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HABITAT AND GEOPHYSICAL CHARACTER, UPPER COLORADO RIVER The width, depth and physical characteristics of the Upper Colorado River vary greatly by geographic area and also vary with quantity of flow. Some of the deeper areas recorded in the study were Cataract Canyon with a maximum depth of 92 ft and Black Rocks and Westwater Canyons with maximum depths of about b0 ft. In the Green River a maximum depth of about 60 ft was recorded for Gray Canyon and up to 28-ft depths were recorded for the lower Green River in Labyrinth Canyon. Average depths for cross sections of the different river strata varied from about 2 ft to over 60 ft. Various habitats in the upper Colorado River were quantified at key locations using computer modeling developed by the FWS Instream Flow L Group (IFG) methodology (3~a appendix G ). Three hydraulic stations on the Green River were modeled;~Mineral Bottom (RM 56), Ouray (RM 248), and Gray Canyon (RM 141); and three stations were modeled on the Colorado River at Black Rocks (RM 136), Potash (RM 53) and Moab (RM 73). Analyses were made to evaluate potential habitats suitable for young a~-3s~a~ `- a.~ squawfish and j::~e~il~e-~ adult humpback chub at these sites. Flow changes experienced in the Upper Colorado River were analyzed by looking at hydrologic time series at the six hydraulic stations mentioned above. Actual USGS gaging station flows for 1960-1980 were compared to modeled flaws depicting: (1) virgin flow conditions without any development in place, and (2) developed flow conditions with 1980 development projected from historic conditions. Temperature modeling was also done to determine effect of flow changes on river temperatures at the key hydraulic stations. Generally, the IFG hydrologic time series analysis shows that there have been significant peak flow reductions on both the Green and Colorado Rivers. The Colorado under present-day conditions has had the average monthly peak flow reduced by about 407 while the Green River has experienced an average peak monthly flow reduction of abaut 25 to 40x. Changes in instantaneous peaks or daily maximum flows has changed by even a greater degree. These changes in peak flow have correspondingly resulted in higher low flows during the fall and winter through the release of reservoir storage waters. River morphology and habitat is primarily determined by peak flow levels, magnitude and duration. What has occured in both the Colorado and Green River with reduced peak flows is that these rivers no longer move the volume of sediment material through the system that they once did. This has resulted in mare silt/sand areas, braided channels and aggradation of the main river channel. The result has been fewer deep r ns, fewer clean gravel-rubble areas and a reduction in the frequency duration of overbank flows, especially on an annual basis. v With reduced peak flows, water temperatures are wa~ming more quickly during the spring runoff period in the Coloradgft~'The opposite `-" occurs in the Jensen area of the Green River with the cold water released from Flaming Gorge Reservoir depressing temperature during the summer. G