In the last decades, the conservation of soil and water resources has become a major environmental concern worldwide. The authors used an anthropogenic radiotracer (137Cs) to assess the magnitude and the spatial distribution of soil movement in a 80 ha agricultural watershed located in Southeastern Québec (Canada). This project is part of a larger one aimed at increasing our understanding of erosive processes, in order to minimize their impacts on soil resources and downstream hydrosystems. Using this technique, it was possible to establish a budget and a map of soil redistribution. The results could help farm operators to implement a resource conservation strategy. It was also possible to assess the environmental impact of this soil movement, by estimating the magnitude of the net output of sediment from the studied watershed. This was estimated at an average of 237 tons per year, over a period of some 33 years. The output represents 93% of the total soil displacement within the watershed boundaries. Although slightly higher, these results are consistent with those obtained, over a time period limited to 2 years, by water quality monitoring at the watershed outlet and by model simulation. This intensive study, involving more than 500 sampling points for 137Cs, has yielded valuable information on the assessment and the spatial representation of erosive processes. Such information is needed to improve the management of watersheds and can help identify the areas most at risk and establish priorities for corrective actions. In this way, nonpoint pollution of downstream hydrosystems may be controlled more efficiently.