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Show Chapter 2 The River and Its Tributaries The Colorado River rises on the western slope of the Continental Divide in north central Colorado and flows southwest nearly 1,400 miles to empty into the Gulf of California. All the principal trib- utaries of the upper basin rise in the bordering mountain ranges. From the high Rockies where the main stream and its tributaries are fed by melt- ing snows, the Colorado runs through the deep canyons in the central portion of the basin to the gentle slopes of the southern deserts, draining an area of 242,000 square miles in the United States. Its tributaries extend into the seven Western States of Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming. The river flows in deep canyons through Colo- rado into Utah, where it joins its major tributary, the Green River-a stream longer than the Colo- rado above their confluence-which gathers the waters of the entire northern end of the Colorado watershed. The main stem continues southwest through a series of canyons to Glen Canyon, Utah, a short distance above the Utah-Arizona boundary, where it is joined by the San Juan. This tributary rises in the mountain slopes of southwestern Colorado and flows westward through the mountains and plateaus of Colorado and north- ern New Mexico and Arizona to meet the Colorado in Glen Canyon. Immediately below the State boundary is Lee Ferry, Ariz., which serves as the dividing point between the upper basin and the lower basin.1 Beyond this point, the Colorado 1 The Colorado River Compact provides principally for a division of the available water of the Colorado system between the "upper basin" and the "lower basin" at Lee Ferry, which is defined as a point on the Colorado River one mile below the mouth of the Paria River. The near- est stream gage to this point on the Colorado is at Lees winds through the Grand Canyon. The major tributaries in this area are the Virgin River (which at one time flowed directly into the Colorado, but now enters Lake Mead from the north) and the Little Colorado, which joins the main stem from the south about 50 miles downstream from Lee Ferry. Emerging from the canyon country at the south- east comer of Nevada, the Colorado flows through broad valleys bordered by mesas. The Gila River, the main tributary in this region, rises in south- west New Mexico and drains most of southern Arizona before joining the Colorado River near Yuma. The Colorado then flows across the border to Mexico and through its delta-a distance of 110 miles-to the Gulf of California. Floods in the upper and lower basins are derived from two different sources. The principal cause in the upper basin is the melting of snow in the spring, augmented in some cases by runoff from rainfall. Except for small local areas, floods on tributaries in the lower basin are usually the result of rainfall from general winter storms or local sum- mer storms of high intensity. Most runoff in the Colorado River Basin originates in the upper basin; stream flow records indicate that 76 percent of the runoff at Yuma originates in that part of the basin above the mouth of the Green River. During the period 1897 to 1943, the average annual natural flow of the Colorado was estimated to be 16,270,000 acre-feet at Lee Ferry. Up- stream depletion in the same period, due to irriga- Ferry which lies just above the mouth of the Paria River. Lee Ferry, a few miles below the Arizona-Utah boundary, is a natural point of demarcation. Here all the waters of the entire upper system, including the Paria River and the return flow from irrigation diversions, converge to form a single stream. 911610-51- -25 363 |
