In a 2004 study by The Nature Conservancy that compared current and historical data, the results showed that 35% of Arizona natural perennial flowing rivers had been altered or lost altogether as a result of dams, diversions, and groundwater pumping. http://azconservation.org/projects/water
The Verde River is one of Arizona’s last-remaining perennial river systems and contains the state’s longest stretch of continuous areas of riparian habitat, over 150 miles. The riparian habitat supports an amazing diversity of wildlife; 270 species of birds, 94 species of mammals, and 76 species of native amphibians and reptiles use the watershed at some point in their life cycles. It drains the central highlands, flowing into the Salt River and then into Horseshoe Dam north of Phoenix metropolitan area. Above the dam, the Verde is perennial to the headwaters with mostly natural flood flows, floodplains, and banks.
Arizona’s major perennial rivers historically supported lush riparian zones, which changed significantly with dams. The lower Colorado River below Hoover Dam is heavily channelized and dammed to serve towns and agriculture in Arizona, California, and Mexico. Development, unnatural shorelines, and invasive plants have replaced the natural riparian habitat. What was once the largest and most productive wetland in North America, the Colorado River Delta, is now a dry, cracked mudflat. No water. No riparian vegetation.
The Gila River drains southwestern New Mexico and central/southern Arizona. In Arizona, the Gila is pumped and dammed to store water for cities and agriculture. It rarely flows to the Colorado.
The Salt River collects water from the White Mountains, historically joining the Gila west of Phoenix. Five dams now constrain the Salt. At Granite Reef Dam, the entire Salt River is diverted to serve Salt River Project (SRP) customers; it no longer flows into the Gila.
The Santa Cruz River once had perennial flow past Tucson. Now, only a few sections are wet with treated effluent. Near Tucson, diversions and groundwater pumping by cities and agriculture changed a vital river into a dry wash deeply incised into the desert.
Despite decades of coordinated effort by Friends of the San Pedro River and others, last year, only 29% of the river was wet. Groundwater models predict that the San Pedro will be completely dry in a century.
Groundwater mining, surface water diversions, grazing, development, and population growth now threaten many more streams. The Nature Conservancy projects that seven more rivers will be threatened by 2050: the Agua Fria, Babocomari, San Pedro, upper Verde, and Little Colorado rivers and Lower Cienega and Arivaca creeks.