One is surely sediment erosion. All the small gravel that would usually end up in the delta being collected behind the dam. I.e like here: https://en.wikipedia

One is surely sediment erosion. All the small gravel that would usually end up in the delta being collected behind the dam. I.e like here: https://en.wikipedia.org/wiki/Restoration_of_the_Elwha_River#Sediment_load https://en.wikipedia.org/wiki/Restoration_of_the_Elwha_River...