Sediment Load

The majority of a stream's sediment load is carried in solution (dissolved load) or in suspension. The remainder is called the bed load.

Dissolved load. Earth material that has been dissolved into ions and carried in solution is the dissolved load. It is usually contributed by groundwater. Common ions are calcium, bicarbonate, potassium, sulfate, and chloride. These ions may react to form new minerals if the proper chemical conditions are encountered during flow. Minerals may also precipitate in trapped pools through evaporation.

Suspended load. The suspended load is the fine‐grained sediment that remains in the water during transportation. For example, a flooding river is muddy and discolored from the large amounts of sediment suspended in the water. The suspended load is generally made up of lighter‐weight, finer‐grained particles such as silt and clay. Most of the sediment in a stream is carried as suspended load. It does not contribute greatly to stream erosion, since it is not in frictional contact with the stream bed.

Bed load. The heavier, coarser‐grained earth material that travels along the bottom of the stream is the bed load. Traction occurs when these fragments move along by rolling and sliding. Turbulent or eddying currents can temporarily lift these larger grains into the overlying flow of water—the grains advance by short jumps or skips until the surge diminishes and then fall back to the bottom because of their greater weight. This process is called saltation.

Capacity and competence. The maximum load of sediment that a stream can transport is called its capacity. Capacity is directly proportional to the discharge: the greater the amount of water flowing in the stream, the greater the amount of sediment it can carry. A stream's competence is a measure of the largest‐sized particle it can transport; competence is directly proportional to a stream's velocity, which can vary seasonally. Because of increased capacity and competence, a single flood event can cause more erosion than a hundred years of standard flow.