Urbanization of the 150-square-mile Irondequoit Creek basin in Monroe and Ontario Counties, N.Y., continues to spread southward and eastward from the City of Rochester, on the shore of Lake Ontario. Conversion of forested land to other uses over the past 40 years has increased to the extent that more than 50 percent of the basin is now developed. This expansion has increased flooding and impaired stream-water quality in the northern (downstream) half of the basin. A precipitation-runoff model of the Irondequoit Creek basin was developed with the model code HSPF (Hydrological Simulation Program--FORTRAN) to simulate the effects of land-use changes and stormflow-detention basins on flooding and nonpoint-source pollution on the basin. Model performance was evaluated through a combination of graphical comparisons and statistical tests, and indicated 'very good' agreement (mean error less than 10 percent) between observed and simulated daily and monthly streamflows, between observed and simulated monthly water temperatures, and between observed total suspended solids loads and simulated sediment loads. Agreement between monthly observed and simulated nutrient loads was 'very good' (mean error less than 15 percent) or 'good' (mean error between 15 and 25 percent). Results of model simulations indicated that peak flows and loads of sediment and total phosphorus would increase in a rural subbasin, where 10 percent of the basin was converted from forest and grassland to pervious and impervious developed areas. Subsequent simulation of a stormflow-detention basin at the mouth of this subbasin indicated that peak flows and constituent loads would decrease below those that were generated by the land-use-change scenario, and, in some cases, below those that were simulated by the original land-use scenario. Other results from model simulations of peak flows over a 30-year period (1970-2000), with and without simulation of 50-percent flow reductions at one existing and nine hypothetical stormflow-detention basins, indicated that stormflow-detention basins would likely decrease peak flows 14 to 17 percent on Allen Creek and 17 to 18 percent on Irondequoit Creek at Blossom Road. The model is intended as a management tool that water-resource managers can use to guide decisions regarding future development in the basin. The model and associated files are designed to permit (1) creation of scenarios that represent planned or hypothetical development in the basin, and (2) assessment of the flooding and chemical loads that are likely to result. Instream stormflow-detention basins can be simulated in separate scenarios to assess their effect on flooding and chemical loads. This report (1) provides examples of how the model can be applied to address these issues, (2) discusses the model revisions required to simulate land-use changes and detention basins, and (3) describes the analytical steps necessary to evaluate the model results.