In environmental risk analysis, many explanatory factors, often with highly complex interaction relationships, need to be taken into account. This is the case, for example, in the relation between agricultural practices and the quality of superficial aquatic ecosystems. Suspended solids are responsible for clogging salmonid spawning areas and hence for reductions in their populations. We studied the risks of erosion across two small watersheds in Normandy (France) on the scale of the agricultural plot the level on which good practices can be applied. Plots were defined according to quantitative as well as qualitative criteria: connectivity to the stream, slope, plant cover, presence of embankments and erosion of the banks in the case of plots beside the river. By combining the ELECTRE III multicriteria analysis method to a geographical information system (GIS), it was possible to discriminate zones that present a risk with respect to particle emission and then to present the results in map form to provide decision-making aid for local actors.