Stream Water Flow

Water flows downhill due to Earth's gravity (force of attraction between two masses) pulling it. Streams, like rivers, are gravity-driven bodies of moving surface water that drain water from the continents. Water scientists, called hydrologists, refer to all bodies of running water as streams, no matter their size so, in one sense, rivers are large, well-established streams). In everyday communication, it is common to refer to streams as smaller than rivers.

Streams transfer water that falls on the land as precipitation (rain, snow, sleet, and hail) to the oceans. Streams, again like rivers, constantly shift their courses and change length. The stream is carried along a defined path, called a channel. Water flowing in stream channels is a powerful sculptor that carves landscapes and molds sediment (particles of rock, sand, and silt). It wears down mountain ranges and cuts deep canyons through solid rock. Stream waters support vibrant communities of plants and animals, and they have been the lifeblood of human civilization for thousands of years. Streams shape the land and are also integral to the hydrologic cycle (circulation of water on and around Earth).

Erosion and deposition

Streams are the main agent of erosion (wearing away) on land. Water in fast-moving streams is usually turbulent. The flowing water is filled with swirls and small localized whirlpools of swirling water called eddies. Turbulent water picks up particles of sediment that have weathered from rock and soil and carries them downstream. (Weathering is the breaking up of rocks by physical and chemical processes, such as being exposed to the actions of water, ice, chemicals, and changing temperature.) Faster-moving water can carry more sediment in the water, and can push larger stones along the bottom of the channel. Some mountain streams move huge boulders, while sluggish lowland (low country and level) streams carry only fine grains of silt and mud. The sand grains and larger rock fragments that slide and bounce along stream beds wear away solid rock. In a straight stream, the fastest-moving water and area of greatest erosion is generally in the middle of the channel. Where a stream bends, the strongest current (a moving mass of water) is on the outside of the curve.

When water slows down, it drops its sediment load, causing sedimentary deposits to form along stream courses in areas of slow-moving water. The slower the current, the finer the sediment it deposits. In straight channels, stream water lays down sediment along the stream banks. In bending channels, sedimentary deposits called point bars form on the inside of the bends. Individual sediment grains travel downstream like hitchhikers. Sometimes the grains are picked up by a strong current or flood that moves them far downstream, but usually they don't go very far in a single trip. The grains of sand on a beach each made a long trip with many stops before they arrived at the ocean. Whether an individual grain of sediment moves depends on the speed of water currents that vary as the amount of water moving through a stream changes. As water currents become faster they can move larger grains.

Stream waters also erode rocks by dissolving its minerals, which causes them to crumble. Chemical weathering, also called dissolution, occurs when the slightly acidic water chemically alters the minerals in rocks, which causes them to break down. Clear stream water carries the chemical components (parts) of the rocks' minerals called ions (electrically charged particles). When conditions in the water change (the water slows or cools), the ions recombine into solid mineral crystals. This form of sedimentary deposition is called precipitation. Limestone, salt, and gypsum form by precipitating from water. Ocean animals like corals and shellfish take in ions and use them to build their shells. Some types of rocks, including chalk and flint (also known as chert) form from the remains of organisms.

Victoria Falls

Victoria Falls is a curtain of thundering water where the mighty Zambezi River tumbles over high cliffs in central Africa near the border between Zambia and Zimbabwe. It is the largest, and arguably the most beautiful waterfall on Earth. (Angel Falls in Venezuela is the world's tallest waterfall.) Victoria Falls is one of the seven wonders of the natural world. Travelers journey there to see its incredible wall of tumbling water and clouds of billowing mist that shimmer with rainbows. The roar of falling water can be heard 20 miles (32 kilometers) away, and a rainforest filled with rare plants and animals grows in the mist.

Legendary British explorer David Livingstone (1813–1873) was the first European to see what local people called "the smoke that thunders" in 1855. In his journal Livingstone wrote of the falls, "No one can imagine the beauty of the view from any thing witnessed in England ... scenes so lovely must have been gazed upon by angels in their flight." Livingstone named the falls after Queen Victoria.

Victoria Falls is a striking example of the power of flowing water. The Zambizi River carved the chasm beneath the falls by eroding away a weak rock layer. The cliff behind the falls is composed of rock that better withstands the force of the water. Victoria Falls has been moving upstream as the turbulent whitewater in its base erodes away the base of the cliff.

Graded streams and base level

All streams strive to reach a constant slope (incline) called a graded profile by eroding and depositing sediment. The profile (side-view) of a graded stream (a stream with a graded profile) is steep near the uphill end and gently sloping near the point at the end where a stream pours its water into a larger body of water. The position of the downstream end of the profile is determined by the water level at the outlet, called base level. Streams cannot erode below base level. Almost all stream systems run to the sea, so sea level is the ultimate base level for most streams.

Flash Floods

Flash floods are deadly. They can occur with little or no warning after a heavy rain, a dam or levee (a protective barrier built along the banks of a stream to prevent flooding, often made of dirt) failure, or release of a log or ice jam in a river. In many ways, flash floods are like all floods. They happen when water overfills a stream channel and spills into areas that are usually dry. Like all floods, they often damage property, drown crops, and pollute drinking water. Unlike other floods, flash floods rise to catastrophic levels within hours or even minutes, and leave people little chance to escape rushing water.

Intense rainstorms that cause flash floods often occur in small area within a larger stream system. The surge of water rushes down through the system through areas that received no rain, causing flash floods to often occur under sunny skies and in dry regions. Flash floods are a particular hazard to hikers and other people in the canyons of the American Southwest. Summer thunderstorms can temporarily turn the dry, steep-walled canyons of the desert southwest into raging streams. The rushing flood waters travel downstream so quickly that downstream victims do not receive warning, even with modern communication systems. Flash floods kill more people than any other weather-related events in the United States.

Conditions change constantly in all streams, and the process of readjustment by erosion and deposition is ongoing. As conditions change along its course, a stream will readjust its profile by eroding sediment in some places and depositing it in others. If base level falls, stream waters cut down into the land surface. If it rises, they deposit more sediment. If the movements of the underlying plates of Earth's crust rise (geologic uplift) to steepen the upper part of a stream, it will erode down to regain its graded profile. Streams also attempt to level out obstructions along their path. They work to tear down dams, both natural and man-made, by erosion and filling the reservoir behind it with sediment. Lakes are, therefore, only temporary features of stream systems, and dams are interrupting the natural flow of a stream's water.

Laurie Duncan, Ph.D.

For More Information

Books

Press, Frank, and Raymond Siever. "Streams, Transport to the Oceans." In Understanding Earth. New York: W. H. Freeman and Company, 2003.

Websites

"Flash Flood!" Central Iowa Weather Cooperative's WeatherEye.http://weathereye.kgan.com/cadet/flood/ (accessed on August 16, 2004).

"Flash Floods. . .The Awesome Power! A Preparedness Guide." NOAA National Weather Service.http://www.nws.noaa.gov/om/brochures/ffbro.htm (accessed on August 16, 2004).

"Victoria Falls." Zambia National Tourist Board.http://www.zambiatourism.com/travel/places/victoria.htm (accessed on August 16, 2004).