A significant part of the cost of most highway projects is attributable to drainage facilities such as storm drains, highway culverts, bridges, and water quality and quantity control structures. A geographic information system (GIS) for hydrologic data development for design of drainage facilities is presented. The system was developed to reduce the analysis time and improve accuracy by integrating spatial data describing the watershed with hydrologic theory. A grid-based GIS to estimate potential extreme peak discharges, watershed parameters, peak discharges for different frequencies, isochrone lines, and runoff curve numbers is presented. Data from the State of Texas were used in an example application.