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Several Best Management Practices (BMPs) were implemented under Water Quality Management Plans (WQMPs) in the Big Cypress Creek Watershed in Texas, USA, where nonpoint source pollution was a serious concern. Major sources of pollution were poultry litter applications and sediment erosion. The objective of the study was to evaluate the impact of implementation of WQMPs on nonpoint source pollution through a modeling approach. The Soil and Water Assessment Tool (SWAT) was applied to quantify the impacts of implementing WQMPs on sediment and nutrients. A pre-BMP scenario representing conditions of the watershed prior to the implementation of WQMPs, and a post-BMP scenario representing the conditions of the watershed after implementation of WQMPs were simulated to estimate the reductions in nonpoint source pollution due to WQMP implementation. Results are presented as percentage reductions in sediment and nutrient loadings. Estimated reductions were about 1% for sediment, about 2% for nitrogen and about 8 to 9% for phosphorus loadings at three locations along the Big Cypress Creek. Higher reductions in phosphorus at watershed level were due to poultry manure management. This study showed that a modeling approach could be used to evaluate the impacts of water quality management programs in large watersheds.