Chinook Salmon habitat evolution following river restoration, drought, and flood

Abstract

How rivers restored for aquatic biota such as Pacific Salmon (Oncorhynchus spp.) will evolve under shifting management and climate regimes is unknown. To inform this, we tracked the evolution of a highly impacted, regulated river reach following habitat restoration and subsequent drought and flood hydrology using a suite of data to better understand how created habitat evolves over time. Topographic change detection, riffle creation and persistence, substrate, and Chinook Salmon spawning and rearing habitat were evaluated for the three epochs. Sediment transport regimes and velocity outputs from the numerical flow model were used to interpret channel change associated with the flood season the site experienced. Salmon spawning observations and habitat suitability modeling demonstrated that habitat improved following restoration for both salmon spawning and rearing. Chinook Salmon spawning activity and the relative site utilization compared to escapement increased post Project. These benefits were largely maintained during drought and flood events, despite an absence of upstream coarse sediment supply. We hypothesize this was because the restoration design incorporated riffle-pool maintenance theories and the use of emergent islands that served as a local sediment source. Extrapolations of bioturbation by spawners suggest that if population targets for the river are met, topographic change due to spawning activity could exceed that caused by river flow under drought conditions and provide a modest contribution during flood years. Thus, this study demonstrates that river restoration can not only improve physical habitat, but also engage ecosystem engineers such as spawning salmonids to contribute to future habitat evolution.