Based on the de St Venant Equations, the Preissmann's implicit finite difference scheme, and the double sweep algorithm, a mathematical model for one-dimensional subcritical unsteady flow in the Third Navigation Channel at the Gezhouba Water Control Project, China, has been developed. The model has been calibrated and verified using the prototype test data. All conceivable factors affecting wave propagation have been studied parametrically. The side discharge method is considered to be the best way to reduce the amplitudes of both water surface elevation and velocity variations caused by unsteady flow due to the emptying of the lock chamber. Using side discharges to distribute the discharge from the lock chamber along the channel, instead of concentrated discharge at the upstream boundary, improves greatly the navigation conditions.