Watershed
The drainage basin or watershed of the Colorado River encompasses 246,000 square miles (640,000 km2) of southwestern North America, making it the seventh largest on the continent. About 238,600 square miles (618,000 km2), or 97.0% of the watershed, is in the United States. Parts of the U.S. states of Arizona, California, Colorado, New Mexico, Nevada, Utah, and Wyoming and the Mexican states of Baja California and Sonora are in the Colorado River watershed. Most of the basin is arid – defined by the Sonoran and Mojave Deserts and the expanse of the Colorado Plateau – although significant expanses of forest are found in the Rocky Mountains, the Kaibab, Aquarius, and Markagunt Plateaus in southern Utah and northern Arizona, the Mogollon Rim through central Arizona, and other smaller mountain ranges and sky islands. Elevations range from sea level at the Gulf of California to over 13,000 feet (4,000 m) in the mountains of Colorado and western Wyoming, with an average of 5,500 feet (1,700 m) across the entire basin.
Climate varies widely across the watershed. Mean monthly high temperatures range from 77.5 to 105 °F (25 to 41 °C) and lows from 48 to 10.5 °F (9 to -12 °C), with extremes of up to 120 °F (49 °C) in the desert regions of the watershed to −50 °F (−46 °C) in Rocky Mountain winter storms. Annual precipitation averages 6.5 in (170 mm), ranging from over 40 in (1,000 mm) in some areas of the Rockies to just 0.6 in (15 mm) along the Mexican reach of the river. The upper basin generally receives snow and rain during the winter and early spring, while the lower basin is characterized by intense but infrequent summer thunderstorms brought on by the North American Monsoon.
As of 2010, approximately 12.7 million people lived in the Colorado River basin. Phoenix in Arizona and Las Vegas in Nevada are the largest metropolitan areas in the watershed. Population densities are also high along the lower Colorado River below Davis Dam, which includes Bullhead City, Lake Havasu City and Yuma. Other significant population centers in the basin include Tucson, Arizona, St. George, Utah and Grand Junction, Colorado. Colorado River basin states are among the fastest growing in the U.S.; the population of Nevada alone increased by about 66 percent between 1990 and 2000 as Arizona grew by some 40 percent.
The Colorado River basin shares drainage boundaries with many other major watersheds of North America. The Continental Divide of the Americas forms a large portion of the eastern boundary of the watershed, separating it from the basins of the Yellowstone River and Platte River, tributaries of the Missouri River, on the northeast, and from the headwaters of the Arkansas River on the east. Both the Missouri and Arkansas Rivers are part of the Mississippi River system. Further south, the Colorado River basin borders on the Rio Grande drainage, which along with the Mississippi flows to the Gulf of Mexico, as well as a series of endorheic (closed) drainage basins in southwestern New Mexico and extreme southeastern Arizona.
For a short stretch, the Colorado watershed meets the drainage basin of the Snake River, a tributary of the Columbia River, in the Wind River Range of western Wyoming. Southwest of there, the northern divide of the Colorado watershed skirts the edge of the Great Basin, bordering on the closed drainage basins of the Great Salt Lake and Sevier River in central Utah, and other closed basins in southern Utah and Nevada. To the west in California, the Colorado River watershed borders on those of small closed basins in the Mojave Desert, the largest of which is the Salton Sea drainage north of the Colorado River Delta. On the south, the watersheds of the Sonoyta, Concepción, and Yaqui Rivers, all of which drain to the Gulf of California, border that of the Colorado.
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