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channel narrowing in Desolation and Gray Canyons, a second objective of this study was to determine if channel narrowing had occurred, and if it had caused a significant effect on discharge/habitat relationships. The purpose of this report is to (1) report measurements of available habitat in 4 representative reaches of Desolation and Gray Canyons at a wide range of discharges, (2) identify and describe geomorphic changes in these canyons, and (3) evaluate how historical changes in geomorphology have altered habitat/discharge relationships over time. For purposes of development of flow recommendations, our results must be integrated with the results of ecological studies which identify the relative importance of the different habitats in the life history of the target species. STUDY AREA _~ Desolation and Gray Canyons are located in eastern Utah where the south-flowing Green River has eroded its course into the Tavaputs Plateau. ~ Although 2 names are attached to the 151- F-j km reach between Sand Wash and Swasey's Rapid (Fig. 1), the reach is more appropriately divided into 3 primary geomorphic divisions; an upstream reach with very low channel gradient and wide channel width, a middle section with. steep gradients and abundant debris fans, and a lower section with moderate gradient and very narrow valley and channel widths (Fig. 2; Table 1). Between Sand Wash (RK 347.6) and Jack Creek (RK 305.8), channel gradient is very flat and the channel plan form is a series of fixed meanders where alluvial terraces occur on alternating banks of the river. This section of the river is the continuation of the southern Uinta Basin and fixed meanders extend upstream to Ouray. The second geomorphic reach is located approximately between Jack Creek and the Roan Cliffs (approx. RK-253). The dip of the ,~ sedimentary rocks of the region is to the north, and the Tavaputs Plateau increases in elevation to `1 the south. As a result, the depth of Desolation Canyon increases downstream. Downstream from ,-- 4 Jack Creek, channel gradient steepens. Bedrock becomes more resistant; and debris flows become more frequent and deliver coarse sediment to the channel. The Roan Cliffs mark the southern limit of Desolation Canyon. 'The third major geomorphic division occurs downstream from this point, where the river establishes its course in the Mesa Verde formation. This reach's southern limit is the Book Cliffs near Swasey's Rapid (RK 212.4) and the town of Green River. The four reaches used in this study were established by UDWR biologists (T. Chart, ~ UDWR, per. comm.). The study reaches are located between river kilometers 301.7 and 293.6, 3 286.9 and 278.4, 262.8 and 253.8, and 238.5 and 230, where locations are measured in kilometers upstream from the confluence of the Green and Colorado Rivers. We refer to these t ~ reaches as the Cedar Ridge, Surprise Carryon, Joe Hutch, and Coal Creek study reaches, k ; respectively. Each study area is approximately 8 km in length and has a slightly different geomorphic setting determined largely by the dominant bedrock.at river level (Table 1). In the upstream parts of Desolation Canyon, the river's course is established in the lacustrine Green River formation. Deposited in the Paleocene Lake Uinta, this formation consists of alternating shales, carbonate mdstones and sandstones. This formation is highly susceptible to weathering, and the river has eroded a wide alluvial valley. There are few debris `-`~ fans, and the channel is shallow and wide. The channel banks are vertically-accreted floodplains, i .~ and terraces are comprised of fine sands and silts. These deposits are covered with dense mature :;; ~__ A-2