Salmon are critical species in North America, cycling nutrients and providing for ecological, economic, and cultural needs. These mostly anadromous fishes migrate from the ocean into freshwater to spawn and die, releasing nutrients and energy inland. However, for migratory salmonids, dams present a sincere threat. Even small dams fragment habitats, prevent movement, reduce biodiversity, limit resilience to climate change, and alter ecosystem processes. While studies show that dam removal can be successful in restoring connectivity, the long term recovery process is not fully understood.
Kiffney, P.M., Lisi, P.J., Liermann, M., Naman, S.M., Anderson, J.H., Bond, M.H., Pess, G.R., Koehler, M.E., Buhle, E.R., Buehrens, T.W. and Klett, R.S. 2023. Colonization of a temperate river by mobile fish following habitat reconnection. Ecosphere 14(2): e4336.
One long term study of Cedar River in Washington State monitored coho and Chinook salmon, as well as mountain whitefish (a non-anadromous salmonid species) for 20 years after dam removal. Landsburg Dam, a small hydroelectric dam, was removed in 2003 after over a century of use.
Researchers used several datasets and mathematical models to measure restoration dynamics in the 15 years following dam removal. Fish were counted and identified at a fish ladder via underwater cameras. Populations were also measured by snorkel counts, traps, and the mark-recapture method.
The study found that the abundance of adult salmon dispersing increased post-restoration. Adult Chinook salmon reached carrying capacity approximately a decade after the dam was removed. However, adult coho salmon abundance continued to increase after fifteen years, showing that carrying capacity still hadn’t been reached. Biomass of all three study species increased post-restoration, particularly that of mountain whitefish, plateauing after 8-10 years.
Restoration also affected the community assemblages at the site. Before restoration, only two salmonid species were present above the dam, compared to six afterwards. Total salmonid summer biomass nearly doubled after 15 years, with summer densities comparable to those of undammed streams. As density increased, fish dispersed upstream from core habitat to spawn. Patches of cool deep water upstream may have served as stepping stones for whitefish.
By the end of the study, restoration had led to a 200% increase in salmonid diversity above the dam. Researchers suggest that this restoration effort was effective due to nearby source populations and the availability of above-dam, high-quality habitat.
Upstream population expansion was slower farther from the restoration site, emphasizing the importance of site prioritization based on these factors. Both abiotic and biotic factors contributed to recovery dynamics – for instance, water level and warm temperatures limited which areas of the river system deep bodied fish could access.
The combined effects of climate change and dams are especially risky for river connectivity, but even after dam removal, climate factors such as water temperature remain important for fish movement. For planners considering dam removal, riparian restoration could take things one step further, creating multi-taxa climate corridors.
One crucial takeaway: while many recovery trajectories plateaued after 15 years, some factors were still increasing constantly. These included adult coho counts, freshwater coho productivity, and total above-dam salmonid biomass. This finding emphasizes how long recovery can take, and that the successes of restoration continue beyond when many studies end. Given the importance of salmonids, restored connectivity is worth the wait.
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