Beavers (Castor canadensis) influence habitat for juvenile salmon in a large Alaskan river floodplain

Our aim was to determine how beavers affect habitats and food resources for juvenile salmon in the Kwethluk River in western Alaska. Habitat modification by beavers was quantified using 3 years of satellite imagery to assess the amount and spatial distribution of potential juvenile rearing habitat. Macroinvertebrate community composition and juvenile salmon abundance in beaver ponds, spring brooks with and without upstream beaver dams, and main channel shorelines were quantified to determine beaver influence. Presence of beaver dams and time‐series measures of water levels were used to assess hydrological connectivity and fish access between the sites and the river as modified by beavers. Of the off‐channel aquatic habitat, 87.5% was altered by beavers damming spring brooks. All beaver‐free and beaver‐influenced juvenile salmon habitats had similar physical characteristics [by non‐metric multidimensional scaling (NMDS) ordination], indicating that all are suitable as juvenile salmon rearing habitat. Aquatic macroinvertebrate community composition differed between beaver ponds and spring brooks (by NMDS ordination) with differences driven by larval stoneflies in spring brooks compared to cladocerans, copepods and freshwater clams in beaver ponds. Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon were predominant in all habitat types except late‐successional ponds. Total fish species and salmon species richness was 2–3× higher, the proportion of young‐of‐the‐year salmon was over 50% compared to <5%, and densities of juvenile salmon were 5–7× higher in spring brooks and early‐successional ponds compared to late‐successional ponds. Early‐successional ponds had high hydrological synchrony values (closely tracking water fluctuations in the main channel), while mid‐ and late‐successional ponds, being farther from the main channel and with more dams blocking flow paths, had lower and highly variable synchrony values. Almost no movement of juvenile salmon occurred past dams at base flow. However, summer and autumn flooding mediated movement past dams, allowing individuals to ‘escape’ or enter early‐successional ponds. Beavers reduced habitat connectivity and added variability to macroinvertebrate assemblages within habitats by damming floodplain spring brooks. Nonetheless, juvenile salmon were able to effectively inhabit and move between early‐successional ponds and spring brooks in the Kwethluk River, although the presence of beaver dams strongly limited the use of late‐successional ponds on the large alluvial river floodplain.