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Water Forecast Drops; Corps Makes Reservoir Refill Changes by CBB Staff Columbia Basin Bulletin - March 19, 2004

A falling Columbia River Basin water supply forecast signaling a drier year than was expected earlier is causing the U.S. Army Corps of Engineers to adjust its mid-April flood control targets.

The late season adjustment means that Basin reservoirs will likely not fill to those targets on time and that could affect spring river flows, but the Corps is still predicting the reservoirs will refill by June or July and the stored water can still be used for summer flow augmentation.

The Northwest River Forecast Center's March 15 mid-month water supply forecast dropped to 92 million-acre feet, January through July at The Dalles Dam. That's 86 percent of the 1971-2000 average, a drop from 87 percent predicted in the March final forecast (March 5) and a drop from the 100 maf or 93 percent predicted in the February final forecast (Feb. 9).

The Corps sets flood control elevation targets at reservoirs based on the latest monthly water supply forecasts. The higher the water supply, the lower it sets the target elevation in order to make room in the reservoir to store more runoff. However, with the falling water supply forecasts, those elevation targets must rise.

"The water supply is diminishing," Cindy Henriksen of the Corps told the Technical Management Team this week. "There hasn't been as much precipitation and the snowpack isn't building as expected. Flows (into reservoirs) are pretty low and so the flood control elevations are going up."

In fact, the drop in water supply in just one month caused the April 10 flood control target at Lake Roosevelt behind Grand Coulee Dam to rise by 17 feet to 1,272 feet. The reservoir elevation currently is at 1,259 feet and is not likely to rise quickly because Grand Coulee outflows are needed to keep the river flow at Hanford Reach above its 70,000 cubic feet per second minimum flow target. Flows at Priest Rapids Dam, the dam upstream of Hanford Reach, have been just above 72 kcfs this week.

"If you want 1,272 (feet), you will need to dry something up," said Tony Norris of the Bureau of Reclamation. "We will likely miss April 10. There is no March miracle this year."

Up to this point, the Corps had set flood control elevation targets at a lower level and had been operating dams to meet those targets. With higher elevation targets now, some fishery managers think the Corps missed an opportunity to store more water that could have been used for spring flow augmentation.

Ron Boyce of the Oregon Department of Fish and Wildlife said it would have been better to initially plan for higher flood control elevations and draft quickly later if the water supply proves to be more than expected.

"Hindsight is 20-20," Henriksen said. "Based on the information we had at the time, we'd probably make the same decisions again. But the water supply went the wrong way."

The Corps had also been evacuating Dworshak reservoir in February to meet a low flood control target, but, with the new water supply forecast, "our target went up 20 feet in one day," Henriksen said.

"It seems we're often in this situation," Boyce said. "If we don't get some precipitation, spring flows will be rock bottom."

The Northwest River Forecast Center also is predicting lower stream flows due to below average February precipitation and deficient snow water equivalent accumulations.

"It is noteworthy that the Columbia River in Canada and the Pend Oreille River basin in Montana are most deficient in precipitation and snow water equivalents. Two-thirds of the runoff for the Columbia River at The Dalles comes from this area. Except for most Oregon basins and the Big and Little Wood basins in central Idaho, streamflow volumes have decreased 5 to 15 percent since February 1st," the Forecast Center said.

The Corps' March 2004 QADJ model runs, which predict the likelihood of meeting flow targets and of refill at reservoirs based upon the latest water supply forecasts, are showing that flows at Priest Rapids Dam will meet the 70 kcfs flow target through April 15 and will likely meet the 135 kcfs flow target through June.

Other predictions are:

• The 100 kcfs flow target for Lower Granite Dam on the lower Snake River occurs during April in 17 of 59 years and during May in 32 of 59 years, given the current water supply forecast. It meets the 84 kcfs flow target during June in 43 of 59 years, and the 52 kcfs July target in 27 of 59 years, and it never meets the 52 kcfs flow target in August.
• McNary Dam flows will meet the 228.3 kcfs target in all years April through June and will meet the 200 kcfs target during July in 16 of 59 years. It never meets the 200 kcfs target in August.
• Bonneville Dam will meet its 125 kcfs flow target in March and April in nearly all years.

Related Sites:
Technical Management Team: www.nwd-wc.usace.army.mil/TMT/index.html
Water Supply Forecast: www.nwrfc.noaa.gov/water_supply/ws_fcst.cgi

Related Page:
No Flow at Night at Snake River Dams to Fill Reservoirs by Mike O'Bryant,Columbia Basin Bulletin, 12/5/3
Zero-Flow Scenario Debated for Lower Snake River Dams by Bill Rudolph, NW Fishletter, 12/19/3