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<br />alluvial valley is inset into a shallow canyon within the Green River formation (Fig. 6). <br /> <br />. <br /> <br />The channel has typically migrated across a broad alluvial valley which is confined by <br /> <br />broadly spaced bedrock walls. However, the river is incised in!o entrenched meanders in <br /> <br />several reaches in the lower course of the river. The river channel itself is inset within a <br /> <br />series of alluvial terraces overgrown by dense stands of tamarisk (Tamarix sp.), Russian <br /> <br />olive (spp.), and a few cottonwoods (Populus sp.). Although the banks and flood plains <br /> <br />are predominantly made up offine-grained alluvium, the bed of the river is <br /> <br />predominantly gravel. <br /> <br />Approximately 33 kIn downstream from the study reach, the exposed bedrock at <br /> <br />river level changes from the Green River to the Uinta formation. The river, from Taylor <br /> <br />Draw Dam to the mouth, has its steepest slope of 0.0044 for the 3 kIn across the <br /> <br />transition. Downstream from this point the river flows for 20 kIn through entrenched <br /> <br />I <br />meanders eroded into the Unita formation. The distance between canyon walls increases <br /> <br />and in the last 30 kIn of its course, the river flows across a 2-kIn wide alluvial valley. <br /> <br />The slope nearing the confluence is 0.0005. <br /> <br />METHODS <br /> <br />Several methods were pursued in order to determine channel maintenance flows. <br /> <br />Flood Frequency Analysis <br /> <br />Seventy-seven years of annual peak floods measured at the Watson gage were <br /> <br />used to calculate the mean annual flood, and the return period of other apnual peak <br /> <br />9 <br />