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e · Relative variation in bed material size over the course of the high flow season. · Relative variation in bed material size within the study reaches. · Evaluation of the change in bed material size since 1963. The results of the bed material size analysis are as follows: The mean Dso bed material size in each study reach is shown in Table 1. The Green River bed material is predominantly medium sand except where impacted by side canyon floods or talus slopes. The Ouray mean bed material size is 0.34 mm and the Canyonlands study reach mean bed material size is 0.27 mm. There is also some variation in sediment size across the channel cross section. The decrease in the sediment size follows a normal variation in the downstream direction (Williams and Wolman, 1984). The Canyonlands reach has a slightly smaller silt and clay percentage than Ouray. There are almost no silts and clays in the bed material of the river. There were some fines in portions of the cross section where velocities were low in the spring and fall. Comparably, the bank and island sediment samples contained significant percentages of silt and clay, indicating deposition in low velocity environments. e All the cross sections in the Ouray reach were sand bed except for cross section WYB- 12, which has some cobbles near the left ban\<. There are a number of cross sections in the Ouray reach in which is flow against or very near bedrock on one side. These include OCW-3, SHB-9, WYB-12, WYB-13, LEB-24, JOB-28, JOB-29 and JOB-31. It is possible that portions of these sections could scour to bedrock during high flows. In Canyonlands, cross section BVB- 19 at Millard Canyon was located across the cobble, boulder bar created from the Millard Canyon floods. The Canyonlands cross sections which appear to be directly contacting bedrock are: MCB-l, VAB-6.5, UNB-ll (talus slope), ANB-14, QAB-l~ (talus slope), QAB-17, QAB- 18, BVB-19 (cobble riffle), BVB-20 and BVB-21. ' Channel bed forms in the two study reaches were essentially the same. Ripples and small dunes were observed during the base flow. At high flow, medium to large dunes were observed with the possibility of transition to plane bed between the dune structures at some cross sections. The dunes may have also separated due to limited bedload transport over hardpan or bedrock conditions. Scour to bedrock was observed at several cross sections. Response to High Flows During the passage of seasonal runoff, alluvial channels are scoured and filled rather quickly. The Green River bed material consists mostly of sand. During high flows the position of the thalweg may shift across the channel. In portions of the Green River where the channel has been relatively stable, the thalweg location at high flows is predictable and therefore the e 18