The South Branch of the Potomac River experienced extreme flooding in November 1985. A flood with an estimated return period of 400 years caused $33 million in damage in Petersburg, West Virginia. Subsequently, a local flood protection study was conducted by the U.S. Army Corps of Engineers to evaluate options for flood protection (USACE 1990). A key component of that study was the use of HEC-2 to simulate water surface elevations for a variety of flow magnitudes under existing and pre-flood conditions. Three major goals are pursued in this study. The first is to use the two-dimensional numerical model, RMA2, to simulate water surface elevation and flow velocity for the 50-yr, 100-yr, and 400-yr floods in order to compare the results of the two-dimensional model with those previously obtained using the one-dimensional HEC-2 model. Our purpose is to evaluate what additional information useful for the design of flood protection plans may be obtained from the two-dimensional simulations. The second goal is to examine the effect of proposed levees on flow during the 50-yr, 100-yr, and 400-yr floods. Finally, an objective is to examine the interaction between river and flood plain morphology and the spatial variation in flow during flooding, and in particular, to explore the effect of alternating expansions and contractions in the width of the flood plain.