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Spring Storms: Some Columbia Basin Reservoirs See Turnaround in Refill Potential by Staff Columbia Basin Bulletin, June 4, 2010

Rain gauges have gotten a big shot during the final week of May, in addition to a precipitation blitzkrieg across the Columbia River basin's midsection to start June.

Example: Idaho's Clearwater River Basin.

In mid-April, the U.S. Army Corps of Engineers estimated that there would be about 35 percent chance this year of refilling the reservoir behind Dworshak Dam on the North Fork of the Clearwater River in west-central Idaho.

The reservoir is a popular summer time playground for boaters and anglers, and a valuable source of water to cool the lower Snake River later in the summer for migrating salmon and steelhead. The North Fork flows into the Clearwater River and then the lower Snake, where in late summer water temperatures climb to levels that are unhealthy for salmon and steelhead.

The 35 percent prediction was based on the fact that the mountains above the Clearwater River basin contained only about half of their average "snow-water equivalent," and the April 1 Corps water supply forecast was for April through July runoff volumes at 52 percent of average. In mid-April, Dworshak pool was 1,531 feet elevation and inching upward the full pool goal of 1,600 feet by July 1.

By Thursday, things were looking considerably brighter.

"We're fast approaching a 100 percent probability of refill," the Corps' Steve Hall said after a forecast storm materialized, dropping more than 2 inches in nearby Lowell and over an inch in Orofino. There is always uncertainty when considering weather forecasts, but at week's end the reservoir was filling rapidly.

The dam operators had been idling along, allowing only about 1,100 cubic feet of water per second out of the dam while inflows into the reservoir slowly increased as a result of the annual spring meltdown. In recent days the reservoir elevation had been rising about 1 foot per day to reach 1,583 -- just 17 feet from full -- by the end of the day Wednesday.

A spate of storms during the last week in May raised the Clearwater drainage's precipitation total for the month from 94 percent of normal May 24 to 108 percent of normal. And then Tuesday the leading edge of something akin to a pineapple express arrived to dowse the region.

"We're getting closer to two feet per day right now" Hall said of inflows that were expected to peak at 28 kcfs on Thursday according to Northwest River Forecast Center projections. Rain totals were lesser to the north and particularly so to the south, but the central part of the Columbia River basin received record rainfall in many places.

The Corps, which operates Dworshak, still must balance the desire to refill the reservoir this month with the need to reserve enough space for flood control.

"They were really in the bull's eye and we've got another storm coming in tonight (Thursday)," the U.S. Bureau of Reclamation's Ted Day said of the Tuesday storm that swept across Portland and directly east into the heart of the Columbia River basin.

That single storm "made a significant difference in the water supply in a number of places," Day said.

Wetter than normal conditions that prevailed in April and May have helped improve what was a relatively dire situation. The NWRFC's May 27 early bird forecast says that runoff past The Dalles Dam on the lower Columbia is expected to be only 66 percent of average from January through July. And the Snake River forecast, as measured at Lower Granite Dam in southeast Washington, is only 60 percent of normal even after the inclement weather of recent weeks.

The Bureau is seeing its system of storage reservoirs filling fast in recent weeks. The rain has dampened irrigation demand and thus eliminated early draws on reservoir water. The Payette River system of reservoirs in Idaho is approaching full, Day said. A month earlier it was predicted that that system would not fill this year.

The Bureau's upper Snake reservoirs have so far loosed 158,000 acre feet of water, all of it in May, as part of its obligation to supplement flows in the lower Snake and Columbia for migrating salmon and steelhead. Day said that the Bureau expects to provide 427,000, all from willing sellers, this year from its Idaho reservoirs for flow augmentation.

Refill of the upper Snake system is possible though still in doubt, as is topping off the Boise River system in Idaho and the Yakima River system in Washington.