Sediment is listed as one of the principle pollutants of surface waters of the
The channel-evolution model CONCEPTS was used in combination with the upland erosion model AnnAGNPS which provided upstream boundary conditions of flow and sediment to determine total, suspended-sediment loadings and the relative contributions from upland and channel sources for a number of watersheds. In each case, sediment emanating from streambanks made up more than 60% of the total, suspended-sediment load at the watershed outlet. These findings have important implications for developing mitigation measures to reduce sediment loadings.
The magnitude of the reduction in suspended-sediment loadings required to reach TMDL targets was determined by comparing the “actual” load or yield (load/km2) from a given watershed to a “reference” or “background” value produced by “stable” or non-impacted streams. Historical flow and sediment-transport data from about 2,900 sites nationwide have been used to determine the distribution of average, annual suspended-sediment yields and yields at the 1.5-year recurrence interval for each Level III ecoregion. Rapid geomorphic assessments (RGAs), including evaluation of stage of channel evolution were conducted along with an analysis of the gauging station record to determine the relative stability of the reach during the period of sampling. Suspended-sediment yields from each ecoregion were then sorted into stable and unstable groups with the median value for stable sites used as the “reference” or “target” value in developing TMDLs for that ecoregion. As expected, median values for stable streams vary by ecoregion by several orders of magnitude.