Floodplain Information Report Volume 3 Gunnison River from Delta to the Confluence with the Colorado River

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2.3 Basin Characteristics The Gunnison River is a major tributary of the Colorado River. It has its origination at Almont, Colorado, at the confluence of the Taylor River and East River in Gunnison County. Those rivers have their headwaters in the Sawatch Range to the east and in the Elk Mountains to the north. Additional tributaries originate from the east in the Sawatch Range and from the south. The Uncompahgre River, which joins the Gunnison at Delta, is a major tributary with its headwaters in the northern slopes of the San Juan Mountains. The watershed area at the downstream limit of the study at Grand Junction is 8010 square miles. The elevation within the watershed ranges from over 14,000 feet above Mean Sea Level at the headwaters to 4550 feet at Grand Junction. The average slope within the study reach is approximately 9 feet per mile with a maximum of 13 feet per mile in the upper reach and a minimum of 6 feet per mile at Grand Junction. The climate in the Gunnison River basin is classified as semi -arid and is characterized by low precipitation, 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 8 inches near Delta to more than 40 inches in the mountain regions. Occurrence of precipitations is highly variable, with a large part of the total concentrated in several months. The annual precipitation. occurs as snow from October to April. Rainfall can occur as convective -type thunderstorms during the late spring and summer months generally over the lower elevations of the basin, making August, September and October normally the wettest months of the year. The Gunnison River basin, from its headwaters to the confluence with the Colorado River, is shown in Figure 3. 2.4 SummM of Drainage Areas The drainage areas for selected hydrologic points were determined through planimetering. The results of the planimetering were then compared to known drainage area values at USGS gage locations. The drainage areas at these selected hydrologic points are shown in Table 2.