Covering more than 58,000 square miles, the watershed of the Gila River includes most of Arizona south of the Mogollon Rim. The river itself rises on the west side of the Continental Divide in western New Mexico; flowing westward into Arizona, it receives the waters of the San Carlos River within the San Carlos Reservoir, northwest of Mount Turnbull. Continuing westward, the Gila takes in water from the San Pedro River (flowing north from Mexico), flows south of Chandler, Arizona, and then angles northwestward, passing between South Mountain and the Sierra Estrella, just south of Phoenix. In southwest Metro Phoenix, it merges with the Salt River; the latter courses westward through the heart of the city after gathering flow from numerous tributaries that drop from the edge of the Mogollon Rim (among these are the Verde, White and Black Rivers and Canyon Creek).
Just west of its junction with the Salt, the Gila receives the waters of the Agua Fria, which rises east of Prescott and flows southward through western Metro Phoenix; en route, the Agua Fria is dammed to form Lake Pleasant. Below the mouth of the Agua Fria River, the Gila dips southward, passing Gila Bend, and then enters Painted Rock Reservoir; beyond that reservoir, it continues westward through low desert terrain, joining the Colorado River at Yuma, Arizona. At its mouth, the Gila has flowed 650 miles and dropped 5500 feet from its source in the Gila National Forest of New Mexico.
As one might expect in the Sonoran Desert, flow volumes through the Gila River and its tributaries are highly seasonal, fed primarily by the monsoon rains of summer, by mountain springs and by winter snows across the Mogollon Rim. Reservoirs along the Gila, while providing irrigation for agriculture in its valley, have all but eliminated its contribution to the Colorado, further ensuring that America's great western river will never again reach the sea (unless, of course, human civilization fails and our massive dams succumb to the Colorado's untamed torrent).
