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Andrews and Nelson downstream part of the secondary channel where some sediment is deposited. Calculations of bar evolution using a discharge of 550 m3/s reveal a similar pattern of scour and fill, except the magnitude of bed elevation changes is greater and the transition from Scour to fill over the bar surface and in the secondary channel are shifted downstream. Calculations of bar evolution using a discharge somewhat less than 475 m3/s indicate that the magnitude of bed elevation changes would be smaller everywhere and the transition from Scour to fill over the bar surface and in the secondary channel would be shifted upstream. Predicted evolution of channel topography in the Ouray reach at a constant discharge of 50 m3/s for a period of 2 days is illustrated in figure 9. At this discharge, the bar apex stands nearly 1 m above the water surface and there is no flow through the secondary channel. The entire river flow is confined to the primary channel. A substantial quantity of sediment accumulates in cross section 16, located upstream of the bar apex. Progressing downstream to cross section 19 through 23, the thickness of accumulated sediment decreases. Comparison of changes in bed elevation at these cross section after various intervals of time shows that the zone of most rapid deposition is moving downstream. At a discharge of 50 m3/s, the reach averaged bed-material transport rate is about one-tenth of the transport rate at a discharge of 475 m3/s. Nevertheless, the rates of scour and fiU are locally quite large compared to values computed at a discharge of 475 m3/s. The local rate of scour and fill depends upon the divergence of the sediment flux rather than its magnitude. As shown in figure 5, divergence of the boundary shear stress, and, hence, the rate of scour and fiU increases . significantly over the upstream face of the bar as discharge decreases. Strong local divergence of the sediment flux is a result of a significant disequilibrium between the flow and channel topography. The d~ree of disequilibrium is not unexpected because initial bed topography was essentially in equilibrium with a discharge of ...275 m3/s. Discussion of Results A comparison of aerial photographs taken of the Ouray reach between 1963 and 1987 shows that the bar we investigated has changed very little durin~ this period. The general size and shape of the bar which is exposed at low discharge, has \ remained essentially the same. Although vegetated islands exist in this portion of \ the Green River, vegetation has not become established on this bar. Finally, the i location of the bar has remained fIXed, whereas channel bars are frequently observed (Ito migrate downstream in relatively straight reaches. As noted above, the Green River floodplain in the vicinity of the Ouray reach has been constructed primarily Iby the aggregation of vegetated islands. Therefore, some bars, although perhaps a lrelatively small percentage, evolve from an unvegetated bar, such as the one we ~vestigated, to become a vegetated bar and finally, are incorporated into the oodplain when the secondary channel aggrades to near the bankfull. elevation. nsequently, the stability of the bar we have investigated is perhaps greater than jone would expect. '. The apparent stability of the bar indicates that the overall rate of bar evolution )is relatively slow compared to the length of time covered by the aerial photographs. Calculated rates of bar evolution, however, indicate that the bar topography adjusts <: rapidly over a period of several days to a few weeks when the discharge changes. \Maximum computed rates of scour and fill are on the order of a few tens of icentimeters per day. To some extent, the lack of long-term evolution in bar jtopography is due to a balance between fiU at one range of discharge against scour I'at another range of discharge. For example, sediment was eroded from the primary \ channel at flows greater than 275 m3fs and deposited in the primary channel at 481 '.~;.. ;..: