Dams are set up in an area that a large amount of surrounding area naturally flows water into.

That’s why dams are constantly letting water out, even when it hasn’t rained.

So like, so X is the total amount of water before the dam overflows. You’d want to keep normal level at X - 25%. But if a storm is coming in they do pre-emptively open the floodgates and go down to X - 50% or whatever.

One of the big problems is these large storms last so long, days rather than hours. And even if they open the floodgates at the dam to help the lake, it makes it worse for people along the river it feeds into.

There’s no simple solution, we already started doing all that decades ago. It’s what was hiding the problem and why so many ignored it and it got so bad.