The Susitna River supports all five species of Pacific salmon (Oncorhynchus spp.), including globally significant populations of king and sockeye salmon (O. tshawytscha and O. nerka). Though known to be strong, the Sustina River’s salmon populations are surprisingly unquantified. Less than half of all of Alaska’s streams have been evaluated for salmon presence or abundance and the Susitna is no exception.
The Susitna River supports Alaska’s fourth largest king salmon population and second largest recreational king fishery. Their abundance makes Susitna kings a globally important population given precipitous declines in the Lower 48. Contrary to assumptions made during dam studies in the 1980s, king salmon do in fact migrate, spawn, and rear far upriver of the proposed Susitna-Watana dam site.
The Mat-Su Basin supports one of the top ten remaining sockeye salmon populations in the world. It also experienced a higher percentage increase in sockeye salmon abundance than any other region in the world between 1962 and 2005. Sockeye are the Susitna’s most commercially important salmon population.
Threats to Salmon from the Susitna dam
The risk of impacts of a large dam to the Susitna’s salmon populations are significant and vast. Lessons from the Columbia River Basin and other once-robust salmon rivers in the Lower 48 provide insight into possible consequences of Susitna Hydro.
Because king salmon are documented migrating above the dam site, they could suffer the most impacts in the upper river. Although fish passage facilities may be constructed, migration, spawning, and rearing habitat would be lost as a result of the 42-mile long reservoir. Even if passage can be designed, constructed and operated, the effects of the reservoir on salmon adapted to a riverine habitat are likely to be detrimental.
Experience in the Lower 48 shows that impacts to fish downstream of dams can be at least as severe as upstream effects—and often much more so. Load-following operations are proposed for the Susitna-Watana dam, meaning river flows would vary hourly, daily, and seasonally according to electricity demand.
Daily fluctuations in water depth, velocity, and channel.
Rapid changes in habitat (riffles may become pools, off-channel habitat may become inundated by increased mainstem flows or, conversely, dewatered at least twice daily).
Ice may not form as easily or last as long downstream of the dam. Studies from the 1980s predicted frozen conditions could be reduced by up to 12 weeks per year. Susitna salmon have adapted over millennia to specific ice seasons.
Fine sediments, a major component of glacial rivers like the Susitna, and organic matter which provides habitat and nutrients for aquatic life will be trapped behind the dam degrading habitat downstream.
Alterations to groundwater which can effect habitats essential to spawning, incubating, rearing, and overwintering salmon in off-channel and mainstem habitats.
Algae, the base of the aquatic foodweb, die or break loose with changes in water.
Aquatic vegetation—important cover for rearing salmon— cannot grow when water levels change too.
Aquatic insects are arguably the most important component of fish diets, and are delivered by river flow as they drift downstream. Patterns of aquatic insect drift are greatly altered by hydropower dam operation.
Direct impacts to fish:
Rapidly changing flows causing either scour or sedimentation may disrupt redds (nests) and egg incubation.
Changes to natural flow patterns alter access to critical off-channel habitat as well as fish migration, potentially altering timing of spawning and smolt outmigration.
In the Cowlitz River in Washington State, about 60% of king salmon fry were estimated to become stranded and die as a result of load following operations.
Unnatural conditions in and below reservoirs behind dams often make ideal environments for non-native species such as Northern pike (Esox lucius), or native species such as Lake trout (Salvelinus namaycush) which can become predators of native salmon and other fishes already struggling to adapt to a new environment.
After nearly a century of hydropower operation in the Lower 48, despite all attempts to mitigate the salmon losses they caused, not a single salmon stock has fully recovered.
Excluding states, tribes, and local government efforts, eleven federal agencies unsuccessfully spent over $3 billion from 1982-2001 attempting to recover Pacific salmon in the Columbia River Basin alone.
To mitigate effects of hydropower, salmon in the Columbia River Basin are often transported by truck, barge, or other man-made means up or downstream of hydro- power facilities. This is an incredibly expense procedure.
Hatcheries are widely used to compensate for salmon losses. Hatcheries, apart from being extremely expensive to operate, are not a substitute for wild salmon runs. Evidence from the entire North Pacific Ocean indicates genetic and ecological impacts of hatchery salmon make them a poor mitigation strategy.