Soil degradation by water erosion remains excessive in Brazil. The contribution of potential sources was investigated in a 2032-km2 cultivated catchment. Cropland is unambiguously the main source supplying sediment to the Guaporé River (91 ± 15%). Cropland supply about 2.17 Mg ha−1 yr−1 of sediment yield to the investigated river. Local magnitude of cropland contribution increased with the drained cropland surface area. There is an urgent need to better plan land use and farming practices in similar areas of Southern Brazil.