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NW Fishletter #395, July 1, 2019

[8] Columbia Basin Runoff Occurring Early This Year

A Northwest River Forecast Center analysis shows this year's spring runoff is occurring earlier than usual in many parts of the Columbia River basin. In some places, like the Willamette River at Salem, Ore., and at Lake Coeur d'Alene in Idaho, more than 80 percent of the runoff has already occurred.

National Weather Service hydrologist Geoffrey Walters analyzed the percentage of April and May runoff that contributed to the projected total April-to-September water supply, and compared it to the April and May runoff since 1980 at several locations. "Based on data analysis, there are indications we are in an early runoff period," he told participants at a June 6 water supply briefing.

The webinar marked the Forecast Center's last water supply briefing this year, with the detailed Columbia Basin climate summaries scheduled to resume next January.

Overall, Walters said, the water supply forecast is similar to what forecasters predicted in early May. He said that May brought above-normal temperatures throughout most of the Columbia Basin. Precipitation was below normal in the north and above normal in the Snake River basin, following a trend that has held for most of this water year since October.

The water supply for the next four months is expected to be well below normal in north central Washington and in the upper Kootenai River basin, Walters said. Both are part of the upper Columbia Basin, where precipitation since October has been 72 percent of normal. The water supply forecast for April through September in the upper Columbia is also below normal, and ranges from 69 percent of normal at Libby Dam to 96 percent of normal at Lake Coeur d'Alene.

In the Snake River, precipitation in May was normal, and an above-normal water supply is forecasted. Water supply is expected to be 100 percent at Dworshak Dam, and 120 percent at Lower Granite Dam, coming mostly from the middle Snake River basins, Walters said.

And on the west side of the Cascades, water supply is below normal in Washington and above normal in Oregon, with the Rogue River basin expecting 138 percent of the 30-year average. "Again, it's the haves and have-nots," with northern basins lacking water and southern basins with more than enough, he said. "The Oregon basins can largely thank the early April precipitation event that came through to drive that water supply forecast up," he added.

The Snake and upper Columbia basins come together at The Dalles, where the forecast for this year's April-through-September water supply is close to normal, at 94 percent, Walters said. "A combination of the upper Columbia being below normal and the Snake being above normal kind of canceling each other out and causing The Dalles to be near normal despite the lack of snowpack we continue to experience above Grand Coulee Dam," Walters said.

Whether they have the snowpack or not, nearly all locations are experiencing an early runoff, Walters said.

At The Dalles Dam, about 50 percent of this year's projected runoff occurred in April and May. That compares with a median percentage for April and May runoff since 1980 at 40 percent of the total runoff, his data showed. April and May's runoff at The Dalles is most likely to be between 40 to 48 percent of the total water supply, and is unlikely to comprise less than 32 or more than 58 percent of the total supply.

Results of Walters' analysis were different in different locations, just as the historical timing of runoff at different locations varies greatly. But his analysis shows that all locations examined except one--Jackson Lake Dam in Teton, Wyo. on the Snake River--are experiencing an early runoff this year. At Jackson Lake, about 40 percent of this year's projected water supply had melted out in April and May, compared to a median of 45-percent runoff during those two months.

All locations analyzed in the upper Columbia were experiencing early runoff, and Lake Coeur d'Alene offered an extreme example, with about 81 percent of the total projected water supply already running off in April and May. Lake Coeur d'Alene often experiences more runoff in April and May compared to other locations, however. Runoff for the two months generally comprises between 68 and 78 percent of the total water supply.

The biggest deviation from the median percentage of April and May runoff was found on the Willamette River, where about 80 percent of this year's projected runoff occurred over the past two months.

The median amount of April-May runoff since 1980 is just over 60 percent, and the two-month runoff is unlikely to comprise more than 70 percent of the total water supply, his analysis shows. Walters said that, once again, the early April precipitation in the southern portions of the Columbia Basin really drove the Willamette River's early runoff volume.

Responding to a question from NW Fishletter, Walters replied in an email, "Until this water year (though projected) Apr-May volume contributions have never been greater than 75%" in the Willamette River since 1980."

He said he hasn't analyzed whether the early runoff is a trend over the last couple of years, or whether it's likely to continue.

When asked about the implications of an early runoff, Walters replied, "That really depends on what your application of using our forecasts is for. It can potentially affect how reservoir regulators operate their dams, It could potentially affect how irrigators operate their diversions. It just definitely depends on who you are and what you're using our forecasts for." -K.C. Mehaffey

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NW Fishletter is produced by NewsData LLC.
Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
Phone: (206) 285-4848 Fax: (206) 281-8035

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