https://en.wikipedia.org/wiki/Tarbela_Dam#Lifespan

The Sanmenxia Dam in China lost 17% of its capacity in the first 18 months of its operation due to sediment accumulation [1]

Countless other dams have already been dismantled due to sediment build up, and many more will follow, considering that around 19000 large dams worldwide are over 50 years old [2]

[1] https://en.wikipedia.org/wiki/Sanmenxia_Dam [2] https://e360.yale.edu/features/water-warning-the-looming-thr...

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.

I always assumed they would clear the area of trees first anyways. Why let the wood rot when you can cut it down and use it?