Water Supply Forecastby CBB Staff
The Northwest River Forecast Center (RFC) adjusted its water supply forecasts for Columbia River basin sites downwards from the forecast it had made just 10 days earlier, a situation that will leave salmon managers and river operating agencies with fewer alternatives for in-season management as the salmon migration season progresses.
Earlier this week, the RFC released it's May Final forecast that predicts a water supply at The Dalles Dam of 79.5 million acre feet, January through July, just 74 percent of average. That's down from the RFC's May Early Bird forecast of 81.6 maf, 76 percent of average, and places the year at 37th in 44 years of water supply.
In first place is 1997 when the water supply was almost 159 maf. The lowest was in 1977 when the water supply was a dismal 53.29 maf, while 2001 ranked as the second worst water supply year at 58.19 maf.
The RFC predicted in January 2004 a water supply at The Dalles Dam of 102 maf, January through July. Since then, the forecast has declined every month. The February final forecast at The Dalles was 100 maf. That dropped to 92.9 maf in March and to 84.2 maf in April.
"Back in January the forecast was 92 to 98 percent of average, so we've had quite a reduction from January to now," the U.S. Army Corps of Engineers' Cindy Henriksen told the Technical Management Team (TMT) at its meeting this week. TMT is responsible for providing in season guidance for river operations during the salmon and steelhead migration.
The water supply at Grand Coulee Dam dropped to 52.2 maf, 83 percent of average. That's down slightly from the May Early Bird forecast of 52.5 maf, and considerably lower than the 61.7 maf the RFC had predicted in January.
In other areas of the Columbia River Basin, the water supply forecast at Lower Granite Dam on the lower Snake River is 13.1 maf, April through July, or 61 percent of average, down from the 14.3 maf of the May Early Bird forecast, according to Henriksen. The RFC predicted the Lower Granite water supply for another period of time, January through July, at 18.8 maf, 63 percent of normal. The May Early Bird forecast was 20 maf and the January prediction was 27.6 maf.
Due to a falling water supply forecast for Dworshak Dam on the North Fork of the Clearwater River, Henriksen said the Corps would have to lower the outflow beginning May 17 at the dam to 3,100 cubic feet per second (it now fluctuates between 2.4 kcfs and 4.4 kcfs) in order for the reservoir to refill by June 30. The May final forecast fell to 2.1 maf, she said. That's down from 2.66 maf, January through July, 75 percent of normal.
Still, the U.S. Army Corps of Engineers is predicting that three of four of the Columbia River Basin's primary storage reservoirs will refill by June 30, a refill date set by the NOAA Fisheries 2000 biological opinion of the Federal Columbia River Power System. Libby Dam in Montana is the exception. Given the current water supply in each of the separate river basins, the Corps predicts that Libby would refill in just 15 of 59 years, while Hungry Horse in Montana would refill in 58 of 59 years, and Grand Coulee and Dworshak reservoirs would refill in every year.
At last week's TMT meeting, salmon managers asked for increased outflows at Grand Coulee Dam so that Priest Rapids Dam could sustain flows of 135,000 cubic feet per second due to an increased presence of juvenile steelhead in the mid-Columbia River. However, a study by the Corps this week indicates that flows at that level through May and June are not sustainable if the dam is to refill by June 30. Refilling Grand Coulee's Lake Roosevelt by June 30 is a high priority for both salmon managers and dam operators, and a recommended action by the BiOp. But the flows may have to drop to 91 kcfs in June to ensure the refill, Henriksen said.
Modeling by the Corps last week also showed that flows would likely have to decline in June, but the modeling also showed that flows of 140.6 kcfs could be maintained through May at Priest Rapids Dam and would have to decline to 128.2 kcfs in June in order to complete the refill. Modeling this week, however, showed that the 136 kcfs flows at Priest Rapids in May would have to drop to 91 kcfs in June.
That will translate into lower flows further downstream at McNary Dam later this spring and summer. Although the BiOp flow target is 220 kcfs, the Corps now expects flows lower than it predicted last week. They will average 198 kcfs in May and 147 kcfs in June. The BiOp flow target drops to 200 kcfs in July, but flows are expected to be 135 kcfs and to drop to 127 kcfs Aug. 1 through Aug. 15 and to 103 kcfs for the remainder of August. All are lower than last week's predictions.
The Corps predicts flows at Bonneville Dam to average 212 kcfs in May, 157 kcfs in June, far lower than the 206 kcfs it predicted last week, 142 kcfs in July, 133 kcfs Aug. 1 through Aug. 15 and 108 kcfs through the remainder of August.
Technical Management Team: www.nwd-wc.usace.army.mil/TMT/index.html
Water Supply Forecast: www.nwrfc.noaa.gov/water_supply/ws_runoff.cgi
learn more on topics covered in the film
see the video
read the script
learn the songs