Water flowing down a slope under the influence of gravity and confined to a channel is a river or stream. Geologists use the term stream to indicate any size channel, but in some places rivers are large, streams smaller, and brooks and rills even smaller. Streams receive their water from precipitation, melting snow, and groundwater that seeps into the channel from below the surface. They lose water by evaporation, sinking into the ground, and by discharge at their terminus or mouth. The size of a stream at any given place is dependent on the drainage area above that point. The discharge is the amount of water flowing past the point for a specific amount of time and is indicated as cubic meters per second. For large streams the amount of water can be measured in millions of cubic meters per second.
Rivers and streams are powerful transporting agents as they roll and push material on their bed down the channel. This is described as the bed load, but rivers and streams can also carry large amounts of suspended matter, usually fine grained, giving the river a muddy look. In addition, rocks can skip along the riverbed in short hops, a process called saltation. Rocks that sit motionless on a streambed are too heavy for the stream to move under the existing conditions. Rivers also carry dissolved materials derived from the weathering of rocks. All of this material—bed load, suspended and dissolved materials—is simply known as the load; the total amount a stream can carry is termed its capacity. The largest-diameter particle a stream can carry is defined as its competence, which is determined by velocity and is dependent on the discharge and steepness of the stream channel or gradient. Higher gradients cause rivers and streams to flow faster.
Rivers carve the valleys they are in, and, except for glacially carved valleys, the tributaries meet the main stream at the same elevation. Many streams encounter resistant layers of rock that form ledges in the channel, causing the water to drop precipitously downward over rapids and if high enough over waterfalls.
From the air or a map, it is clear that stream networks have a drainage pattern of connecting tributaries, a pattern that reflects the underlying geology. Dendritic patterns are treelike and are developed on homogenous layers of rock that are horizontal, while parallel drainage develops on homogenous rocks with a steep slope. A trellis pattern develops on inclined layers, and radial drainage on circular mountains with a central peak such as a vol cano. So-called deranged drainage is located on a recently glaciated surface; it has many lakes, and the streams have not had time to develop a clear pattern. Over time rivers evolve and cycle through stages that generally begin with down-cutting as the primary erosional direction, eventually becoming lateral as erosion reduces the landscape's relief.
A profile along the length of a stream will usually be steeper at its headwaters and gentler downstream, forming a concave curve. The velocity of a stream may not decrease downstream, even though the gradient is less steep, because it is offset by other factors such as increased volume of water and changes in channel width and depth. For example, if the channel width decreases, and all other factors remain the same, a given volume of water will flow faster through a narrow channel than one that is wider.
As a stream flows toward its mouth, more and more tributaries join it, increasing the amount of water flowing in the river, increasing the discharge, and increasing the width of the channel. The doubling of the velocity increases the amount of sediment that a stream can carry by a factor of twenty, while also increasing the particle size.
The drainage basin contains all the streams that contribute to the main river and is separated from the adjacent basin by a drainage divide. It can be imagined as shaped like a large spoon, steep on the sides and nearly flat in the middle.
The lowest elevation of a stream is its base level, at its mouth, where it terminates in a body of water or in an interior basin, thus making base level the lowest elevation to which a stream can erode. As a stream slows down, it drops its sediments, larger particles first, until its velocity is zero, when all the sediments eventually settle out. As a stream flows down its channel, the sediment load
becomes progressively well sorted by size, density, and composition as the particles are weathered and eroded during transportation. Thus the sediments, for the most part, are of the same size and composition.
When a river enters a body of water, a large fan-shaped pile of sediments is deposited, forming a delta. Here the river breaks up into a number of branches called distributaries. During flooding, one distributary is usually preferred over the others, bringing sediments to that section of the delta. Over time all the distributaries will be flooded, enlarging the delta in all its sections.
