10.2: Stream Transport
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The volume of water that passes through a stream in a given period of time is called the dischargeor flow rate. The discharge (Q) of a stream can be calculated by multiplying the width of the channel (w) by the depth of the channel (d) by the stream velocity or speed (v). This formula,
\[Q = wdv\]
is usually expressed in cubic meters per second, or cubic feet per second (cfs).
The discharge of a river usually increases downstream as more and more tributaries, or small streams, join the major river. The Mississippi River is an example of this phenomenon. In equatorial regions of the world, streams sometimes flow from humid regions to arid regions, causing the discharge to decrease downstream because of high potential evapotranspiration rates. The Nile river is an example of this phenomenon, called an exotic stream.
Streams are an important mechanism in the transport of sediment. If you have ever visited the Mississippi river, the Green River in Utah, the Colorado River, or the American River near Sacramento, you may have noticed that you cannot easily see to the bottom of the river because the water is very cloudy or muddy. These rivers all have a large dissolved load and suspended load, typical of large, placid rivers.
- Dissolved load is the material carried in water as dissolved minerals such as calcium from limestone bedrock.
- Suspended load is the sediment held up by running water, like silt and clay particles. These particles are so small that they stay suspended in the water until the water is nearly stationary.
- Bed load is the larger pieces of sediment which are moved by the river, but cannot be suspended in the running water, such as sand, gravel and boulders. These materials roll and bounce along the bed of the river. The higher the discharge in a river is, the larger the bed load can be. Flood events, where the discharge rate is very high, results in large transport of bedload materials.
When the bed load exceeds the streams capacity (its ability to carry materials as bedload), the stream channel becomes tangled with material which used to be bedload. This results in a braided stream.