctic Ocean and may affect ocean circulation in a similar way, which would, in turn, affect world climate.

Historical Background and Scientific Foundations

Rivers are one of the two primary means by which water that falls on land as rain and snow returns to the sea. The other is evaporation followed by rainfall over the ocean. Globally, about 29,000 mi³ (119,000 km³) of water fall as precipitation on land every year. Of this amount, about 17,600 mi³ (72,000 km³) return to the atmosphere by evaporation, while the rest—11,400 mi³ (47,000 km³)— recharge underground water tables or flow to the sea through rivers. At any given moment, rivers contain about 517 mi³ (2,120 km³) of water.

Continental runoff through rivers—water reaching the oceans—has increased globally during the twentieth century, even though human beings have increased the amount of water they divert from rivers for irrigation, industry, and domestic use. The continents have not seen identical changes in runoff and precipitation. In the Americas, Africa, and Asia, both runoff and precipitation have increased, while in Europe, precipitation has increased while runoff has decreased. (There are no rivers in Antarctica that run to the sea; runoff occurs more slowly, in the form of glaciers.) There are several possible reasons for global shifts in runoff over the last century, including climate change, increased climate variability, deforestation, and global dimming (which reduces evaporation from landscapes and open water).

WORDS TO KNOW

ATLANTIC CONVEYOR BELT: The north-south circulation that dominates movement of water in the Atlantic Ocean and is a key part of the global meridional thermohaline circulation and of the climate system. Moves warm waters toward the Arctic, where they cool, sink, and move southward along the ocean floor.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosyn-thesis are no longer present and contributing to carbon storage.

EVAPOTRANSPIRATION: Transfer of water to the atmosphere from an area of land, combining water transfer from foliage (transpiration) and evaporation from non-living surfaces such as soils and bodies of water.

GLOBAL DIMMING: Decrease in amount of sunlight reaching Earth's surface caused by light blockage by clouds and aerosols. Global dimming increased from 1960 to 1990, reducing sunlight reaching Earth's surface by 4%, but this trend reversed after 1990 in most locations.

NORTH ATLANTIC OSCILLATION: Alternating annual variations of atmospheric (barometric) pressure near Iceland and the Azores (an island group in the eastern Atlantic): corresponds to fluctuations in the westerly winds over the Atlantic and influences other aspects of climate in the Northern Hemisphere. The Icelandic Low shifts westward while the Azores High shifts eastward, and vice versa: storms track eastward between these two rotating systems, like paper being run between a pair of rollers.

PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F(0°C) for several years.

THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.

TRANSPIRATION: The process in plants by which water is taken up by the roots and released as water vapor by the leaves. The term can also be applied to the quantity of water thus dissipated.

WATER INSECURITY: Lack of access to clean and dependable water supplies. Climate change alters the precipitation patterns on which water security largely depends; forecasted climate changes are likely to greatly increase the number of people suffering water insecurity worldwide, especially in parts of Africa, Asia, and South America.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

YOUNGER DRYAS: A relatively recent episode of abrupt climate change. About 12,900 years ago, conditions in the Northern Hemisphere cooled in about a decade (extremely rapidly), in some locations by 27°F (15°C). The cold period persisted for about 1,300 years and then reversed, also suddenly. The causes of the Younger Dryas are not well understood, but the event does show that Earth's climate is capable of extremely rapid and dramatic shifts.

Scientific study of river flow intensified after the publication of the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report on climate change in 2001. Specific effects of climate change have been observed in rivers in particular regions. For example, rivers fed by meltwater from permafrost, glaciers, and snow, have seen increased flow. Most dramatically, in the Arctic, permafrost has been melting and precipitation has been increasing as a result of warmer temperatures. The average annual water flow into the Arctic Ocean from the six largest Eurasian rivers—the Yenisey, Lena, Ob', Pechora, Kolyma, and Severnaya Dvina, which include three of Earth's largest rivers—increased by 7% over 1936–1999, a change of about 0.5 mi³ (2 km³) of flow per year. Total flow is now about 31 mi³ (128 km³) per year greater than in 1936, when routine measurements of these rivers began. These changes are the result of a combination of human-caused (anthropogenic) global warming and recent changes in the pattern of oceanic circulation known as the North Atlantic Oscillation, which may or may not be affected by anthropogenic climate change.

