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Federal and state agencies across the U.S. are currently tasked with Total Maximum Daily Load (TMDL) development to ensure compliance with the Clean Water Act of 1972. In the northwestern part of the country, the TMDL effort is particularly challenging due to the complicated nature of expansive forested watersheds, steep mountainous topography, and orographic precipitation. This is especially true for sediment, which is a primary pollutant of concern. Modeling, in combination with field source assessments, has commonly been used to estimate watershed sediment yields and associated source contributions. However, even with widespread use of these methods, little has been done to evaluate the prediction performance of modeling tools in forested mountainous regions. The purpose of this article is to present an eight-year simulation period (1985-1992) for the 1,709 km2 Lamar River watershed in Yellowstone National Park where simulated loads from the Soil and Water Assessment Tool (SWAT) were compared with observed suspended sediment discharge data. Based on Nash-Sutcliffe model efficiencies of >0.81 and >0.86 for daily and monthly streamflow, and >0.51 and >0.78 for sediment, our findings suggest that SWAT is suitable for simulation of sediment in mountainous and snowmelt-dominated terrain. Two ancillary lines of evidence were used to support this conclusion: (1) a comparison of simulated landscape sediment yields with that of regional literature studies, and (2) confirmation of simulated landscape and bank erosion source contributions with that of ratios established using radionuclide tracers.