The intakes for the turbines are two giant "tubes" running down the slope on the south side of the reservoir. They're screened on top, and, IIRC, have gates to choose at which level the water is allowed in. They can therefore take water from a much higher pressure than the water backing up at the discharge. How can that stall the turbines?
The turbines use most of that head up doing work turning the generator. The discharge is at a much lower pressure.
