4.3. Diversity Metrics in Boundary Habitat
Our discovery of geomorphically-complex boundary habitat was especially significant in the discussion of landscape mosaics as a measure of riverscape complexity. We defined boundary habitat in this study as habitat found along the edges of the upstream-expanding valley plug. Overall, we found this habitat to be the most complex, containing all of the geomorphic units we had identified as metrics of diversity (pools, riffles, backwaters and large woody debris structures; Bottcher, 2009) as well as the most diverse array of inundation types. These findings support our initial suggestions that: 1) boundary habitat is the most diverse within this system (Remiszewski, 2022) and 2) the valley plug has contributed to ecosystem habitat diversity, especially when compared to our simplified and degraded reference reaches. Especially significant is the presence of pool habitat, as prior studies have identified that the lower SRR is incredibly pool limited (Walsworth, 2011).
Pool habitat offers refugia from predation for native fishes, as well as thermal refugia during increasingly hot summer months (Murphy, Pavlova, Thompson, Davis, & Sunnucks, 2015). Direct observation demonstrated pools are frequently the only wetted portions of the lower SRR during seasonal drying and, during spot sampling that occurred simultaneously with seasonal drying in the SRR, the only fish remaining for many kilometers of the lower SRR were captured in pools below habitat features such as beaver dams and BDAs (personal communication, 2021). The presence of large quantities of pool habitat within boundary sites, compared to other habitat types, means that these are some of the only reaches of river offering persistent habitat for fishes during drought. Additionally, the large quantities of large woody debris found in boundary reaches offer rare and vital refugia for both adult and larval native fishes, ensuring the persistence of spawning and rearing habitat in an otherwise extremely habitat limited system (Bottcher et al., 2013; Walsworth et al., 2013).