9.3: How Do Streams Move Sediment?

How Do Streams Move Sediment?

The running water in a stream will erode and move material within the stream channel Links to an external site.. The transported material includes 1) the dissolved load Links to an external site., dissolved substances taken into solution during chemical weathering, 2) the suspended load Links to an external site., tiny silt and clay particles that are kept in suspension by the water’s flow, and 3) the bed load Links to an external site., visible sand and gravel-sized sediments that typically travel along the stream bed. Here, grains move either via hopping (saltation Links to an external site.), rolling, or sliding (traction Links to an external site.) (Figure 9.5). The measure of the total sediment a stream can carry is called stream capacity Links to an external site..

Stream competence Links to an external site. reflects the ability of a stream to transport a particular size of particle (e.g., boulder, pebble, etc.). An increased velocity of water flow increases stream competence, whereas a decreased velocity of flow decreases stream competence (Figure 9.6).

While the dissolved, suspended, and bed loads may travel long distances, they will eventually settle out and be deposited. These stream deposited sediments, called alluvium Links to an external site., are typically deposited during flood events. This is because to more effectively transport sediment, a stream needs energy. This energy is mostly a function of the amount of water and its velocity. A fast-moving stream is more capable of carrying much more and larger sediment. As a stream loses energy, it will slow down, which is why deposition occurs.

Under normal conditions, water will remain in a stream channel. When the amount of water in a stream exceeds its banks, the water that spills out of the channel will rapidly decrease in velocity. A decrease in velocity results in the deposition of the larger sandy material the river carries along the channel margins. These ridges of sandy alluvium are natural levees (Figure 9.7). As numerous flooding events occur, these ridges build up under repeated deposition. These levees are part of a larger landform known as a floodplain Links to an external site.. A floodplain is the relatively flat land adjacent to the stream that is subject to flooding during times of high discharge.