Unsteady flow in open channels can be simulated by solving the Saint Venant equations to predict the discharge and water level in the channel during the future time series under the given condition. On the other hand, the operation problem is used to compute the inflow at the upstream section of the channel or a schedule of operation for the regulating structures of the delivery system to get a predefined water demand at the downstream end of the channel. This type of problem is also known as the inverse computation of open channel flow. The finite difference Preissmann implicit model was used to solve the operation-type problems in open channels, based on the saint Venant equations. The final linear system of equations was solved by the double-sweep algorithm. The computed results using the finite difference implicit model showed more accuracy and less oscillation than that obtained by an explicit model for all the numerical tests. The finite difference Preissmann implicit model was successfully applied to a nonprismatic reach of El-Mansouria canal between Meit-Ghamer and Sanayet regulator.