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

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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.