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An Overview / 5 riparian zone extending for hundreds of meters from the stream channel. Change in elevation (with its concomitant effects on frequency of inundation) appears to be the most significant factor associated with the distribution of . riparian plant communities and their species composition (Szaro 1989). In the Southwest, low-lying (less than 1,000 m in elevation) riparian plant commu- nities are often dominated by velvet mesquite (Prosopis velutina) or Goodding willow (Salix gooddingii). Mid-elevation (less than 1,500 m) riparian commu- nities are often dominated by species such as sacaton (Sporobolus wrightii) or Arizona sycamore (Platanus wrightii), while higher-elevation communities may be dominated by blue spruce (Picea pungens). Within an elevation range, other factors contribute strongly to determining the composition of the riparian plant community, including fluvial geomor- phic processes (Osterkamp 1978; McBride and Strahan 1984), with elevation above the stream channel being a major cofactor (Nixon et al. 1977; Hupp 1982; Hupp and Osterkamp 1985). Stream bearing, stream gradient, flow regime, and geology also affect the characteristics of riparian plant commu- nities at a local level (Zimmerman 1969; Szaro 1989). Human-related activities have contributed to the decline of these valu- able ecosystems and their associated rivers. Many of the cienegas, mesquite bosques, and forests of cottonwoods and willows that once dominated por- tions of southeastern Arizona have fallen under the weight of impacts from agriculture, groundwater pumping, introduced exotics, livestock grazing, fu- elwood harvesting, and flood control measures, among others (Bahre 1991). Beginning in the 1860s, cottonwood and willow forests along the lower Colo- rado River were extensively cut to provide fuel for steamer travel (Ohmart et al. 1977). With the improvement of pumping technology in the twentieth century, the pumping of groundwater to meet the thirst of our major urban centers has led to groundwater decline, which, in turn, has contributed to the deterioration of riparian ecosystems in many areas, including the disappear- ance of mesquite bosques along the Santa Cruz River near Tucson and the Gila River near the Casa Grande National Monument (Betancourt and Turner n.d.). The construction of dams has led to the demise of stands of cottonwoods along the lower Salt River in central Arizona (Fenner et al. 1985) and the lower Verde River (McNatt et al. 1980) near Phoenix, Arizona (fig. 1.1). The creation of in-stream reservoirs, the diversion of water for cropland irrigation and electrical production, the demands of an increasing human population, and a variety of other human-related impacts have greatly altered the flow regime and channel dimensions of portions of the Platte River system in Colo- rado, Wyoming, and Nebraska (Minnich 1978). Overuse by livestock has also