The Colorado River runs through Grand Canyon National Park and forms a natural water border between Arizona and California.
The Colorado River is the primary river of the American Southwest, draining somewhere in the vicinity of 242,000 square miles of land, from the states of Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada and California. The Green River is the primary tributary of the Colorado River, and until 1921 the Colorado River did not technically begin until the Grand and Green Rivers joined together in Utah. In that year the Grand River was renamed as the Colorado River, at the request of the State of Colorado.
The headwaters of the Colorado River are located in Rocky Mountain National Park in Colorado. By the time the river enters the Grand Canyon, at Lee's Ferry, its altitude has fallen to 3,110 feet, dropping over one mile since its beginning.