In the remainder of the twenty-first century, given a global surface air temperature rise of 2.52–10.44°F(1.4–5.8°C) by 2100 (actual warming may be more than this range, but is unlikely to be less), runoff through the largest Eurasian arctic rivers alone would increase by 18–70% over present amounts. Increased flow from North American arctic rivers, increased rainfall over the Arctic Ocean, and increased meltwater from the Greenland ice cap will all add to this Eurasian freshwater flow. Such large changes in the amount of freshwater entering the Arctic Ocean may slow the formation of the North Atlantic deep water (NADW), that is, the cold, dense water at the bottom of the North Atlantic that flows south as part of the global overturning thermohaline circulation of the oceans. This circulation transports heat from the tropics toward the poles, affecting climate globally but most strongly in the Northern Hemisphere. Some slowing of the Atlantic conveyor belt in the twenty-first century has already been pronounced likely by the IPCC.

A feasible range for increased Arctic Eurasian river flow by 2100 is 0.01–0.04 sverdrup (a sverdrup is a million cubic meters per second), and some studies have found that if freshwater entering the North Atlantic were to increase by 0.06–0.15 sverdrup, formation of the NADW would cease altogether, that is, the Atlantic conveyor would stop. This would likely produce severe and hard-to-predict changes in the climate of the Northern Hemisphere. Although the halting of the Atlantic conveyor has not been predicted for the twenty-first century, the similar magnitudes of these figures show that the increased amounts of freshwater flowing through Arctic rivers may have a direct impact on global climate.

In the Arctic, increased rainfall has been the main cause of increased river flow. In other cases, such as rivers fed by melting glaciers in the Himalayas, flow has increased because warming temperatures are causing glaciers to melt and shrink. Another factor tending to increase river flow is the effect of increased carbon dioxide (CO₂) on plants. Human burning of fossil fuels has caused atmospheric CO₂ to increase from its preindustrial (pre-1750, approximately) value of about 280 parts per million to 383 parts per million as of mid-2007.

CO₂ causes most green plants to grow faster while absorbing less water. The tiny openings in leaves called stomata partly close in response to elevated CO₂, slowing release of water vapor to the air. Decreased uptake of water by plants as a result of higher atmospheric CO₂ allows more water to run off through soils to rivers, increasing global river runoff. Since reduced evapotranspiration (the sum of evaporation and transpiration) from plants reduces cooling of vegetated landscapes, this effect will also tend to increase land surface temperatures. A 2006 study by N. Gedney and colleagues found that although changes in weather could partly account for the changes in river runoff seen in the twentieth century, the direct effect of CO₂ on plant evapotranspiration had to be invoked to account for all the changes seen.

Impacts and Issues

Computer simulations of how ongoing climate change may affect river flows in the future show a patchwork effect worldwide, with some areas experiencing increased river runoff and others seeing decreases. In general, Europe and most of the 48 contiguous U.S. states will see decreased runoff, as will most of southern and western Africa. Eastern Africa, Australia, eastern and southern Asia, and parts of South America will see increased runoff. Higher latitudes (more northerly regions), including Scandinavia, Alaska, Siberia, most of Canada, and central Asia, will experience increased runoff.

Seasonal variations in river flow in areas with little or no snowfall will tend to become more dramatic, with peak flows higher and low flows lower. Glacier-fed rivers, especially in the Hindu Kush-Himalaya region and the Andes of South America, are already experiencing increased flows while glaciers melt in the early 2000s, but will experience gradually decreasing flow over the next few decades as mountain glaciers shrink or disappear. Human settlements depending on these rivers for water will experience water insecurity and will be forced to curtail water use, with possible effects on health and agriculture.

In areas with increased flow and increased extremes of flow, floods will become more common. The frequency of 100-year floods, averaged across all river basins worldwide, has been increasing since at least 1993.

See Also Great Conveyor Belt; Hydrologic Cycle; Rainfall.

BIBLIOGRAPHY

Periodicals

Clark, Peter U., et al. “Freshwater Forcing of Abrupt Climate Change During the Last Glaciation.” Science 293 (July 13, 2001): 283–287.

de Wit, Maarten, et al. “Changes in Surface Water Supply Across Africa with Predicted Climate Change.” Science 311 (March 31, 2006): 1917–1921.

Gedney, N., et al. “Detection of a Direct Carbon Dioxide Effect in Continental River Runoff Records.” Nature 439 (February 16, 2006): 835–838.

Peterson, Bruce J., et al. “Increasing River Discharge to the Arctic Ocean.” Science 298 (December 13, 2002): 2171–2173.

Schiermeier, Quirin. “The Outlook for Amazonia Is Dry.” Nature 442 (August 17, 2006): 726–727.

Larry Gilman