Dworshak looking at status quo on flood operation

Flood control operations at Dworshak Dam are not expected to change under the updated Columbia River Treaty between the U.S. and Canada, despite the United States taking on more responsibility to manage high spring flows.

The two countries reached an agreement in principle earlier this year that outlines how flood risk, hydropower operations, and fish and wildlife flows will be managed for the next several decades. Even though the treaty revamp has yet to be finalized, its new flood risk management provisions will be used to manage spring runoff in 2025.

For the past 60 years, the treaty committed Canada to storing up to 8.9 million acre-feet of water to reduce flood risk at places like Portland, Ore., the Tri-Cities and Vancouver, Wash. But that commitment expired in September and, if the treaty had not been updated, it would have required the U.S. to manage all of its own flood risk by significantly lowering its reservoirs throughout the Pacific Northwest.

Under the new terms, Canada is committed to storing 3.6 million acre-feet each spring.

To make up the difference, Lake Roosevelt behind Grand Coulee Dam and the Columbia River behind John Day Dam will be drawn lower during flood years than they have been over the past six decades. Spring flows generally peak sometime between April and May.

Grand Coulee Dam, west of Spokane and north of Moses Lake, stores a large amount of water and produces a massive amount of hydropower, compared to other dams in the Columbia River system south of the U.S.-Canada border.

“Grand Coulee will likely experience deeper and longer duration drawdowns in some years,” said Steve Barton, chief of the Army Corps of Engineers Water Management Division during an online presentation Wednesday. “Flows downstream of Grand Coulee will generally remain the same.”

The dam can store 82 feet of water. Flood risk managers will draw Lake Roosevelt down about 7 feet lower than normal during moderately wet years. During dry and average years and during the wettest years, Grande Coulee and Lake Roosevelt will be operated similarly to the way it was before treaty modernization. Drawdowns during the wettest years, which occur about 10% of the time, won’t change much because former flood control rules required Lake Roosevelt to be lowered dramatically when spring runoff was expected to reach flood levels.

John Day is not routinely relied on to provide flood control storage even though it can be lowered as much as 9 feet to capture runoff. But it will become more common.

“Before September 2024, this operation was needed approximately 10% of the years. Moving forward, we expect to see this in about 20% of years,” Barton said.

During high flows, river transportation companies may be required to use more tug boats to move barges, increasing costs. But flood control operations are not expected to disrupt commerce on the Columbia River.

Tom Conning, a spokesperson for the corps said Dworshak Dam and Reservoir will continue to perform much as it has in the past.

“That’s mainly because we already fully use the reservoir’s storage for flood risk management during wet years,” he said.

Nor will treaty modernization lead to changes at lower Snake River dams.

“Operations of the lower Snake River dams are not affected by Columbia River risk management operations. Likewise, they have no storage aside from their operating pools, and thus have no bearing on the main stem flood risk management,” said Gen. William C. Hannon Jr.

Eric Rothwell of the Bureau of Reclamation said increased flood control drawdowns could briefly affect recreation by leaving some boat ramps on Lake Roosevelt high and dry; making it more expensive to pump irrigation water from the river upstream of John Day Dam and from Lake Roosevelt to Banks Lake that feeds the Columbia River Project in central Washington; and exposing inundated Native American cultural sites, making them vulnerable to looting and damage from erosion.

Higher flows resulting from the treaty changes could increase total dissolved gas levels below dams. High gas levels can harm fish.

Barker may be contacted at ebarker@lmtribune.com.