A discharge control system for a reservoir having a pressure-communicating valve in a valve chamber with the inlet of the pressure-communicating valve connected the reservoir outlet aperture and the outlet of the pressure-communicating valve connected to a discharge aperture in the containment structure for the valve chamber. An auxiliary reservoir outlet aperture enables water to flow from the reservoir into the valve chamber, submerging the pressure-communicating valve. Water flows from the valve chamber through a valve chamber outlet aperture that is substantially the same as the auxiliary reservoir outlet aperture and is located at substantially the same height as the center of the reservoir outlet aperture. One or more supplemental auxiliary reservoir outlet apertures are located at incrementally greater heights in the containment wall for said reservoir to assure that water will enter and leave the valve chamber at the same rate under all conditions.