A one-dimensional unsteady flow model has been applied to the tidal Potomac River, including its major tributaries and marginal embayments, between Washington, D. C. and Indian Head, Md. The computer model has been calibrated to simulate the combined effects of tide, freshwater inflows, and wind conditions governing flow in the system of channels. The comprehensive flow data provided by the model can be used to better understand the physical, geochemical, biological, and other processes that affect the river's water quality.