Improved watershed modeling, uncertainty analysis and statistical techniques for TMDLs have been identified as immediate TMDL science needs by the National Research Council (US EPA 2002). Recently approved sediment TMDLs in Iowa use versions of the Universal Soil Loss Equation (USLE) with conservation input to account for uncertainty. The uncertainty associated with these USLE estimates cannot be explicitly quantified and the margin of safety (MOS) required by the TMDL program is considered to be implicit in the watershed modeling process. This project proposes to test the following hypothesis: The uncertainty associated with sediment TMDLs can be quantified by applying the process-based Water Erosion Prediction Project model (WEPP) and an explicit computation of the margin of safety (MOS).