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2. PHYSICAL SETTING The Yampa River located in northwestern Colorado and southern Wyoming is a major tributary to the Green River and has a drainage area of about 8,000 miles2. The little Snake River, a major tributary to the Yampa River, delivers about 27 percent of the water and neariy 70 percent of the sediment load (Andrews, 1978). The Yampa Canyon reach of the river extends from Deeriodge Park (RM 45) to the confluence with the Green River (AM 0). The upper 25 miles of the canyon (RM 20-45) are bounded by the Morgan Formation which is comprised of interbedded limestones, sandstones and shales. The canyon is relatively wide and asymmetrical and the average slope of the river is about 0.3 percent. The lower 20 miles of the canyon are bounded by the Weber Sandstone. The lower canyon is narrow, deeply incised and very sinuous (sinuosity is 2.6). The average slope of the river in the lower reach is 0.1 percent. Bars are located within the lower canyon where stream power drops below critical thresholds necessary to maintain transport of cobbles and boulders during the high magnitude flows (Graf, 1979; Webb et aI., 1988). Stream power minima occur in local reaches that are located immediately upstream of sharp canyon bends and contractions and downstream of canyon expansions. Canyon morphology therefore controls the distribution of the bars within the canyon (Usle, 1986; O'Connor et aI., 1986; Harvey et al., 1993). The majority of the annual sediment load transported through the Yampa Canyon is derived from upstream of the canyon sources (Elliott et aI., 1984; Resource Consultants, Inc., 1991). Wash load comprises about 35 percent of the total load. Tributary derived coarse sediments within the canyon are the primary sources of the bar forming materials. Debris flows in the tributaries are generated by summer thunderstorms, that generally occur after the runoff season in the Yampa River has been completed, and they deliver cobble to boulder size materials to the Yampa River in the upper 25 miles of the canyon. The out-of-phase supply and transport of the bar forming materials in the lower Yampa Canyon is an important component of the bar buDding process. Also important to the observed behavior and sedimentology of the bars in the lower canyon are two south bank tributaries, Johnson Canyon (RM 21) and Bull Canyon (RM 20.3). The drainage basins of both tributaries contain outcrops of erodible Park City and Moenkopi Formations, both of which are capable of supplying considerable volumes of fine grained (silts and clays) sediment as well as gypsum and carbonates to the lower canyon (Untermann and Untermann, 1954). The remainder of the tributary drainage basins in the upper 25 miles of the canyon do not contain outcrops of either formation, or any others capable of producing salts or fine sediments. The fine grained sediments derived from the local tributaries are probably a major source for the mud drapes that are deposited on the bars during recessional f1ows'(Harvey et aI., 1993). 2.1 Resource Consultants & Engineers, Inc.