A GIS-based procedure for assessing and mapping pesticide ecotoxicological risk for surface waters was applied to all active ingredients used in a catchment characterized by intensive agriculture. Chemical concentrations in riverwaterwere calculated for 54 chemicals in 25 drift and 21 runoff events that occurred during the growing season, from March to September. Screening level risk for the aquatic community was estimated using a risk index. The different role of drift and runoff processes, as well as the temporal trends of exposure and risk, were compared for the three classes of pesticides (herbicides, fungicides, and insecticides). High levels of risk are usually associated with runoff events for herbicides and to drift events for insecticides and fungicides. The described approach may serve as a powerful tool for a comparative evaluation of site-specific pesticide risk for surface water. However, for large-scale risk mapping, getting information on pesticide use with sufficient detail would be the major problem.