Introduction
Globally, rivers have been severely altered over the last century due to
human population growth, accelerating economic activity, land-use
alteration, water development and climate change that have all
interrupted fluxes of water, sediment and nutrients (Dynesius and
Nilsson, 1994; Ward, Tockner, & Schiemer, 1999; Syvitski, Kettner,
Correggiari, & Nelson, 2005), simplified the physical structure of
habitats and floodplains (Beechie, Beamer, & Wasserman, 1994;
Hohensinner, Jungwirth, Muhar, & Habersack, 2005), and degraded habitat
and water quality in river systems by the loading of nutrients and
pollutants (Williams, Cook, & Smerdon, 2021). These impacts to
watersheds and rivers have altered riverine ecosystems dramatically and,
as a result, the freshwater biodiversity of North America has become
increasingly threatened. Especially severe impacts of ecosystem
stressors can be observed in the dryland systems of the American
Southwest where water is scarce relative to demand and there has been a
growing urgency over the past few decades amongst managers and
stakeholders for a more holistic approach to river management and
restoration (Laub, Jiminez, & Budy, 2015).
In arid or semi-arid rivers, reduction in the frequency and/or magnitude
of seasonal floods often induces a shift from a braided or anastomosing
to a meandering single-thread channel, decreases channel width, causes
valley alluviation, and is often accompanied by changes in riparian
vegetation communities which favor domination by nonnative taxa
(Vorosmarty et al., 2010; Castle et al., 2014; Udall & Overpeck, 2017).
This degradation process is especially prevalent in the arid American
Southwest, where four of the 14 fish species native to the Colorado
River are considered threatened or endangered under the Endangered
Species Act (ESA; Rinne & Minckley 1991; Laub et al., 2015).
Remediating threats to the persistence of native biota in Southwest
desert rivers will likely require coupled management of flow regimes and
active in-channel restoration efforts in order to preserve and maintain
crucial habitat (Propst, Gido, & Stefferud, 2008; Pennock, Ahrens,
McKinstry, Budy, & Gido, 2021).
The San Rafael River (SRR), a tributary of the Green River in the
Colorado River Basin (CRB), is representative of many desert river
systems, in that an altered flow regime, fish passage barriers, degraded
habitat, and nonnative fish and vegetation have combined to
synergistically alter ecosystem processes and threaten the persistence
of native communities (Macfarlane et al., 2017; Olden & Poff, 2005;
Stromberg, Beauchamp, Dixon, Lite, & Paradzick, 2007). In an attempt to
mitigate degradation, an adaptive restoration and monitoring plan was
developed (Laub, Jiminez, & Budy, 2013), and there have been a few
small-scale restoration projects that include the installation of beaver
dam analogs (BDAs) and post-assisted log structures (PALS), as well as
tamarisk removal (Keller, Laub, Birdsley, & Dean, 2015). However,
mitigation goals in the SRR have tended to focus on reach-scale
restoration, and the potential role of valley plugs and avulsions in
restoration-conservation efforts had not been previously considered.
Additionally, restoration efforts are challenged by the large spatial
extent of degraded habitat (> 64 km), the lack of adequate
funding for system-wide restoration and, most recently, by pervasive
drought and the over-allocation of water within the watershed.
In July 2010, a severe rain event caused flash-flooding in Cottonwood
Wash, a tributary of the SRR (Lyster, 2018). This rain event coincided
with low flows and reduced sediment transport capacity in the SRR,
resulting in the deposition of a valley plug at the confluence with
Cottonwood Wash. The term ‘valley plug’ was coined by Happ, Rittenhouse
and Dobson (1940) to describe the processes and deposits resulting from
an occluded canal or river channel. The initial plug, stretching from
400 m downstream of Cottonwood Wash confluence to 450 m upstream of
Cottonwood Wash confluence, blocked the flow of water in the main river
channel and brought water levels to above floodplain elevation (Utah
State University Water Research Laboratory, 2010). As a result, the SRR
underwent rapid avulsion and transitioned from a single-thread channel
to a multi-thread channel that began at the confluence with Cottonwood
Wash and rapidly expanded upstream (Lyster, 2018). Root masses of
invasive species such as tamarisk (Tamarix ramosissima )
were believed to have stabilized floodplain soils, preventing the
upstream migration of channel headcuts and causing water to remain at
near-floodplain levels for nearly a year. Water retention further
facilitated expansion of this wide, shallow and heterogenous reach of
river, which appeared to be contributing to dramatic habitat
diversification within the system and, as of 2021, had continued to
expand upstream.
The SRR valley plug offered a unique opportunity to examine the impacts
of potential large-scale innovative restoration actions in a
historically degraded desert tributary system. We predicted that the
resulting rapid geomorphic change contributed to both habitat
diversification and increased floodplain inundation, and was actively
creating opportunities for native fish persistence. Our research
objective was to determine if the valley plug had contributed to both an
increased diversity of habitat types and an improved capacity for water
retention against a background of otherwise extremely simplified and
degraded desert river habitat. To address these aims, we calculated
straightforward metrics of habitat diversity utilizing on-the-ground
habitat sampling, as well as manual landscape feature digitization using
Google Earth and drone-collected orthomosaic imagery.