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DRAFT February25, 1998 Allred presented evidence of channel narrowing (from interpretation of aerial photography) between 1930 and 1938 and from 1962 to the present for the Green River channel near Green River, Utah. Channel narrowing at the U.S. Geological Survey gauge cross section occurred during 1930 - 1938, rapid accretion occurred from 1957 to 1962, and further narrowing after 1962. Allred's research at this site indicates that channel narrowing occurs in response to hydrologic changes and vegetation (typically saltcedar) invades and stabilizes newly formed inset floodplain deposits. Characteristics of Debris-Fan Affected Canyon Reaches Schmidt and Rubin (1995) proposed that the debris fan-eddy complex is the fundamental geomorphic channel unit in canyons with abundant debris fans. This assemblage includes, in a downstream direction, a river segment of ponded flow that has a low downstream velocity and that is controlled by a downstream channel constriction, a constricting debris fan, eddies and eddy bars, and a gravel bar. This assemblage occurs at nearly every tributary mouth where debris fans constrict the river. The extent of each channel element varies from site to site. Longitudinal profile, channel geometry, and the occurrence of rapids in the canyons of the eastern Uinta Mountains (Lodore Canyon in reach 1, Whirlpool Canyon and Split Mountain Canyon in reach 2) are each strongly influenced by tributary-fan frequency. Bankfull channel width-to-depth ratio is smaller and gradient is steepest in the reaches with highest fan frequency; and all rapids are caused by debris fans or the gravel bars below debris fans that are composed of reworked debris-fan material. Expansion gravel bars are the other element of coarse-grained alluvial deposits in debris-fan dominated canyons. These bars are located in the expansion downstream from debris-fan-created eddies where uniform downstream flow resumes. The lithology of gravels in these bars indicates that their source is the debris fan immediately upstream and its associated tributary basin (Grams and Schmidt, 1996). Debris fans in Desolation Canyon (reach 3) are predominantly large and of low elevation. Only the small portion of the fan that is active delivers sediment that restricts flow and causes rapids and eddies, while the main portion of the debris fan is so large that it acts more like a meander bend as the river flows around the fan. As discharge increases the total area of eddies increases, however, at bankfull discharge many of the relatively small active portions of the fan are overtopped, and eddy frequency decreases. Although very large debris fans dominate the river corridor, fine-grained alluvium is the most abundant bank material at low discharge, and vegetated fine-grained alluvium dominates at higher flows (Schmidt, Orchard, and Holman, 1996). • 4