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1 1 t 2.3 Basin Characteristics The Yampa River is a tributary to the Green River, which is in turn a tributary to the Colorado River. The Yampa River joins the Green River downstream of the study reach in Dinosaur National Monument. The Yampa has its headwaters in the Elkhead Mountains to the north, in the Park Range to the east, and in the White River Plateau to the south and west. The watershed area at the downstream limit of the study at Deerlodge Park in the Dinosaur National Monument is 7660 square miles, including tributary area in Wyoming. Approximately half of that area (3810 square miles) is in the Little Snake River watershed. The Little Snake River joins the Yampa River about 4 miles upstream of the downstream study limit. The elevation within the watershed ranges from 12,500 feet above mean sea level at the headwaters to 5600 feet at the downstream limit of the study. The average slope within the study reach is approximately 5 feet per mile, except at Cross Mountain Canyon where the slope is approximately 60 feet per mile within the three-mile canyon. The climate in the Yampa River basin is classified as semi-arid and is characterized by low precipitation, large daily temperature range, low humidity, and frequent wind. The primary sources of moisture are air masses originating in the Pacific Ocean from October through April, and air masses from the Gulf of Mexico during the late spring and summer. Normal annual precipitation ranges from approximately 14 inches downstream of Craig to an estimated 80 inches in the higher elevation of the Park Range. Most of the annual precipitation in the high elevations occurs as snow from October to April. Rainfall can occur as convective-type thunderstorms during the summer months generally over the lower elevations of the basin. The Yampa River basin, from its headwaters to Dinosaur National Monument, is shown in Figure 3. 2.4 Summary of Drainage Areas The drainage areas for selected hydrologic points were determined through planimetering. The 1 results of the planimetering were then compared to known drainage area values at USGS gage locations. The drainage areas at the selected hydrologic points are shown in Table 2. 1 1 t