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r r II. Description of the Green River Study Reach At the start of the study reach, a large vegetated sand and cobble bar has evolved downstream of the sharp bend in the river. A close examination of the river channel morphology reveals this bar to be located just upstream of a constriction in the canyon walls. The river course is pinched between the mancos shale escarpment on the south and a small bedrock outcrop on the north. The original bar may have evolved as cobbles and boulders deposited in the backwater upstream of the constriction during extreme floods. The coarse bed material would have formed a bar which was subsequently covered by sand (both fluvial and windborn) and vegetation. Such extreme flooding may have been the result of glacial melt. The flow now splits around this vegetated bar (referred to as the razorback sucker spawning bar or just 'spawning bar') and is actively eroding the talus slopes on each side of the river. The surface of the vegetated portion of the lower bar is approximately 8 ft higher than the river bed indicating that some channel incision may have occurred. There are several old flow paths across north (right) side of the bar; evidence that this part of the bar has been reworked by historical flows. The highest portion of the bar has dense tamarisks and upland vegetation such as sage and prickly pear cactus. Upstream of the bar from cross section E-1 to E-3, the river channel is confined by vegetated banks and has a cobble substrate. This substrate is covered by sand at various flows depending on the incoming sediment load to the reach, the sand on the bed and the river discharge. The sand bedforms appear primarily as sand dunes or sand strings. From cross section E-3 to E-5.1 the river splits around the spawning bar. During low flow a slight riffle appears at the start of the bar just after the adverse slope. At high flow, this riffle disappears. Approximately three quarters of the river discharge is conveyed by the left (south) channel during moderate to high flows around the spawning bar with a thalweg located along the left bank at the bedrock contact. The left channel bed material consists of bedrock along the left bank, and boulders and cobbles across the rest of the channel. Sand may cover a portion or most of the bed at different flow stages. The right channel is also primarily cobbles but appears to have a greater percentage of the bed covered by sand. The right channel bed appears to more actively adjust to changing sediment load conditions than the left channel where the sand sized sediment seems remains in transport. 3