tributary of the Gila River, east-central Arizona, U.S. The Salt River is formed at the confluence of the Black and White (White River) rivers on a plateau in eastern Gila county. It flows 200 miles (320 km) in a westerly direction and empties into the Gila River 15 miles (24 km) west-southwest of Phoenix. The Salt River and its main tributary, the Verde River, are part of the Colorado River drainage basin. The Salt River Irrigation Project includes the Theodore Roosevelt, Horse Mesa, Mormon Flat, and Stewart Mountain dams on the Salt River and Bartlett and Horseshoe dams on the Verde. In pre-Columbian times the broad Salt River valley was cultivated by the Hohokam (Hohokam culture), who constructed systems of irrigation canals.