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• DRAFT February 25, 1998 Thus, flow-habitat relationships regarding nursery habitat availability for Colorado squawfish include both antecedent-condition and flood magnitude components. The recommended flood cycle over a period of years should include both occasional large floods that rebuild channel topography and more frequent lower magnitude floods. The current practice of extending the duration of low-peak floods should be re-evaluated. If the prolonged peak's stage is greater than the bar tops but less than bankfull, then neither the habitats associated with the bars or with the floodplain are available, a condition that was typically of very short duration for the Green River prior to dam closure. The target summer flow should be dependent on existing channel morphology and hence will need to be evaluated annually, although the conceptual model presented could be useful in advance planning of flow needs. Winter flows and their impact on overwinter habitat availability and degradation must be evaluated. Model simulations indicate that shear stress, and hence sediment transport, is a function of depth, so that the discharge necessary to build bars in this sediment abundant system are a function of depth of flow over the bar rather than absolute discharge. Consequently, when antecedent topographies are lower, it is expected that a lower discharge would be necessary to build bars. The flow conditions during this study were insufficient to test this hypothesis but future work could address this issue. • Schmidt, J.C. 1994. Annual Report: Compilation of historic hydrologic and geomorphic data for the upper Colorado River basin. Flaming Gorge Research Program, Study #37, Department of Geography and Earth Resources, Utah State University, Logan, Utah. 5p. Preliminary results of ongoing investigations of the geomorphology and hydrology records of the Green River Basin are: 1. The magnitude of the annual peak discharge of the Green River upstream from the Yampa River has decreased about 60 percent from the 1923-1962 period to the period following closure of Flaming Gorge Dam. At Jensen and Green River, LIT, the changes have been about 25 percent, depending on the recurrence of the flood being evaluated. Floods on the Yampa River have been unchanged during the same periods. 2. The highest floods of this century occurred between in the early part of this century. Instantaneous peak discharge at Green River, UT, exceeded 60,000 ft3/s in 1897, 1909, 1917, and 1921. Peak flows at Greendale are estimated to have exceeded 20,000 ft /s in 1899, 7918, and 1921 (Figure 2). These floods are unprecedented; the fact that floods of this magnitude do not occur at present may be related to climatic change and be unrelated to the existence of dams. The effects of these floods -on habitat availability have not been evaluated. • 25