Holding Water: Redefining Reservoir Rules to Make Room for Environmental Stability

Water storage is vitally important in the arid West. Reservoirs hold water for irrigation, drinking, hydroelectricity and recreation. But nature needs access to water too, so operational instructions for dams require some of the incoming water to be sent downstream to keep ecosystems functional.

This system has inefficiencies, and new research in the journal Nature from a team led by Sarah Null from the Quinney College of Natural Resources says that ecosystems could benefit from holding ecosystem-allotted water in dams for when the environment needs it most.

Dams are hard on rivers. They change the flow and shape of riverbeds and the chemistry of the water. They also block fish from upstream habitat and limit floods critical for building downstream wetlands. But mandated releases from dams, intended to replicate aspects of natural flows, often don’t benefit coldwater-loving fish when summer temperatures warm the surface of the water.

Regulators occasionally call for supplemental water releases to comply with water quality and endangered species requirements. These flow events typically pull water from farms and cities and leave ecosystems a begrudging afterthought, Null said. These factors create impractical inefficiencies and shortfalls. The solution may be in carving out an official piece of the pie for ecosystems in reservoirs from the start, she said.

“Our models show that creating a system that makes ecosystem health a primary objective for water management could actually improve the efficiency of this reserved environmental water, and benefit ecosystems by saving time, effort, money and ultimately water,” said Null, who is from the Department of Watershed Sciences.

Making the environment an official “water user” like cities and farms would allow for dedicated water storage in reservoirs, and let surplus water carry over from one year to the next. This could increase the availability of water for natural systems in especially dry years and allow water to accrue for occasional high-magnitude releases to replicate the high flows that have been part of the natural system in the West.

But it could also mean certain tradeoffs in storage capacity and water availability for other users, according to the research. The trick is finding the sweet spot in the proportion of storage allocation that you set aside for the environment, Null said.

To demonstrate the concept, the team modeled sections of the Sacramento River where protected Chinook Salmon spawn. They found that using a strategy that stored a portion of the reservoir inflow for environmental purposes used water more efficiently, and better met objectives for water temperatures to benefit coldwater-loving fish.

The drought of the past few years offers another illustration of the benefits of a storage-for-ecosystems strategy — environmental demands during those years were lowest during dry summer months, when water could be used for other demands like irrigation, the researchers said. Later, storage allocated for environmental use could be tapped to meet ecosystem objectives while allowing stored water to remain at cooler temperatures, meeting spawning needs to benefit fish.

Allocating a percentage of the inflow and storage space for environmental management could be a promising strategy for meeting multiple important objectives, the authors said. The new research offers an illustration for how a reservoir could be operated with environmental water demands as a primary objective, rather than as a constraint on other water supply operations, they said.

“As the cycle of drought and intense wet periods becomes more pronounced with climate change, these kinds of management changes will become more and more necessary,” Null said. “Ecosystems downstream of large dams will be increasingly vulnerable to climate warming and declining snowpack. If managed strategically, dams could be an essential tool for mitigating that environmental damage.”