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Final Report 3-9 September 2000 Reach 3, between the White River and Colorado River confluences, is about 394 kIn in length (Figure 2.1). The White and Duchesne Rivers at the upper end of Reach 3 add considerable sediment (about 4.4 million metric tons per year) to the Green River. A portion of the flow of the Duchesne River is diverted out of the Green River basin; the combined mean annual discharge of the White and Duchesne Rivers is about 31 m3/s. Before entering Gray and Desolation Canyons in Reach 3, the Green River meanders through the Unita basin and has a low gradient (about 0.0002). Numerous sandbars can be found in this portion of the reach at low flow, and low-elevation floodplain areas are prominent. In Gray and Desolation Canyons, gravel bars are abundant, and many of the banks are composed of coarse debris-flow material or talus. Recirculating eddies are also prevalent, and there are many regions of stagnant flows in these canyons. Three geomorphic divisions occur in the 1Sl-kIn portion of Reach 3 from Sand Wash through Gray Canyon: (1) an upstream division with a very low channel gradient and wide channel width; (2) a middle section with steep gradients and abundant debris fans; and (3) a lower section with a moderate gradient and very narrow channel widths. The average channel gradient through these canyons is about 0.001, and the bed material ranges from sand in the upper portion and in the recirculating eddies to cobbles and boulders in the riffles and rapids formed by debris fans. The Price River is the principal tributary in this part of Reach 3. Downstream from Gray Canyon, the river flows through a broad valley for approximately 62 kIn. The channel pattern is primarily restricted meanders (Section 3.6.1.1), although straight channels occur in some areas. The channel gradient is about 0.0004, and the bed material ranges from sand to gravel and cobble. The San Rafael River is the largest tributary in this part of Reach 3. The lower 148 kIn of the Green River flows through Labyrinth and Stillwater Canyons and has a sinuous river channel with a relatively mild gradient of about 0.0002. Bed material is predominantly sand in this portion of Reach 3, with numerous emergent sand bars at low flow. 3.4 FLOWS IN THE GREEN RIVER DOWNSTREAM OF FLAMING GORGE DAM This section presents a discussion of annual, seasonal, and daily variability in flow in the Green River and effects of depletions and dam operations on these flows. This information is necessary to understand changes that have occurred in flows and how these changes affect ecological conditions and endangered fish populations. Flow data are based on USGS flow gages in the Green River and its tributaries (Figure 3.3). Table 3.3 lists significant gages and the period of record for which data have been collected at those gages. Numerous other gages exist in the Green River basin closer to the mountainous headwaters. Data from Flaming Gorge and Fontenelle Dams (on reservoir storage, inflow, releases, etc.) were also used in some analyses. ~