On desert floors, where the stream terminates, the fan-shaped deposit is called an alluvial fan. Often rivers that flow into desert areas dry up for a period of time, because the water evaporates, sinks into the ground, or there is just not enough water to supply them. When this happens they are called intermit tent streams; sometimes streams flowing through a desert, such as the Nile River in Egypt, have water flowing all year round, because the constant supply from its headwaters is greater than the loss in the desert. These types of streams are described as perennial.
As a stream begins it journey, minor irregularities in the channel, due perhaps to different materials, cause the velocity of the stream to fluctuate, resulting in more erosion where it is faster and deposition where it is slower. Curves, or "meanders," begin to form and enlarge. Not only do they get wider but they also shift downstream, eroding as they do a flat, broad plain covered with sediment. This plain, the flood plain, is covered with water when a stream overflows its bank, when discharge exceeds the capacity of the channel. Often the river will deposit its coarsest (and heaviest) sediment where it overflows the banks, to create a low, ridgelike deposit, the levee. An individual meander does not last forever; during flood stage streams may cut a channel across the meander. The abandoned loop becomes an oxbow lake, separated from and independent of the river.
Flooding occurs when input from tributaries exceeds the capacity of the streams to hold the water. Rainfall frequency, permeability and porosity of the subsurface (which determines the infiltration rate), saturation level, and slope steepness are some of the factors that affect runoff rate. The greater the runoff, the greater the amount of water entering the stream. Vegetation can reduce flooding by being a physical barrier, thus slowing down the flow; it also holds together the soil and at the same time increases its permeability.
During a flood the water level rises; its elevation is called a stage; when the water overflows its banks, that is flood stage, the maximum discharge. It is at maximum discharge that a river crests and moves downstream until it returns to normal flow.
It is surprising that people choose to live on flood plains, but sometimes they are unaware of the hazard: they take a chance, it is scenic, a flood may occur only rarely, and flood control structures make people feel safe. Farmers appreciate the flood because the fine sediments deposited on the flood plain fertilize the soil. Cities are built adjacent to rivers, using them for transportation. There are many factors that create flooding in urban environments: asphalt and concrete reduce infiltration; buildings occupy space, raising the flood stage; storm sewers discharge water into streams; and vegetation is removed.
Silting may also increase the potential for flooding, by reducing the capacity of channels. A number of engineering solutions have been adopted to reduce flooding: retention ponds, storage of flood waters in quarries, flood control dams, dredging of silt from stream chan nels to widen or deepen the riverbed, and raising levees. However, the process of channelization may cause flooding downstream, or severe flooding may result when an unexpectedly high flood flows or breaches the levee. Once the flood occurs, the levees make it difficult for the flood waters to return to the channel.
Flood control dams have adverse effects, hindering navigation and the movements of organisms; they also destroy habitats, and the resulting reservoir creates a new base level into which sediments are deposited, reducing their effectiveness and eventually making them useless. The loading of the earth's crust with water and sediment has in some places pushed it down, resulting in earthquakes at a number of dam sites.
The amount of water flowing in a stream channel can be quite variable throughout the year. In some places along the channel it flows fast, and at others, where it flows over a flat surface, the water may move quite slowly. Organisms living in the water and adjacent to it on the riverbanks must be adapted to changing water conditions. When the river is flowing swiftly, not only does the turbulence add plentiful oxygen to the water; in addition, the animals must be able to attach themselves to a surface or dig themselves into if they are not to be washed away. As the water approaches the mouth of the stream, it can slow down and may contain suspended sediment, reducing the amount of oxygen and limiting photosynthesis in bottom-dwelling plants. In general, however, slower-moving water supports a greater diversity of plants and animals than more swiftly flowing water.
See also: Freshwater; Hydrologic Cycle; Lakes
Gleick, Peter. 2001. "Making Every Drop Count." Scientific American 284 (2): 40-45; Hamblin, W. Ken-
neth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Leopold, Luna B. 1974. Water: A Primer. San Francisco: W. H. Freeman and Company; Leopold, Luna B. 1994. A View of the River. Cambridge: Harvard University Press; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing.
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