Not to be “that guy” on top of you being “that guy,” but it’s not unheard of to completely redirect a dammed river with a chute spillway. I’m gonna pretend the spillway exits that mountain to the right of what we can see lol
And now I've got to be the tertiary guy and point out that if the flow rate into a reservoir is low enough, and the surface area of the lake large enough, evaporation can control the water level. See: aral sea.