Both numerical solutions and experimental measurements are obtained for flow within a reservoir after a partial dam breach. The dam breach is assumed to act as a flow control so that the reservoir solution can be calculated independently of the downstream flood-routing solution. Numerical solutions are obtained by using the method of characteristics to solve the St Venant equations. Boundary conditions at the breach are obtained from a weir equation, and boundary conditions at the upstream end of the reservoir are obtained by either specifying an inflow for subcritical flow, by specifying both the velocity and depth for supercritical flow or by making the level-reservoir approximation when a zero inflow occurs. A surge often, but not always, appears in the solution. Experimental measurements confirm the calculations in almost all respects.