No one said anything about back to Lake level... it is about stalling turbines due to erratic flow and compromised outlets. Plenty enough to get crap back into the turbines.
What "eratic flow"??? You'd need either a lid on the lake creating a vacuum or giant whirlpools sucking air before anything like that could happen.... And you can compromise the outlet as much as you want....the hydro pressure from above will not allow a backflow....like I said, it's against the laws of physics...
