With near-record runoff potential as a result of late season heavy snowpack throughout the Columbia Basin, river managers with the U.S. Army Corps of Engineers and the Bureau of Reclamation are using the region's major storage dams to protect life and property and minimize impacts to fish, navigation, power generation, and irrigation.
Cooler temperatures and wetter-than-normal conditions in the Pacific Northwest mean that the biggest portion of the expected runoff volume is yet to come, with sub-basin snowpacks ranging from 120 percent to more than 200 percent of normal in many places.
"Our projects are in a good position for managing flood risk and minimizing flood damage," said Steve Barton, chief of the Corps' Reservoir Control Center, which directs system flood control operations in the basin. "We're working hard to manage reservoir storage space so that we have room to hold water back at the peak of the runoff, which is yet to occur."
"If we have a gradual warming trend, we could get by without major flooding," said Barton. "If, however, it warms up quickly and we get a lot of rain at the same time, we could see large and sharp rises in runoff."
That snowpack, which is higher than normal in every part of the basin, continues to build.
“Normally we peak out in snow accumulation in mid to late April,” Barton said during a Wednesday meeting of the Technical Management Team, a panel of federal, state and tribal fishery and hydro managers. The TMT was formed to discuss hydro operation measures that might be implemented to improve the survival of salmon and steelhead that migrate up and down through the hydro system.
“There are indications that the water supply forecast has been and continues to trend upward,” Barton said. “We’re having a very delayed maximum to the snowpack.”
The mid-month forecast released Thursday by the Northwest River Forecast Center predicts that runoff past The Dalles Dam on the lower Columbia River will total 125 million acre feet from April through September. Such a total would be 127 percent of average and the fifth highest on a record that goes back 41 years. The May 19 forecast jumped 5 percent from the 122 percent of normal prediction issued just two weeks earlier, on May 6. The Dalles passes water flowing down from the Snake River basin and the mid and upper Columbia.
The new forecast for Grand Coulee Dam in central Washington is 77.4 MAF, or 121 percent of normal, up from a 117 percent of normal estimate made May 6. Runoff from the upper Columbia in British Columbia, Idaho, Montana and northeast Washington flush past Grand Coulee.
The forecast for runoff past Lower Granite Dam on the lower Snake River in southeast Washington is 33.5 MAF or 139 percent of average, up from 130 percent two weeks earlier.
Inflows to Libby Dam’s reservoir in northwest Montana are expected to be 7.71 MAF or 125 percent of average, up from 123 percent.
With this year's high water, increased flows in the Columbia and its tributaries have brought the Columbia River to bank-full at Vancouver, Wash., as measured at the Interstate 5 Bridge. At 15.5 feet, the Columbia is within 6 inches of flood stage.
By next week, higher flows may push the stage slightly above 16 feet, but less than 18 feet, according to the Corps. Impacts at that level are generally minor and most likely to be seen at river access points, low-lying parks and sidewalks, or on agricultural land.
In comparison, during the area's last major flood event in Feb. 1996, the Columbia River topped 27 feet at Vancouver harbor, more than 11 feet over flood stage.
Flows for the lower Columbia River, as measured at The Dalles Dam about 88 miles east of Portland, have been running about 450,000 cubic feet per second. That's the volume of water equivalent to submerging a football field nine feet deep in water every second.
Corps officials caution that if warm weather patterns continue, flows may reach 480 kcfs next week.
Northwest federal and non-federal reservoirs have enough storage space to control about 25 percent of the annual runoff volume in the Columbia Basin.
Reservoirs that do have storage capabilities are filling up fast. Albeni Falls Dam on the Pend Oreille River in north Idaho had inflows as high as 100 kcfs on a day it only sent 67.3 kcfs downriver.
“Managing the lake is going to be a challenge,” Barton said.
Storage reservoirs throughout the system “have been drafted down appropriately for the volume we are expecting,” Barton said.
Dworshak Dam’s reservoir “is filling three feet per day,” the Corps Steve Hall with inflows as high as a daily average of 32 kcfs during this past week and outflows for only 2.4 kcfs. The reservoir bottomed out last month 150 feet below full pool and the goal is to refill by the end of July.
The Corps this week had to Monday through Wednesday suspend the transportation of outmigrating juvenile salmon from Lower Granite Goose and Lower Monumental dams on the lower Snake because the high flows and turbulence created by high spill volumes made it unsafe for barges to approach the projects to load collected fish. A portion of the young fish are collected at the dam and steered into barges for a ride down river through the hydro system. The fish are then released below Bonneville to continue their journey toward the Pacific Ocean.