‘Deadbeat dams’ and their impact on cold-water ecosystems

As California mulls water storage, a new study adds nuance to cold-water conservation.

 

As drought-stricken California considers constructing new dams, a new study finds that many of the state's existing structures despite efforts to prioritize healthy water temperatures are failing the cold-water ecosystems that depend on them.

The study, published in PLOS One, crunched data from 77 cold-water streams across California to characterize their “thermal regime” — that is, their annual temperature fluctuations over an eight- to 12-year period. Salmon, trout and a variety of other cold-water species are sensitive to disruptions in temperature patterns; the temperature range is as important to their life cycle as the amount of water flow. But across the state, the study found, certain dams disrupted these rhythms for up to 31 miles (50 km) of the rivers involved.

Pine Flat Dam in Sanger, California, releases water back into the flow of the Kings River after being held in a reservoir. Except for Shasta Dam, no other regulated river in a recent study showed the natural temperature patterns that cool- and cold-water ecosystems need.
George Rose/Getty Images

In order to protect habitat, water managers deliver cold water to streams from reservoirs. But according to lead author and UC Davis researcher Ann Willis, no one temperature can ensure the health of an ecosystem. For example, the temperatures required for the incubation of salmon eggs are different from what is needed for the fishes’ juvenile growth. “Streams are the temperature they are because of interactions between the water, the trees, the snowmelt and the groundwater creating unique temperature patterns,” she said. “Water temperature is so much more than a single number.”

Willis’ research highlights a difficult reality for conservationists: Out of 27 dams, only one successfully duplicated the crucial temperature patterns that the cold-water ecosystems depend on. “In science, when we have results like that, we call it an outlier, which means it’s the exception that proves the rule,” said Willis. Dams — even those designed to manage cold-water ecosystems downstream  were overwhelmingly unable to mimic the fluctuating temperature patterns.

Historically, water management has focused on releasing water from dams with a single-degree temperature target, or a seven-day average, to maintain cold-water ecosystems. But this simplifies the temperature dynamics, flattening an otherwise dynamic process and disrupting the seasonal patterns important to ecosystems. In other words, the right temperature isn’t always released at the right time.

Scientists and policymakers need to carefully consider thermal regimes when weighing the future of California’s water infrastructure and imperiled cold-water ecosystems, said Belize Lane, a hydrologist at Utah State University who was not involved in the study. “They make a good point: We have oversimplified environmental management targets.” But a more thorough study of the complex stream-level dynamics is necessary, Lane said, especially the potential role the state’s existing dams play in supporting the ecosystems’ temperature needs.

“They make a good point: We have oversimplified environmental management targets.”

Restoring a dynamic annual temperature pattern is crucial, both for preserving California’s salmon and trout — three-quarters of which are at risk of extinction — and for protecting the broader ecosystems that many Californians rely on. Whether that comes with — or without — dams is another question.

Some argue that dams can help manage cold-water ecosystems in a changing climate by storing and releasing cool water. But Willis said the science is clear: Environmental management needs to move away from oversimplified temperature targets.

Watershed management, as a field, has seen its operational strategies called into question before. After scientists noticed the importance of a varied amount of water flow, managers adjusted the timing and volume of water releases. The same holds true for temperature, said Willis: Adjustments need to be made to account for the patterns of cold-water streams and the species that depend on them.

Willis said she is not advocating the removal of all dams. She simply believes that we need to be more deliberate about what our water infrastructure seeks to accomplish. As she put it, we need to identify “deadbeat dams” — and remove them if necessary — while improving existing infrastructure as needed. If we want to enhance the ecosystems that so many species depend on, Willis said, we need to focus more clearly on how science informs our goals.

Increased water storage is on the list as federal infrastructure spending winds its way towards California. Willis hopes that her research will give pause to anyone recommending the expansion of the state’s already staggering dam footprintShe also cautions against relying on the existing approach to sustain cold-water ecosystems: “The science doesn’t support that,” she said.

Theo Whitcomb is an editorial intern at High Country News. We welcome reader letters. Email him at [email protected] or submit a letter to the editor. See our letters to the editor policy.

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