In addition to flood disasters along major rivers, damage caused by the flooding of smaller tributaries is also of considerable significance. Within the European WaReLa-project, water retention measures are being investigated that have positive effects within small watersheds. Geographic information system-based assessment keys were developed on the basis of digital forest site classifications to evaluate the water retention functions that depend on landscape features and land-use. The keys lead to digital maps identifying sensitive forest sites and linear structures. The maps provide a spatial distribution of runoff types and intensities. Forest inventory data are the basis for forestry measures to support water retention. The assessment keys and a tool box of forestry water retention measures are part of a Decision Support System (DSS) that is under development within the WaReLa-project. The DSS includes an evaluation tool for the economic consequences and the eco-efficiency of flood-precaution measures.