Riparian ecosystems in many parts of the Southwest have become degraded and some have disappeared altogether (Smith 1977; Warner 1979b). Degradation of southwestern riparian habitats is well documented. Examples include areas along Dry Creek (McBride and Strahan 1981), the Carmel River (Kandolf and Curry 1984) and the lower San Lorenzo River (Griggs 1984), California; the San Pedro River (Zimmerman 1969), Sonoita Creek (Glinski 1977), the Gila River (Minckley and Clark 1984) and the Santa Cruz River (Betancourt and Turner forthcoming), Arizona; and the Rio Puerco, New Mexico (Elliott 1979). This loss of habitat is due in large part to the effects of human activities (U.S. Council on Environmental Quality 1978; Warner 1979).
The decline of native riparian habitats has affected many wildlife species. Although riparian habitats comprise only a small percent of the total land area of Arizona, they are inordinately important to wildlife (Johnson and Jones 1977; Ohmart and Anderson 1974, 1982, 1988). Studies by Gaines (1977) in California; Carothers et al. (1974), Anderson and Ohmart (1984), Rice et al. (1984) in Arizona; and Hubbard (1971), Hink and Ohmart (1984) in New Mexico confirm the high value of riparian habitat to wildlife. The importance of these habitats to wildlife, combined with their rapid decline, has made the protection and restoration of riparian habitat a prominent conservation issue.
As the human population in the Southwest continues to grow, urbanization will increasingly affect riparian ecosystems, often contributing to their degradation. Riparian areas are directly degraded by development when they are removed to make way for construction and flood control projects. Equally important are the many indirect effects of urbanization on riparian ecosystems, notably through groundwater decline, alteration of natural flood flows, and changes in the way sediment and water run off the land. For example, nearby development can significantly degrade the condition of riparian habitat by reducing the amount of water available for plant use, as flood control projects alter streamflow and groundwater pumping drops the saturated zone of the soil profile below the shallow root systems of phreatophytes.
However, the indirect effects of development on riparian habitat have not yet been studied extensively. Judd et al. (1971) and Stromberg et al.(1992) studied the effects of groundwater decline on mesquite bosques, but these studies are rare exceptions. Increasing our understanding of how development affects the condition of riparian habitats will significantly improve our ability to reduce the impacts of development on healthy riparian areas, as well as to improve the effectiveness of restoration strategies for riparian habitats that are already degraded.
Although many riparian restoration projects have successfully improved riparian habitat conditions, many other restoration efforts have been ineffective. This is often due to a lack of understanding of the causes of degradation and how land use changes can directly and indirectly affect the condition of these important habitats. Improving the effectiveness of riparian conservation strategies can only be accomplished by increasing our understanding of how riparian habitats become degraded.