Understood. I took the OP's "how much water flows past generators" to address whether all the dam's outflow goes through the turbines or whether some is divert

Understood. I took the OP's "how much water flows past generators" to address whether all the dam's outflow goes through the turbines or whether some is diverted around them. The former is the case. The latter only happens when spillways are in use, which is rare.