Salt River valley, irrigated region around the lower course of the Salt River, which rises in mountain streams near the Mogollon Rim of the Mogollon Plateau and flows southwest to join the Gila River in S central Arizona. Phoenix is the main city of the latter Ariz. region. Native Americans used the Salt River for irrigation many centuries ago. In the 19th cent., American settlers began irrigated farming in the valley, and the Mormons used some of the old Native American canals at Mesa, Ariz. The Salt River project, the first large irrigation scheme undertaken under the Federal Reclamation Act of 1902, is one of the most economically successful projects in North America. It began in 1903 when construction started on the Roosevelt Dam in a canyon E of Phoenix. The dam, forming Roosevelt Lake behind it, impounds enough water to irrigate fields for about two years even if no rain falls. Other dams were built between 1922 and 1946 to supply water and power. The region is a rich producer of citrus fruits, lettuce, melons, and other crops. A major factor in the Salt River project's success is the long season without frost; the climate also makes this area an attractive winter resort.