8255 - Laserfiche WebLink

Home Browse Search

Grams and Schmidt 11 streamflow. Cutbanks created by the Green River and tributary streams show that debris fans are poorly sorted, matrix. supported diamictons with angular to sub-rounded, pebble to boulder-sized clasts; the matrix is clay to coarse sand. Eddy bars form in zones of recirculating flow that occur downstream from channel constrictions. Figure 4 is a detailed map of a single fan-eddy complex in Lodore Canyon. Debris fans are the most common cause of channel constrictions, but talus slopes, bedrock outcrops, and gravel bars occasionally cause flow separation and eddy deposition. Although distinct separation and reattachment bars do occur in the canyons of the eastern Uinta Mountains, the eddy bar shown in Figure 4 is more typical. This bar extends the full length of the eddy, from separation to reattachment point; this distance is typically on the order of 1 to 3 channel widths. The area of fine-grained deposition is not limited to a secondary eddy cell, which is typical of separation bars, and the bar lacks a distinct return current channel, distinguishing it from a classic Grand Canyon reattachment bar. This lack of eddy bar differentiation within eddies resembles the tendency described by Schmidt and Rubin (1995) for separation and reattachment bars to appear 'merged' under unregulated flow conditions where sediment transport rates are high. Excavations in undifferentiated eddy bars reveal upstream and onshore migrating dunes and climbing ripples, confIrming deposition in a recirculating-flow environment. Reworking by wave .-action typically forms multiple river-parallel ridges and swales. Expansion bars. are generally about 1 to 3 channel widths in length and form in areas of flow expansion downstream from debris-fan created constrictions and associated expansions (Fig. 3 and Fig. 4). These deposits occur in association with eddy deposits and debris fans, but are located downstream from the eddy and in downstream-directed current. Expansion bars are always gravelor gravel-cored with a veneer of fme-grained alluvium. Channel-mariPn bars are narrow strips of alluvium that may be several channel- widths long and usually OCCll!"..in straight reaches of relatively uniform downstream flow.