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.