Without careful coordination for flood risk management throughout the water year, the Columbia River would have hit a stage of 25 feet at Vancouver in June, a height not seen in the river in springtime since 1956. That level would be considered ‘major flood stage’, as determined by the National Weather Service. Evidence of the same storm’s power was seen further south in the damaging flooding along the Yellowstone River in northern Wyoming.
The U.S. Army Corps of Engineers’ Northwestern Division leads the coordination of this large system of multi-agency dams to reduce flood risk in all parts of the basin, with a specific – though not exclusive - focus on protecting the Portland-Vancouver area. USACE water managers work with partners from the Bureau of Reclamation, Energy Keepers Inc., Idaho Power Company, the Bonneville Power Administration, and British Columbia Hydro (Canada) to accomplish this mission. The United States also has an extensive system of levees to help manage flood risk.
Through their coordinated actions, the team was able to adjust reservoir operations across four states and British Columbia to store water in storage reservoirs and manage flows to reduce flooding in communities throughout the Pacific Northwest.
The flood risk management operation in the Columbia River Basin avoided potentially damaging floods in both the U.S. and Canada. The Columbia River and its tributaries move water across the U.S.-Canada border, with the Kootenai and Pend Oreille rivers flowing north into Canada before merging with the Columbia River and flowing south into the U.S. The entire basin drains an area about the size of France. Through the Columbia River Treaty, an international agreement signed in 1961, the U.S. and Canada cooperate on reservoir operations for flood control and hydroelectric power generation and to allow both countries to create other benefits.
“This year started out as a fairly normal water year in the basin,” said Julie Ammann, chief of USACE’s Columbia Basin Reservoir Control Center. “In early June the water supply forecasts for the spring and summer period were about 99 percent of normal for The Dalles, Oregon, though snowpack in parts of the upper basin was above normal, due to a cold April and May.”
Then, an unseasonably strong atmospheric river took aim at the Pacific Northwest and produced heavy rainfall between June 9 and 12.
“On the 8th, it appeared the system would hit the coast much further north - centered on the border between Washington and Oregon,” Ammann said. “But on the night of June 10th, it became apparent that the system was instead shifting south and would hit Oregon hard and push into the basin.”
That is significant, because weather systems of this size and strength are uncommon in June, according to Ammann. Willamette projects in Oregon were managed to capture high inflows and maintain downstream gauges below flood stage, contributing to reductions at Portland and Vancouver. After hitting western Oregon and Washington, the system pushed over the Cascade Mountain Range and continued to move into Idaho, Wyoming and Montana, where snowpack in the mountains quickly melted and combined with the runoff from the rain, dramatically increasing stream flows.
“The rapidly changing conditions made anticipating inflows difficult,” Ammann added. “Decisions were made using the best available information from the Northwest River Forecast Center and our other partner agencies. Our reservoir regulators had to look ahead to base their decisions on project operations that would prevent the most damage, while being sure to weigh the uncertainty in the forecast and any upcoming precipitation and snowmelt.”
Regulators utilized space in reservoirs across the basin to reduce peaks. “Timing was crucial,” Ammann explained. “Filling up the reservoirs too quickly would mean you could run out of storage space in the reservoirs and have to pass all the water coming in, which could make conditions worse downstream. That is never a situation we want to put ourselves in.”
In the end through daily coordination calls and the operation of storage reservoirs among the various agencies in both countries, flooding in the Lower Columbia and Lower Snake rivers was limited to minor impacts. In addition, flooding was avoided downstream of Libby Dam and the three Treaty dams in Canada. The Pend Oreille subbasin was hit the hardest in the basin, with many areas around Columbia Falls and Kalispel, Montana, experiencing flooding. Much of this flooding was reduced by the dams in that area.
As the weather has dried out, river levels have dropped and project releases have remained a bit higher than normal as some storage dams finish refilling and capture the remaining snowmelt. “This event brought a tremendous amount of precipitation into the basin in a short amount of time, which will mean more water in the rivers this summer and many full reservoirs for recreation. The dams and levees did their job and through the cooperation of many agencies we reduced the impacts of a potentially damaging flood,” said Ammann.