Riparian ecosystems in the semiarid and arid southwestern United States support some of the highest avian species richness and density in temperate North America (Brand et al. 2008). However, these biodiverse ecosystems are threatened by a lack of legal protection coupled with human population growth and subsequent increasing demands for groundwater and surface water (Stromberg et al. 1996; Brand et al. 2011). Groundwater and stream withdrawals have contributed to the loss and alteration of wetland and riparian ecosystems throughout the Southwest (Levine and Stromberg 2001). Water control structures such as diversions, levees, and dams modify flood frequency and duration, surface flow rates, and sediment and nutrient transport (Scott et al. 2000; Levine and Stromberg 2001; Merritt et al. 2010). Additionally, channelization of rivers has led to channel incision, further altering the depth, timing, duration, and frequency of flooding (Scott et al. 2000; Merritt et al. 2010). Such changes in the hydrogeomorphology of rivers have led to the degradation of riparian habitats as the native biota has evolved to cope with the dynamic flows of rivers (Stromberg et al. 1996; Scott et al. 2000; Merritt et al. 2010). These hydrologic and geomorphic changes have impacted vegetation communities of southwestern riparian systems. For example, the recruitment of native plants, like Goodding's willow (Salix gooddingii), Rio Grande cottonwood (Populus deltoides ssp. wislizeni), and coyote willow (Salix exigua), has declined as the hydrologic
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