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Andrews and Nelson the bar will grow to within ",10-15 centimeters of the water surface. In our calculations, deposition on the bar crest ceases when the local boundary shear stress is less than the critical value. In actuality, the mechanics of flow and sediment over the bar crest become very complex when the local depth is less than 10-15 centimeters, and the model does not simulate locally important processes; advection of suspended bed-material and wave-current interactions. Given appropriate flow conditions and bar topography, suspended bed-material, especially the smallest bed particle, can be transported from upstream areas with relatively large boundary shear stress and deposited on the crest of the bar even when the local boundary shear stress is less than critical. As the bar crest approaches the water surface, wind-induced waves on the river become an important factor limiting further growth of the bar. Waves breaking on the bar crest resuspend the bed material, which then can be advected or diffused away from the crest. Especially large waves are generated by interaction with the river current when the wind is blowing upstream. Water waves with amplitudes up to 20 centimeters are rather common in a channel as wide as the Green River i through the Ouray reach. Typically, water waves break when the depth is slightly ': greater than the wave amplitude. Therefore, it appears likely that water waves i become a si~ificant factor inhibiting further growth of the bar crest when the local 1\ flow depth IS less than roughly 20 centimeters. Furthermore, breaking waves may erode the bar crest as discharge decreases after a peak, especially when the discharge decreases slowly. . Wind erosion also may influence the elevation of the bar crest. Outing a majority of the year, the bar crest is emergent and dry. Aerial photographs taken in late summer and fall show that the bai crest is extensively modified by eolian transport. The annual rate of eolian deflation, however, is unknown. A windstorm with a velocity of 7 m3/s, a rather common occurrence in eastern Utah, would deflate the dry bar surface at a rate of 2-3 em/day. Thus, the total eolian deflation between successive spring floods may be as much as a few tens of centimeters. The l combined erosion of the bar surface by water waves and windstorms is probably on . the order of 0.5 meters per year. The predicted rate of sediment accumulation on the bar crest at a discharge of 475 m3/s is about 15 em per day. Between 1963 and 1987, a discharge of 475 m3/s occurred approximately 3 days per year. Therefore, it : is possible that the elevation of the bar crest is maintained approximately constant : over ,a period of years due to deposition from fluvial transport and erosion by water \ waves and windstorms. The si~ificance of bar erosion by water waves and : windstorms deserves further attentIon. i- The downstream migration of channel bars in relatively straight channels with uniform width and cross-sectional area is a commonly observed phenomena. As noted above, channel width and cross-sectional area vary considerably through the Ouray reach, this appears to explain the long-term stability of the location of the bar. Predicted bar evolution at dischar~es of 275 m3/s and 475 m3 Is indicate a slight tendency towards downstream Dllgration. At a discharge of 475 m3 Is, sediment is eroded very slowly from the upstream face o{ the bar and deposited very slowly on the downstream {ace. At discharges less than 275 m3 Is, the zones of both scour and fill shift upstream and, thus, counteract the pattern at larger discharges. When these calculations are repeated using a larger channel width and area in the vicinity of cross sections 18-22, the magnitude of scour and fill over the bar surface at 475 m3/s is greatly enhanced, and the upstream shift in the pattern of scour and fill at relatively small discharges is less pronounced. Therefore, the constriction of channel width and area in the vicinity of cross sections 18-22 probably determines the location of the bar. The constriction of channel width and area in the vicinity o{ cross sections 18-22 also appears to explain the persistence of the secondary channel. As shown in Figure 8, cross sections 16, 18, and 20, the secondary channel scoured rapidly at a discharge o{ 475 m3 Is. Conversely, at discharges less than 275 m37s, model 483