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Draft Fmal Completion Report to UDWR for Contract #93-1070, Amendment 3 13 results ofYu and Wolman (1987) are consistent with Lyons and others (1992) finding that channel widening and narrowing have both occurred since closure of Flaming Gorge Dam, but the net trend has been narrowing due to lower average flows. Numerical Modeling of Flow and Sediment Transport in Natural Channels For this study, it is desirable to predict bar and bed response to high flows and bed evolution during passage of a flood in order to design flood flows that will improve and enhance habitats. The 3-dimensional flow of water in rivers is very complex. Empirical models of river flow reduce this complex system to a simpler 1- or 2-dimension system with empirically-derived factors. An often employed example of this type ofmodel is theHEC-2 model developed by the US Army Corps of Engineers (HEC, 1982). The HEC-2 model is used to calculate water surface profiles for ~ver reaches using cross-sectional data and Manning's n. a channel roughness coefficient This model assumes steady, uniform flow and predicts water surface elevations. However, being a 2- dimensional model, it has no information concerning lateral distribution of hydraulic properties. More recent modeling efforts recognize that 3-dimensional properties such as topographically-induced convective accelerations, secondary circulation, and the distribution of boundary shear stress within the curvilinear 3-dimensional channel are critical to determining patterns of erosion and deposition (for example, Nelson and Smith, 1989a; Smith and McLean, 1984; Engelund, 1974). An orthogonal stream-wise coordinate system appropriate for stream models was developed by Smith and McLean (1984). The N avier-Stokes equations, the equations of motion which consider local and convective accelerations in fluids, were then transformed to the stream-wise system. Smith and McLean (1984) compared modeled results for bottom shear stress and free surface elevation with those measured in Hooke's (1975) flume experiments ina fixed-bed sinusoidal channel. Their model produced the same pattern of boundary shear stress, although the maximum boundary shear stress was underestimated, and the area of low boundary shear stress extended too far downstream. Nelson and Smith (1989a) refined and expanded the model of Smith and McLean (1975). Therevisedmodel allowed variation in channel width, accounted for the presence of bedforms, and predicted sediment transport. Numerical results for boundary shear stress, sediment transport, and vertically-averaged velocity compared well with the field measurements by Dietrich (1982) in Muddy Creek. Nelson and Smith (I 989b) expanded this model using