They don't really release water into the cavern, they release air out (through the turbine stages). The water is never really held back. It's a floating counter

They don't really release water into the cavern, they release air out (through the turbine stages). The water is never really held back. It's a floating counterbalance for keeping the air pressure constant (or mostly constant, considering surface reservoir level differences which would never exceed a tiny fraction of the total water head, unless the facility runs into extreme water shortage)