A thinning snowpack due to climate change could polarize streamflow patterns in rivers on the Olympic Peninsula, presenting adverse effects for fish, according to a new report from the University of Washington and Washington State University.

The report, which the Washington Department of Ecology published last week, summarized projected climate effects on streamflow and water temperature statewide, as well as barriers within Washington water law to potential responses in streamflow management.

The report used a new dataset for streamflow modeling developed by Bart Nijssen in UW Civil & Environmental Engineering, according to Crystal Raymond, a climate adaptation specialist with the University of Washington and an author of the report.

Jonathan Yoder, director of the state of Washington Water Research Center and an author of the report, said the report will contribute to Ecology’s awareness of climate impacts on streamflow.

Yoder said that while the science behind climate-induced snowpack decline isn’t new, the report provides a new level of geographic specificity for projected streamflow patterns.

Rivers on the Olympic Peninsula and the Western slope of the Cascades, specifically, could experience more exaggerated flow patterns than other parts of the state, like rivers in Central Washington, according to the report.

Streamflow is largely dependent on the kind of precipitation, Yoder said. He said snowpack is like a reservoir, storing solid water in the mountains until warmer summer temperatures melt it downstream. That melting maintains healthy flows during the summer, when precipitation is minimal.

But warming temperatures will — and are — changing the type of precipitation falling in the mountains.

“It doesn’t take much warming to raise winter temperatures enough for winter precipitation to fall as rain, rather than snow, and runoff in the winter,” Raymond wrote in an email.

When precipitation falls as rain, it enters streams directly and washes out, raising peak flows during the rainy season. The report mapped projected changes in streamflows during two-year peak flow events, which occur roughly every two years, finding that peak flow changes will be the largest in areas where temperatures are usually near freezing.

A rising snowline will result in bigger floods, according to the report.

And when rainfall floods rivers, it means little is stored for summer months. Low flows in Olympic Peninsula and Western Cascade rivers are expected to get even lower as snowpack declines, but that’s not necessarily the case state-wide.

“In minimum, seven day average flows, that variation between the Olympic Peninsula and the central part of the state, particularly the North-Central part of the state, that’s pretty striking,” Yoder said.

Rivers on the Olympic Peninsula drain in a radial pattern from a centralized — and isolated — pocket of mountains, which makes streamflow patterns different from other parts of the state, Yoder said. Much of the flow in Washington’s Columbia River, for example, comes from snowpack-heavy parts of Canada.

Exacerbations in both minimum and maximum flows pose a challenge for salmon, the report says. Decreased low flows disconnect habitat and threaten upstream migration for summer spawners, while increased peak flows in the spring can scour egg-laden gravels in streambeds, according to the report.

Salmon health and flood potential are two of many reasons Ecology has taken an interest in developing projects to improve streamflow in Washington. In 2018, the state Legislature allocated $300 million to be used for streamflow restoration over the next 15 years. In 2022 Ecology awarded $35 million in streamflow grants to projects across the state, including some in Grays Harbor County.

But, as the report concludes, some Washington water law presents barriers for beneficial streamflow management in some cases. For example, the state Supreme Court decision in Foster v. Ecology and City of Yelm, which prohibits some streamflow altering activities, also “may represent inflexibility for active fisheries management in the context of climate change impacts,” according to the report.”