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Researchers used a model of the Butte Creek watershed, taking into account the dams and hydropower installations along the river, combined with a model of the salmon population, to test the effect of different water management strategies on the fish, feeding in scenarios for climate change out to 2099.

In almost all scenarios, the fish died out because streams became too warm for adults to survive the summer to spawn in the fall. The only option that preserved salmon populations, at least for a few decades, was to reduce diversions for hydropower generation at the warmest time of the year.

“There are things that we can do so that we have the water we need and also have something left for the fish,” says Lisa Thompson, director of the Center for Aquatic Biology and Aquaculture at the University of California, Davis.

“If we leave the water in the stream at key times of the year, the stream stays cooler and fish can make it through to the fall.”

The study is reported online in the Journal of Water Resources Planning and Management.

Summer, of course, is also peak season for energy demand in California. But Thompson notes it might be possible to generate more power upstream while holding water for salmon at other locations.

Hydropower is often part of renewable energy portfolios designed to reduce greenhouse gas emissions, says David Purkey of the Stockholm Environment Institute at UC Davis, but it can complicate efforts to adapt water management regimes to a warming world. Yet it need not be all-or-nothing, he says.

“The goal should be to identify regulatory regimes which meet ecosystem objectives with minimal impact on hydropower production,” he says. “The kind of work we did in Butte Creek is essential to seeking these outcomes.”

There are also other options that are yet to be fully tested, Thompson says, such as storing cold water upstream and dumping it into the river during a heat wave. That would both help fish and create a surge of hydropower.

Salmon are already under stress from multiple causes, including pollution, and introduced predators and competitors, Thompson says. Even if those problems were solved, temperature alone would finish off the salmon—but that problem can be fixed, she says.

“I swim with these fish; they’re magnificent,” Thompson says. “We don’t want to give up on them.”

The study was funded by the U.S. Environmental Protection Agency.

More news from UC Davis: http://www.news.ucdavis.edu/