Improved assessment of flow, sediment, and nutrient losses from watersheds with computer simulation models is needed in order to identify and control nonpoint source pollution. One model, currently under consideration by the U.S. Environmental Protection Agency for watershed assessments, is the Soil Water and Assessment Tool (SWAT). We describe an application of SWAT for the Sny Magill Creek Watershed (SMCW), which covers 7,100 hectares in northeastern Iowa. The goal of this application was to further evaluate the suitability of using SWAT for Iowa conditions. Model output was compared to sediment and nutrient measurements collected at various stream sites within the watershed for validation. The model was generally able to predict flow, sediment, and nutrient losses, considering the limited quantity of available monitoring data. Thus, this study supports the hypothesis that SWAT could be used to estimate rapidly, accurately, and inexpensively the factors important for water quality assessments, such as flow, sediment, and nutrient losses at the watershed level. The SWAT model also provided useful insights regarding the importance of accurate data inputs (rainfall, for example), weaknesses in some of the data collecting methodologies (such as the frequency of the organic-N and -P measurements), and the impacts of best management practices (BMPs) (terraces, for example) on water quality.