[rualca]

> I'm struggling to believe this.

If seems you didn't even did the most basic cursory search on the issue.

Here's literally the first hit on a google search for "dam sediment".

https://www.hydroreview.com/world-regions/dealing-with-sedim...

And think about it: what's a dam reservoir other than a big old sedimentation basin.

https://en.wikipedia.org/wiki/Sediment_basin

Also, think about it: hydro power is not the only use for a dam. They are also typically used for things such as flood control, water supply, and energy storage. All of those usecases rely on the sam's ability to hold water (and it's sediments) and lower discharge, if not totally block it.