Not impossible at all! The main spillway discharge enters the river channel at a 90deg. angle, easy to see how it could flow both ways with enough volume. It will never back up to lake level, but could back up enough to affect the turbines discharge.
Yup, the increased back pressure starts STALLING the turbines, and computer controlled respond by increasing the excitation voltages,....bad electrical catastrophes ensue. In the case of debris actually being inducted, the almost incalculable forces involved can break the shafts / gear trains ......the equivalent of a powerful bomb going off.
GTC
So, if that is all correct, I take it simply that fluctuating backpressure is a very bad thing - and that makes sense. Still, for some reason, I had thought I remembered there being a way to let water pass through without spinning the turbines - but as I said, it was a long time ago when I saw it all close up.
