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I I then southwesterly to its confluence with the Yampa River at Deerlodge Park (Gregory, 1989). Drainage area is approximately 3,730 square miles, and the Geological Survey estimates approximately 21,000 acres of irrigation occurs above the Lily gage. I I I Runoff patterns are similar to those in the Yampa River sub-basin, with snowmelt runoff generally occurring between mid-May and mid-June. Flow typically is low or zero during the months of August and September; thunderstorms occurring in the late summer months are the main source of flow after snowmelt declines. Yampa River at Deerlodge Park. CO I I I I Deerlodge Park is a broad alluvial valley where the Little Snake River joins the Yampa River above Yampa River Canyon. The Yampa River drainage area at this point is approximately 7,6UJ square miles. Although the two sub-basins above Deerlodge Park are approximately equal in size (3,410 square miles at Maybell and 3,730 square miles at Lily), they differ in the delivery of water and sediment to Deerlodge Park and the lower 45 miles of the Yampa River. The area above Maybell contributes 73 percent of the annual streamflow and 27 percent of the annual sediment load at Deerlodge Park. In contrast, the Little Snake River above Lily contributes only 27 percent of the annual streamflow but nearly 69 percent of the annual sediment load (Andrews, 1978). Immediately below Deerlodge Park, the Yampa River enters Yampa Canyon and flows 45 miles at a steeper gradient to its confluence with the Green River. Within Yampa Canyon, the river is entrenched into Permian and Pennsylvanian sandstones and limestones of the Uinta Mountain uplift; little of the transported sediment is derived from these formations (Elliott, et. aI., 1984) I I I I I Green River near Jensen. UT The Yampa River joins the Green River at Echo Park, located approximately 65 miles below Flaming Gorge Dam. The Green River flows westerly though the confmes of Whirlpool Canyon and after crossing the Island Park fault, the slope decreases and the channel widens. Large alluvial islands have formed in this area as the sediment transport capacity of the river has decreased. After crossing the Island Park fault again, the Green River enters Split Mountain anticline, and the slope steepens. The Green flows across the northern abutment of the anticline before bending sharply to the south and cutting through the same beds again in reverse order. The Jensen gage is located a few miles below Split Mountain, and approximately 43 miles below the Yampa River confluence. The Green River drainage area at Jensen is approximately 29,660 square miles, of which approximately 4,260 square miles is noncontributing. Since October 1962, streamflow has been regulated by Flaming Gorge Reservoir located 108 miles upstream of the gage. 4.3 Discharg:e Record Extension at Deerlodge Park I I I I t The period of record for the Yampa River at Deerlodge Park is relatively short, beginning in April, 1982 and ending in September, 1989. The Deerlodge Park gage is at the beginning of Yampa Canyon, and thus provides a good measure of flow conditions for the lower 45 miles of the Yampa River. To extend the period of record available at Deerlodge Park, daily values at the two upstream gages (Little Snake River near Lily and Yampa River near Maybell) were used to estimate the flow at Deerlodge Park. The sum of daily values at the upstream gages is highly correlated with the daily flow at Deerlodge Park (see figure 4.2). Examining the residuals between the predicted and observed flow at Deerlodge Park revealed the regression equation shown in figure 4.2 is no better than using the sum of daily values at the Lily and Maybell gages. Thus, mean daily flow at Deerlodge Park for water years 1922 through 1988 were estimated as the sum of daily values at the Lily and Maybell gages. To retain consistency, the estimated flow during the 1982 through 1988 period is used rather than the actual gage record. 4-3