4.1. Valley Plug Persistence and Expansion
We believe that the valley plug has been able to persist and expaFnd over the past decade due to the SRR being largely a low gradient system in the lower 90 km, with a wide and less rugged valley bottom than other tributaries of the basin (Fortney et al., 2011). Due to this low gradient, the SRR is also a fairly low velocity river with exception to seasonal flooding, and the wide valley bottom offers the river a greater opportunity to meander if given floodplain access. The lack of consistent high velocity flows and the wide, relatively flat valley bottom likely offered the perfect landscape for the valley plug to persist. When a large magnitude flash flooding event resulted in a channel occlusion in June of 2010, the ensuing flooding likely drained much less quickly than it would have in a higher gradient system, allowing the valley plug to evolve and expand.
In addition to its gradient, the SRR has a densely vegetated floodplain, consisting primarily of non-native tamarisk (Macfarlane et al., 2017). Though colonization by nonnative vegetation often carries negative consequences, dense root masses could be a significant contributor to the persistence of the valley plug by maintaining the near-floodplain water surface levels of the newly formed channels and preventing the upstream migration of channel headcuts. Beaver are also prevalent in this system, both presently and were prior to the development of the valley plug, and their presence has been associated with valley plug maintenance and expansion. Prior research monitoring the success of translocated beavers in Utah desert river tributaries, including the SRR, identified that river reaches containing beaver populations experienced a greater density of dams and woody debris structures (Doden, Budy, Avgar, & Young, 2022). Many translocated beavers, as well as resident beaver populations, in the SRR were located within the valley plug itself, where they were known to contribute to drought-specific resilience in this system. This is due to the fact that, even at low flows, beaver dams will continue to pond water, which we directly observed during the summer of 2021 (Bartelt, 2021; personal observation, 2021). We surmise that ecosystem engineering by beaver facilitated an increase of inundated area, thereby extending water residence times during increasingly dry periods and continually expanding the available area for geomorphic change to occur as well as refugia for fishes.