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LJ <br />2.3 Basin Characteristics <br />The Yampa River is a tributary to the Green River, which is in turn a tributary to the Colorado <br />River. The Yampa River joins the Green River downstream of the study reach in Dinosaur <br />National Monument. The Yampa has its headwaters in the Elkhead Mountains to the north, in <br />the Park Range to the east, and in the White River Plateau to the south and west. The watershed <br />area at the downstream limit of the study at Deerlodge Park in the Dinosaur National Monument <br />is 7660 square miles, including tributary area in Wyoming. Approximately half of that area <br />(3810 square miles) is in the Little Snake River watershed. The Little Snake River joins the <br />Yampa River about 4 miles upstream of the downstream study limit. The elevation within the <br />watershed ranges from 12,500 feet above mean sea level at the headwaters to 5600 feet at the <br />downstream limit of the study. The average slope within the study reach is approximately 5 feet <br />per mile, except at Cross Mountain Canyon where the slope is approximately 60 feet per mile <br />within the three -mile canyon. <br />The climate in the Yampa River basin is classified as semi -arid and is characterized by low <br />precipitation, large daily temperature range, low humidity, and frequent wind. The primary <br />sources of moisture are air masses originating in the Pacific Ocean from October through April, <br />and air masses from the Gulf of Mexico during the late spring and summer. Normal annual <br />precipitation ranges from approximately 14 inches downstream of Craig to an estimated 80 inches <br />in the higher elevation of the Park Range. Most of the annual precipitation in the high elevations <br />occurs as snow from October to April. Rainfall can occur as convective -type thunderstorms <br />during the summer months generally over the lower elevations of the basin. The Yampa River <br />basin, from its headwaters to Dinosaur National Monument, is shown in Figure 3. <br />2.4 Summary of Drainage Areas <br />The drainage areas for selected hydrologic points were determined through planimetering. The <br />results of the planimetering were then compared to known drainage area values at USGS gage <br />locations. The drainage areas at the selected hydrologic points are shown in Table 2. <br />5 <br />