Classically, if you have free flow (high level to low, no pump) between two vessels you want to control you use some restricting orifice, or flow over a variable height weir. Both dissipate energy.
Now you could have a turbine in the flow, attached to a generator. If you adjust the torque applied by the generator to the turbine, you can control the flow over some range and extract power. I think this is an old idea, I just don't know the corect term to properly google it.
There are many considerations why this may not be feasible or worth the trouble in a given instant (e.g. hydraulic efficency of turbine over the flow ranges required). The applications I'm thinking of would be wastewater treatment plants with flows 1-10m³/s and heads <1m - so single digit kW most likely. At these heads, Banki or axial flow type turbines would probably be used.
My question is about the possibility - Can you actually control flow by appying breaking torque to a turbine, what does this do to hydraulic efficiency?
Maybe I'll ask about the electrical side in a different question, while certainly not trivial it's out of scope for now.