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An Overview / 7 contributed to the decline of riparian areas throughout the western United States: examples include Rodero Creek, Nevada (Swanson et al. 1987); Hen- ry's Fork, Idaho (Platts et al. 1989); Tonto Creek, Arizona (Alford 1993); and Big Creek, Utah (Platts and Nelson 1989). In New Mexico, roughly 21,000 ha (54,000 acres) of riparian vegetation was removed between 1967 and 1971 in the flawed belief that removal of streamside plants would increase the amount of water supplied to down- stream users (Ohmart and Anderson 1986). Dense stands of nonnative salt- cedar (Tamarix chinensis) and native arrow-weed (Pluchea sericea) have re- placed cottonwoods as the dominant vegetation type along extensive reaches of the Colorado River (Ohmart et al. 1977). And along the Rio Grande in New Mexico, formerly extensive stands of Fremont cottonwoods are being replaced by saltcedar and Russian-olive (Elaegnus angustifolia) (Howe and Knopf 1991). In Mexico, the Santa Cruz River, which is the major supplier of potable water for the city of N ogales, Sonora, is contaminated by high levels of nitrates and coliform bacteria from the untreated waters of local industries (Castella- nos, pers. comm., 1994). Similar problems exist for the San Pedro River, which is contaminated by heavy metals, particularly from copper mining oper- ations near its headwaters in Cananea, Sonora (Moreno 1992). Of the major rivers of northern Mexico, the Colorado River may be the most affected by human insults. The effects of agricultural activities (in particular the use of large quantities of Colorado River water to irrigate vast farmlands such as those of the Imperial Valley, California; San Luis Rio Colorado, Sonora; and Mexicali, Baja California) have combined with urban and industrial contami- nation to dramatically alter the physical, chemical, and biological characteris- tics of the Colorado River delta (Sanchez 1991). For those who have just begun to study riparian ecosystems, background literature on riparian ecology and conservation issues by state is listed in the bibliography. These references will provide a solid foundation for understand- ing the ecology and value of riparian ecosystems and some of the conservation issues that pertain to them. Lessons Learned from Past Riparian Recovery Efforts Riparian revegetation - which involves planting trees, shrubs, forbs, or grasses to replace species that have been lost - is one of several recovery strategies that have been used to address the decline of riparian ecosystems in the western