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The Mediterranean would dry up if the Straits of Gibraltar closed up. That’s some very large fraction of all rivers in Europe plus the biggest in Africa that never actually make it to the Atlantic. Evaporation is the cause.

This is untrue. You aren’t thinking about the effects of sediment, usage, and erosion. When you build a dam and allow the riverbeds to dry, it changes the entire water table of the region.

It also blocks animal migration.

They don't dry; they resume as normal once the dam is complete.

I don't understand that nobody can see this. The lake can't hold all the water to inifinity. Once it's full, the outflow becomes normal. Except now you can regulate it.

Where I live, we then drain those lakes regularly for the water of our cities so the water available down stream is greatly reduced except in tons of flood in which case it is unusually high.