Most natural ¢ows in streams and rivers change slowly with time. Also, man-made channels and canals have gates that permit a greater or lesser ¢ow through their structures in response to changing demands in water requirement. An important problem that has not been solved, and will probably never be completely solved is how controls are to be operated in time to optimize bene’ts to the water users, while minimizing waste, and anticipating changes in demand caused by weather conditions, crop requirements, and supply limitations. Thus, the ¢ow in man-made channels is often controlled so as to be unsteady. The fact is that in the real world, most open-channel ¢ows are not steady state, but unsteady and often only modestly so.