4.4. Riparian Vegetation as an Indicator of Habitat Persistence
A large base of research points to drought as the major driver of shrinking vegetated zones in arid and semiarid systems (e.g., Stromberg et al., 2007; Garssen, Verhoeven, & Soons, 2014; Andersen, 2016); however, further research indicates the role of ephemeral wetlands in ecosystem persistence in the face of extreme drought (Leigh, Sheldon, Kingsford, & Arthington. 2014; Sandi et al., 2020). The presence of dryland wetlands allows arid or semiarid riverscapes to exist as isolated refugia during periods of extreme drought, with low flow channels providing short-term connections between otherwise isolated sections of the river network. Additionally, the presence of riparian vegetation reduces the rates of evaporation, contributing to water retention in dryland systems (Rodrigues, Gomes Costa, Raabe, Medeiros, & Carlos de Araújo, 2021).
We demonstrated the valley plug and resulting overflow from the main channel into the valley bottom allowed the riparian zone to increase substantially over 12 years. Though we cannot make a definitive claim that this system has experienced increased resilience, visual estimates and delineation of the riverscape demonstrate the valley plug has experienced an increased capacity for water retention beyond what is typical during drying. We observed from imagery collected in 2021 that the system was retaining overflow inundation well into the expanded riparian zone through late-August, prior to summer monsoon flooding. Given what is known about the importance of riparian vegetation, as well as dryland wetland systems in arid and semiarid regions, it is logical to suggest that this valley plug is contributing to habitat persistence in the face of amplified drought in the American Southwest.