Coe, Treva. 2001. Contrasting discharge patterns, juvenile salmonid use, and fish community structure in off-channel floodplain habitats, Queets River, Washington. M.S.
In unconstrained alluvial rivers, dynamic channel/floodplain interactions form and maintain diverse networks of off-channel aquatic floodplain habitats. I examined the effects of the spatiotemporal variability among floodplain habitats on fish community structure and juvenile salmonid distribution, abundance and size in an unconstrained reach of the Queets River, Washington, during summer low-flow. Methods included habitat characterization and electrofishing in 10-20 m long segments. I classified each of 20 channels by channel type (abandoned mainstem, overflow channel, terrace tributary) and position (active and marginal floodplains, terraces), and each of 41 contiguous wetted habitat patches by patch connectivity relative to the mainstem Queets River (persistent, discontinuous, isolated). 1997 and 1998 summer low-flows were 181% and 63%, respectively, of the long-term average. Lack of rain and low flows during summer 1998 reduced availability (42% reduction in wetted area) and connectivity of floodplain habitats. There was high persistence (presence of species) and stability (relative species abundance) in the fish community both across habitat types and between years. Reticulate sculpin dominated most sites, followed by young-of-the-year (YOY) coho and trout. YOY coho and trout were found in most available habitats, although trout were more common in persistent patches on the floodplain and coho were more common in isolated patches, especially in terrace tributaries. Significant relationships were found between habitat parameters and YOY trout CPUE in both years (r2=0.32, r2=0.43) and with YOY coho CPUE in 1997 (r2=0.53). Largest YOY coho occurred in abandoned mainstem channels (83 mm, 6.1 g), while largest YOY trout occurred in terrace tributaries (66 mm, 3.6 g). Fluctuations in discharge, moderated by differences in emergence timing among salmonid species, appeared to have led to differences in importance of floodplain habitats between years and species. Colonization of habitats deep within the floodplain clearly presented a greater risk in 1998, increasing vulnerability to stranding, predation and exposure to extreme environmental conditions. In 1997, however, when flows remained high in the mainstem Queets well in to July, there was greater access to potentially productive, high quality floodplain habitats.