I think it depends on the useful life of the dam, mainly determined by silting, which in turn depends on geology, soils, vegetation, gradients, erosion, rainfal

I think it depends on the useful life of the dam, mainly determined by silting, which in turn depends on geology, soils, vegetation, gradients, erosion, rainfall patterns, hence suspended load in tributaries, and the possibilities for sluicing out the silt. For example, the Three Gorges dam seems to have a problem with silt accumulation that could cause a dramatic reduction in reservoir water capacity. I suspect many such dams will end up as concrete carcasses at the head of muddy lagoons - maybe not so bad for wildlife in the long run.