the answer is
plants...
dams are man made...winds, they blows the soil away which causes erosion..rivers, they also cause erosion because when they flow, they carry the soil in the direction they r going n ump it somewhere else.
but plants, plants help breaks down soil n even makes more because the roots goes into the rocks n craks it up blah blah..and they hold the would from moving, like grasses do
I dont think my explanation was that good but I hope I helped a bit :)