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484 Topographic Response of a Bar calculations predict that the secondary channel will fill slowly. The rapid rate of scour at a discharge of 475 m3/s apparently has been sufficient to maintain the secondary channel. When the calculations of bar evolution at 475 m3/s is repeated using a larger channel width and area in the vicinity of the constriction, the rate of scour in the secondary channel decreases. Thus, it appears that the constriction of channel width and area is sufficient to prevent a net accumulation of sediment in the secondary channel over a period of years. Conclusions The conclusions of our investigation are: 1. The fully nonlinear flow model described by Nelson and Smith 11988a,bl predicts flow structure, sediment transport rates and temporal evolution of channe topography all of which are in good agreement with observed conditions in the Green River near Ouray, Utah. 2. Over the past 30 years, construction and operation of a reservoir, as well as natural variation in the snowmelt runoff, have caused substantial and persistent year-to-year differences in the magnitude of discharge equalled or exceeded less than 10 percent of the time in the Green River near Ouray, Utah. Annual sediment loads have varied accordingly. . The quantity of sediment transported by a given discharge, and the approXimate balance between sediment supplied and tra.risported within appreciable reaches, however, have not been affected. The bankfull channel of the Green River has adjusted slowly to the large changes in annual flood discharges. 3. The flew velocity field and distribution of boundary shear stress are - determined primarily by bed topography and longitudinal variations in channel width. At all discharges investigated, streamline curvature exceeded channel curvature. Stream line curvature increased significantly as discharge decreased. 4. Channel topography changed very slowly at a discharge of 275 m3/s, which was approximately the discharge when the topography was surveyed. Hence, the baseline topography appears to be nearly the steady state bed configuration for 275 m3/s. '-.. 5. Channel topography adjusts relatively quickly to discharges appreciably I different from 275 m3/s. Initially, divergence of the bed-material transport rate , were locally large. Extensive scour and/or fill occurred at most Cross sections at i discharges appreciably greater or less than 275 m3/s. Divergence of the sediment ;---lransport field tended to decrease as bed topography evolved. 6. Compared to the baseline configuration, topography of the channel bar was greatly enhanced following a simulated flow of 475 m3/s for 2 days. Sediment is deposited on the bar surface and is eroded from the primary and secondary channels, except in the downstream part of the secondary channel where some sediment is deposited. 7. Compared to the bankfull topography, bartopograpby was diminished followin~ a flow of 50 m3/s for 2 days. The primary channel aggraded ...1 m along. most of Its length, while the bar crest and secondary channels were emergent. 8. Model calculations predict that sediment will be deposited on the bar crest at all discharges large enough to cover the entire bar to a depth of several centimeters or more. Erosion of the bar crest was not indicated at any discharge. Actual deposition of sediment on the bar crest probably is slightly greater than predicted by the model, because the advection of suspended sediment from areas of relatively large boundary shear stress into areas of relatively small boundary shear stress is neglected. The bar crest is most likely deflated by wind-induced waves and/or wind transport of sand when the bar crest is emergent and dry.