The Gila River and its main tributary, the Salt River, would both be perennial streams carrying large volumes of water, but irrigation and municipal water diversions turn both into usually dry rivers. Below Phoenix to the Colorado River, the Gila is usually either a trickle or completely dry, as is also the lower Salt from Granite Reef Diversion Dam downstream to the Gila, but both rivers can carry large volumes of water following rainfall. A long time ago, the Gila River was navigable by large riverboats from its mouth to near Phoenix, and by smaller craft from Phoenix nearly to the Arizona - New Mexico border. The width varied from with a depth of . The natural discharge of the river was roughly per year, with a mean flow of about at the mouth. The river's present discharge near the mouth is less than per year, with an average flow of just . Overdraft from the Gila River system has prompted the construction of the Central Arizona Project, which delivers some annually from the Colorado River to supplement water supplies in the basin.