Not time to get complacent about flood risk
June 18, 2011
Columbia-Snake river water supply forecasts, and reservoirs, continue to rise as a result of continued above-average precipitation and enormous mountain snowpacks that continue their slow meltdown.

A “mid-month” forecast released Thursday by the National Weather Service’s Northwest River Forecast Center estimates that a total of 135 million acre feet of water will flush down through the system from April-August and through the lower Columbia’s The Dalles Dam.

That would be 137 percent of the 1971-2000 average and the third largest volume on the NWRFC’s 41-year record. The largest was 140.91 MAF in 1997 and the second largest runoff volume was 139.68 MAF in 1974.

The new forecast is for a total of 81.4 MAF (127 percent of average) past the mid-Columbia’s Grand Coulee Dam in central Washington and 37.3 MAF (155 percent) past the lower Snake’s Lower Granite Dam in southeast Washington. The Snake joins the Columbia near Kennewick, Wash.

According to data compiled by the NWRFC, precipitation in the Columbia-Snake basin upstream of The Dalles from June 1-13 was 170 percent of average.

The flows past Bonneville Dam, which is the lowermost hydro project in the system, have been at or near 500,000 cubic feet per minute since May 27. It is a goal of hydro operators to manage flow during spring runoff to no more than 500 kcfs in order to prevent extensive flooding in the Portland-Vancouver area. The Columbia’s elevation has been slightly above minimum flood stage since late May but is forecast to drop below that 16-foot level within the next few days.

The chief of U.S. Army Corps of Engineers’ Reservoir Control Center, Steve Barton said Wednesday that estimates are that about one-third to one-half of the snowpack has melted.

“The moral of the story is we still have quite a bit of the snowpack to come off,” Barton told the Technical Management Team, a group of fishery and hydro officials that convenes to discuss dam operations that might be implemented to benefit migrating juvenile and adult salmon and steelhead.

Unseasonably cool weather this spring has resulted in a changed runoff timeframe.

“It slid the hydrograph over by about three weeks,” Barton said.

The Federal Columbia River Power System’s water storage is about 55 percent full, with projects such as Grand Coulee and Libby dams having been held lower than normal to preserve flood control space in anticipation of the late spring-early summer runoff. Barton said that flows at Portland-Vancouver would have been eight feet higher if not for the system flood control management. Grand Coulee is on the Columbia in central Washington and Libby in on the Kootenai River, a Columbia tributary, is in northwest Montana.

Despite the flood control, inflows to Libby’s reservoir “have been pretty impressive” in recent days and it is “filling at a fairly steep clip,” Barton said. Like Libby, the reservoir backed up by Dworshak Dam in west-central Idaho is rising at a rate of about two feet per day. At the end of the day Thursday Dworshak’s reservoir was 26 feet below full pool.

The Corps’ Seattle District has flood teams out in three basins in western Montana, northern Idaho and central Washington, and the Corps' Emergency Operations Center remains in 24-hour operation as rivers remain above flood stage.

The Pend Oreille basin team is assisting Bonner County with emergency work on a 300-foot section of the River Road Levee in Cusick, Wash., and on a segment at Dufort Road upstream of Albeni Falls Dam in Idaho. The team completed work to protect the Johnson Creek Levee, strengthened the Lightning Creek levee and provided sandbags and sandbag training there.

In the Clark Fork basin, the Corps has closed a breach in a levee in Sanders County in Plains, Mont. The County will continue to monitor and work on the levee as the high water persists.

The Okanogan team is continuing to monitor sites in the basin on the Okanogan, Similkameen, and Methow Rivers in Washington. The flood team finished a protective berm along Salmon Creek to protect the town of Conconully and completed repairs to a 100-foot section of the Okanogan City levee in Okanogan, and a 1,800-foot segment of the Twisp levee.

The Corps has distributed more than one million sandbags to date in western Montana, central Washington and northern Idaho, and provided pumps to two local communities. In addition, the Seattle District has distributed hundreds of thousands of sandbags to southern Idaho and eastern Montana.

The Corps continues to operate its reservoirs to reduce flood risk. Outflows from Libby Dam are around 25,000 cubic feet per second, and inflows are about 54,000 cfs. The Libby Dam outflow rate should help to keep Bonners Ferry below flood stage.

This morning inflows into Lake Pend Oreille were 124,000 cubic feet per second, and Albeni Falls Dam continues its free flow operation, releasing more than 117,000 cfs. Flood stage at Newport is 100,000 cfs. The lake elevation is about 2,064.2 feet and rising. Flood stage on the lake is 2,063.5 feet.

Although upper Snake River flood waters may appear to be receding over the past few days, Corps hydrologists at the Walla Walla District warn residents there is still plenty of snow up in the mountains yet to melt.

“Now is not the time to become complacent about flood risk -- it’s not over, yet,” said Steve Hall, a hydrologist in the Walla Walla District’s Reservoir Regulation and Hydrology Section. “The snowpack is still really high in the upper basin. There’s a lot more water volume headed for the rivers in the next few weeks.”

Most high-elevation SNOTEL gauge stations are showing at least 10 inches more water than previously recorded maximums for this time of year, according to the National Resource Conservation Service snow survey data.