2.2. Sample Design
We conducted a preliminary field study in 2020 where we selected six 300 m sample reaches that were representative of: a) the historically degraded habitat found in the lower SRR, b) boundary habitat located along the edges of the valley plug and c) habitat located within the valley plug. Each of the three habitat types were represented by two of the six sample reaches. In each reach, we measured mesohabitat composition by estimating the area of geomorphic units within each reach (i.e., glide-runs, pools, backwaters, riffles and large wood accumulation) and calculated percent-area of each type.
In analysis of geomorphic change over time, as well as habitat persistence in the form of floodplain inundation, we used modified methods from the Riverscape Inundation Mapper tool (RIM; Bartelt, 2021). Based on these methods, we conducted relatively rapid and manual digitization of landscape features to analyze readily available (e.g., Google Earth) and easily acquirable (e.g., consumer-grade drones) high-resolution aerial imagery. The digitization of visible features from high-resolution orthoimagery is a widely used method in landscape analysis (e.g., Carbonneau, Fonstad, Marcus, & Dugdale, 2012; Carbonneau et al., 2020; Donovan et al., 2019; Green, Hagon, Gómez, & Gregory, 2019), and we used the approach to examine geomorphic change resulting from the valley plug and the persistence of this feature over time.
To establish a baseline of geomorphic condition in the lower 90 km of the SRR, we chose two degraded and laterally-confined or partly-confined reference reaches, one upstream of Spring Canyon and the other upstream of Moonshine Wash (Figure 1). Selection of these reference reaches was access-limited due to a lack of roads and the rugged terrain of the region. The selected reaches were similar in length to the valley plug (7.36 km) and were intended to represent simplified and degraded habitat, offering context for metrics of change within the valley plug. For each reference reach, we followed a five-step process where we 1) acquired basemap imagery, 2) digitized features that represent: a) valley bottom extent, b) inundation, c) geomorphic units and d) riparian and upland vegetation, 3) split the valley bottom and metrics into 13 equally spaced segments, 4) quantified the metrics from each segment and 5) calculated measurements of variance for each site (see Table 2). We repeated these data capture methods over two time periods: 2009 and 2015, chosen because these time steps allowed us to examine significant change over time pre- and post- valley plug formation. We then repeated these data capture methods for the Cottonwood Wash valley plug over three time periods: 2009 (pre-valley plug), 2015 and 2021.