Gila (hē´lə), river, 630 mi (1,014 km) long, rising in the mountains of W N.Mex. and flowing W across Ariz. to the Colorado River at Yuma, Ariz.; the San Francisco River is its main tributary. The Gila valley was occupied by the ancestors of the Pima and Papago ethnic groups, who farmed the region by irrigation. The ruins of their dwellings are preserved in Casa Grande Ruins and Gila Cliff Dwellings national monuments (see National Parks and Monuments, table). In the river's headwater region are Gila National Forest and the government-preserved "unimproved" Gila Wilderness Area. The Gila and its tributaries have many dams to provide flood control, hydroelectricity, and water for irrigation in the arid Southwest (see Salt River valley). Coolidge and Painted Rock dams are the largest dams on the Gila River. Gila monsters (poisonous reptiles) are numerous in the Gila valley.