Abstract: The objective of this study was to investigate the phenomena of subcritical and supercritical spatially varied flow in rectangular expansions such as that in an erosion gully. Based on momentum and continuity principles, equations were developed to predict such flow phenomena. Computations of the water surface profiles for subcritical spatially varied flow were carried out by applying direct step numerical calculations. Because of the presence of standing waves, the method of characteristics was employed in the analytical analysis of the supercritical flow phenomena. Of primary importance in this study is the effect of varying amounts of lateral inflow rates. Other parameters varied during the experimental tests were the slope of the channel and the main channel discharge. Comparison of the analytical and experimental results indicated that for both subcritical and sμpercritical spatially varied flow, water surface profiles can be predicted with good accuracy. Funding information: The work upon which this report is based was supported in part by funds provided by the United States Department of the Interior, Office of Water Resources Research, as authorized under the Water Resources Research Act of 1964.