The Impervious Surface Analysis Tool (ISAT), which is available as a geographic information system extension, is used to calculate the percentage of impervious surface area within user-selected geographic areas (e.g, watersheds, municipalities, subdivisions). In small watersheds, the correlation between an increase in impervious surfaces and a decrease in water quality has been well established. People use the information derived from ISAT to predict how different management scenarios might impact local water quality.