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Final Report 3-1 September 2000 3 HYDROLOGY AND GEOMORPHOLOGY OF THE GREEN RIVER Physical factors such as climate, geology, and physiography affect the distribution and abundance of organisms in and along rivers. These factors affect vegetative cover, runoff patterns, runoff rates, and volume and seasonality of river flow. Geology and vegetative cover also affect sediment dynamics and water quality within the river . Water temperature varies according to climate, season, topography, and water source. These physical factors strongly influence the life history of native species, which are adapted to the particular conditions characteristic of the watershed in which they evolved. Physical conditions are not constant along most rivers. Large rivers typically pass through different climatic zones and areas of divergent geology. Flow patterns and geologic conditions create unevenly distributed habitats of low or high velocity within the channel and seasonally flooded habitats in areas outside the channel. High flows during the runoff period are particularly important for creating and maintaining habitats for riverine organisms, because these flow events reshape sediment deposits, scour vegetation, and flush accumulated fine sediment from the streambed. Occasional flooding of floodplain areas creates temporary, but productive, habitats and can result in substantial inputs of biomass and energy to the river for sustaining aquatic food webs. Any discussion of the flow needs of endangered fishes in the Green River system would be incomplete without an understanding of these important physical processes. Therefore, a description of the hydrologic and geomorphic characteristics of the Green River from Flaming Gorge Dam to the Colorado River confluence is presented in this chapter. Wherever possible, an attempt is made to identify critical flow values necessary for maintenance of natural processes important to the endangered fishes both within and outside the river channel. Such processes include deposition and erosion of sand bars, creation and maintenance of low-velocity habitats, channel narrowing, vertical accretion of banks, and overbank flooding. 3.1 CHARACTERISTICS OF THE GREEN RIVER BASIN The Green River basin, which is located in Wyoming, Colorado, and Utah, occupies a total area of 115,800 km2. The Green River is about 1,230 km long and originates in the Wind River Range of Wyoming, flows south through Colorado and Utah, and joins the Colorado River in Canyonlands National Park (Figure 3.1). Elevations in the basin range from nearly 4,200 m in the mountainous headwaters to 1,200 m at the Colorado River confluence. The Green River is the largestitributary of the Colorado River. Nearly half of the flow of the Colorado River at its confluence with the Green River is from the Green River basin.