In fact, in most cases, the majority of sediment deposits furthest from the dam, and most of it doesn’t reach the dam until the reservoir is pretty much full. And that’s a very tough thing to do because of how variable those rates can be, because sediment doesn’t flow the same in a reservoir as it would in a river, because of the constraints it puts on operations (like the need to draw reservoirs down) and because of the complicated regulatory environment surrounding the release of sediments into natural waterways. You can fight fire with fire by building check dams to trap sediment, but then you’ve just solved reservoir sedimentation by creating reservoir sedimentation.