The computer programs addressed in this chapter simulate flow conditions in natural and improved streams and rivers and associated flood plains and in human-made channels. Some of the programs can also be applied to reservoirs, estuaries, and coastal waters. Model input includes channel geometry and roughness data and either steady-state or time-dependent inflow rates. Steady, varied flow models compute flow depths as a function of location along the channel. Unsteady flow models calculate discharges and flow depths as a function of time and location. Some programs model movable as well as fixed channel beds. Movable boundary models simulate erosion and sediment transport processes as well as flow.