8255 - Laserfiche WebLink

Home Browse Search

Grams and Schmidt 17 frequency of fan-eddy complexes ranges from 1.6 per kIn in Lodore Canyon to 2.3 per kIn in Whirlpool Canyon. In Lodore Canyon, 59 percent of all fine-grained alluvium and 74 percent of all gravel are stored in the fan-eddy complexes (Table 3). Eddy-deposited sand bars contain about 42 percent by area of all fine-grained alluvium in the canyon reaches and less than one percent in the meandering reaches (Table 3). Comparison of gravel and boulder lithologies between debris fans and expansion gravel bars within individual fan-eddy complexes indicates that the debris fan is the primary source of the material for the gravel bar immediately downstream. Debris fan boulders are composed of lithologies from the contributing tributary basin, dominated by the most resistant lithologies found in that basin. For example, the debris fan at Rkm 563.6 (Fig. 4) is dominated by Uinta Mountain Group quartzite (70 percent) and shale (25 percent), Lodore Formation sandstone (5 percent), and rare cobbles of Madison Limestone. The expansion gravel bar immediately downstream has a similar ratio of lithologies with the more resistant formations more prevalent than the less-resistant shale (Table 4). The outcrop of Madison Limestone is much larger and the outcrop of Uinta Mountain Group shale is much reduced 1.5 kIn further downstream in the tributary basin that contributes to the debris fan at Triplet Falls; these differences are reflected in the ... composition of the downstream expansion gravel bar (Table 4). The sandstone and limestone cobbles in these bars must be derived entirely from the immediately adjacent upstream basins, because they are the upstream-most tributary basins in Lodore Canyon to contain outcrops of these rocks. 6.3 Gradient The stair-stepped 10ngitudinalprof1le of the Green River (Fig. 6) is related to variations in lithology and debris fan occurrence. The average slope of the Green River is 0.0021 in the canyon reaches and 0.0008 in the meandering reaches (Table 5). The bedrock resistance along each reach at river level is also indicated on Figure 6. The rocks, of highest resistance class are coincident with the reaches of steepest average gradient.