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0 DRAFr February 25, 1998 Yampa River confluence as the Yampa contributes more than 10 percent of the average annual flow for this reach. Reach 3, Green River upstream of Colorado River confluence to White/Duchesne Rivers, is about 394 km in length. It is begins at the junction of the Green River with the White and Duchesne Rivers as both tributaries contribute more than 10 percent of the average annual flow of the Green River. Geomorphic Reach Description Longitudinal Profde The Green River encounters geologic formations of differing erosional resistance because the river crosses regional geologic structures perpendicularly. Formations vary between hard, cliff- forming rocks such as the metasediments of the Uinta Mountain Group exposed in the Canyon of Lodore which dominate reach 1 and erodible, slope-forming rocks such as the Mancos Shale exposed near the town of Green River, Utah in reach 3. Alluvial valley width and bankfull channel width alternate between narrow and wide segments which are controlled by the stratigraphic arrangement of resistant and erodible bedrock formations that in turn result from the region's geologic history. The alluvial valley ( the Uinta Basin) in reach 2 of the Green River is typically more than 2 km wide where the river crosses erodible formations, but the valley may only be 200 in wide where resistant formations are crossed. Channel width is greatest where the alluvial valley is wide, but the difference between wide and narrow channels is much less than is the difference between wide and narrow valleys. Bankfull channel width may be nearly 300 in in the widest alluvial valleys, but is less than 75 in in some narrow canyons. The longitudinal profile of most large rivers is generally concave up (Leopold and others, 1964), with larger bed material and steeper gradients in the headwaters and lower gradients and smaller grain size downstream. The longitudinal profile of the Green River downstream from Flaming Gorge includes steep and flat segments and these segments do not systematically decrease in gradient downstream. Narrow alluvial valleys may have flat or steep gradients. For example, channel gradient in Labyrinth and Stillwater Canyons in reach 3 is very low although the channel is confined in a narrow alluvial valley. Low-gradient reaches of the river have sandy substrate, but the median grain size of exposed bars exceeds 10 mm in steep gradient segments (Schmidt, 1996). Channel Planforms The Green River consists of a series of linked segments of three channel planform types without a systematic downstream change from one planform to the next. The channel planforms are restricted meanders, fixed meanders, and canyons with abundant debris fans. Restricted meanders only occur where the channel crosses erodible shale and the alluvial valley is wider than 1.5 km, such as the Uinta Basin of reach 2 near Vernal, Utah. Fixed meanders are found in Stillwater 2