Sediment is important because it helps to maintain the dimensions and shape of a river, and also provides habitat for fish and other aquatic organisms. As the depth of water increases in a river, more stress is applied to the river bed causing clay, silt, and sand particles to be lifted up into suspension. This material is generally considered the suspended sediment load because it essentially mixes with the river water. This is what makes the water cloudy during floods. The heavier material that bounces along the bottom of the channel is called bedload.
Bedload consists primarily of gravel and cobbles. The load of sediment can be measured and is usually expressed in tons per day. As discharge increases, the sediment load increases. Figure 1.10 contains sediment load data for Big Creek, a small tributary to the North Fork Flathead River. Typically, the suspended load makes up the larger proportion of total sediment carried by a stream or river. As shown, sediment loads can be quite variable. At an estimated bankfull discharge of 850 cfs, Big Creek may transport over 10 tons of sediment per day.
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|Figure 1.10: Big Creek Sediment Load. Source: Craig Kendall, Flathead National Forest|
In general, the amount of sediment carried by rivers over time is in balance with the amount of water. The balance of sediment and water allows a stream or river to be in equilibrium, which means it maintains a certain shape and pattern over time. Changes in either sediment or water volume can cause a river to change. For example, when water is diverted from a river, sediment may build up downstream. By contrast, a large dam prevents sediment movement, which may cause a river to scour its bed and banks downstream. Sediment can also be a pollutant. If too much fine sediment washes into streams as a result of erosion, it can fill the spaces between gravel and impact aquatic insects and fish.