Sediment is transported into streams by the erosion of uplands as a stream naturally evolves. The rate of erosion can be increased by natural disturbances, such as fires and floods, and by human disturbances, such as agricultural and construction activities. These disturbances can change the geomorphology of a stream by increasing the erosion of stream banks and beds, which serves as an additional source of sediment.
The Natural Resources Conservation Service has estimated rates of annual soil erosion from cropland, by major hydrologic unit, using the universal soil loss equation (U.S. Department of Agriculture, Natural Resources Conservation Service, 1997). Unit-area annual erosion rates from cropland for 1992 ranged from 2,200 (tons/mi2 of cropland)/yr (hydrologic unit 06020004, Sequatchie River Basin) to 5,200 (tons/mi2 of cropland)/yr (hydrologic unit 06030005, area contributing to Pickwick/Wilson reservoirs). These rates were multiplied by the percentage of cropland in each major hydrologic unit in 1992 to obtain an estimate of sediment input from cropland soil erosion to the hydrologic unit (table 6); estimates ranged from 51 to 540 (tons/mi2)/yr.
Quantification of the various natural and human inputs of sediment presents a complex challenge because few sources of ancillary data can be related to the numerous potential sources of sediment. An extensive analysis of the sources of sediment was not compiled for the selected monitoring basins because the limited data set of instream sediment loads (at three tributary sites) was insufficient for input/instream load comparisons. Furthermore, only one of these three sites drained basin areas large enough to compare with the estimates of sediment inputs based on major hydrologic unit.
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