Typically at the dam, there is an outlet control structure. The main structure will have a preset opening (orifice) that maintains a normal pool elevation (water level). This structure will also have either a valve or a sluice gate. The valve opens and closes just like your bathroom faucet, but just a much larger diameter. A sluice gate is vertical barrier that can be raised or lowered to control the water level and to release excess stormwater. Sluice gates allow a lot of water to exit quickly, while a valve would take much longer. I have not seen plans for the dam and structure, but one can see the outfall channel on Google Earth. Once the water level is lowered, mother nature does a good job of refilling the reservoir.