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reduced by 28% and 33%, respectively. The CROSS modeled data showed that in 6 of the 17 years natural flow in August and September was depleted by 50% or more. Description of Study Area and River Strata The study area encompasses the Yampa River from its confluence with the Green River at Echo Park in Dinosaur National Monument upstream to Craig, Colorado (Figure 2.5). This area of the Yampa River was stratified by Miller et al. (1982) into 8 strata based upon geomorphology, gradient, tributary input, etc. These same strata were used in our study area. Strata 1 through 4 were located downstream of Cross Mountain Canyon and the remaining strata were located between Cross Mountain Canyon and Craig, Colorado. Strata 5 and 7 were not included in the study because they were relatively short river reaches. Brief descriptions of the strata sampled in this study are found below and in Table 2.1. Stratum 1 extends upstream from RM 0 at the Green River confluence to RM 20 at Harding Hole. The river is bedrock confined in the canyon channel with frequent hydraulic controls provided by debris fans from local side channels. Substrate ranged from sand and cobble to large colluvium depending on local sources and hydraulic conditions (i.e. smaller substrate above Warm Springs Rapid and larger substrate below). Wider areas in the canyon have provided opportunities for formation of Pleistocene Era alluvial terraces. Stratum 2 extends from RM 20 upstream to RM 45 in the upper Yampa Canyon and contains the greatest slope of all strata studied. This reach is less sinuous than Stratum 1. The higher gradient of the Yampa River is masked somewhat by the backwaters formed by tributary debris fans responsible for the many steep rapids in this reach. Substrate is larger consisting of cobble and small boulder as well as large colluvium. Stratum 3 extends from RM 45 upstream to the confluence of the Little Snake. This is a lower gradient reach in a wider alluvial valley dominated by sand transported from the Little Snake River. 23