Sediment

Erosion is the wearing down of a landscape over time. It includes the detachment, transport, and deposition of soil particles by the erosive forces of raindrops and surface flow of water.

A land area contains rills and channels. Raindrop impact can detach soil particles on unprotected land surfaces between rills and initiate transport of these particles to the rills. From the small rills, the particles move to larger rills, then into ephemeral channels and then into continuously flowing rivers. Entrainment and deposition of particles can occur at any point along the path. When erosion occurs without human influence, it is called geologic erosion. Accelerated erosion occurs when human activity increases the rate of erosion.

Erosion is a matter of concern to watershed and natural resource managers. Two of the main reasons reservoirs are built are water supply and flood control. Erosion upstream of a reservoir deposits sediment in the bottom of the reservoir which lowers the reservoir’s water-holding capacity and consequently its usefulness for both of these purposes. The soil surface is the part of the soil profile highest in organic matter and nutrients. Organic matter forms complexes with soil particles so that erosion of the soil particles will also remove nutrients. Excessive erosion can deplete soil reserves of nitrogen and phosphorus needed by plants to grow and extreme erosion can degrade the soil to the point that it is unable to support plant life. If erosion is severe and widespread enough, the water balance of a watershed can be altered—remember that most water is lost from a watershed via evapotranspiration.

Erosion caused by rainfall and runoff is computed with the Modified Universal Soil Loss Equation (MUSLE) (Williams, 1975). MUSLE is a modified version of the Universal Soil Loss Equation (USLE) developed by Wischmeier and Smith (1965, 1978).

USLE predicts average annual gross erosion as a function of rainfall energy. In MUSLE, the rainfall energy factor is replaced with a runoff factor. This improves the sediment yield prediction, eliminates the need for delivery ratios, and allows the equation to be applied to individual storm events. Sediment yield prediction is improved because runoff is a function of antecedent moisture condition as well as rainfall energy. Delivery ratios (the sediment yield at any point along the channel divided by the source erosion above that point) are required by the USLE because the rainfall factor represents energy used in detachment only. Delivery ratios are not needed with MUSLE because the runoff factor represents energy used in detaching and transporting sediment.