As part of the Columbia River Basin Agreement, the U.S. Army Corps of Engineers will send more water over spillways instead of through turbines during its annual “spring spill” at dams on the lower Snake and Columbia rivers.

The Corps will also expand spills in the fall and early spring.

“This is an important component of the agreement with Tribes, states, and other parties to implement a long-term, durable path forward,” Tim Dykstra, Northwestern Division Fish Policy lead, said in a press release. “We’ve been using spill as an important tool for fish passage since the mid-1990s to improve migratory conditions for juvenile salmon and steelhead and we will evaluate the impacts of the increased spill to inform our decisions in the future.”

Spill operations targeting juvenile salmon and steelhead generally start April 3 on the lower Snake River and April 10 on the lower Columbia River. The Corps uses the operations to decrease the time it takes juvenile salmon and steelhead to move through the system of dams to the Pacific Ocean as well as provide another non-turbine passage route. Many of the dams also have alternative ways to move past the dams.

Four projects on the lower Snake River and McNary Dam on the Columbia River on March 1 began spilling four hours each day through surface passage routes to primarily assist migrating adult steelhead.

Beginning March 21, those projects, plus John Day Dam on the Columbia River, will begin spilling through surface passage routes 24 hours each day until regular spring spill begins with the goal of providing additional benefits for both juvenile and adult salmon and steelhead.

Spring spill levels vary at each of the eight dams, but all projects have an upper limit that protects water quality from high levels of total dissolved gas, or TDG, according to the Corps. Even though this additional water is valuable for fish passage, managers must reduce spill amounts to keep from exceeding 125% TDG limits that state water quality agencies set, and EPA approved in Washington, which adds more intricacy to water management in the basin.

Under the litigation agreement:

• Bonneville, McNary, Ice Harbor, Lower Monumental and Lower Granite dams will spill up to the 125% TDG levels 24 hours per day.

• The Dalles Dam will spill 40% of the river over the spillway.

• John Day Dam will spill 40% of the river over the spillway during the day and up to the 125% TDG levels at night.

• Little Goose will also spill 16 hours per day up to the 125% TDG levels and reduce spill for eight hours to performance standard spill levels, or 30% of the river. This is to benefit adult fish passage.

High levels of spill can hinder adult passage as it can confuse fish, making it difficult to locate fish ladders, said Tom Conning, public affairs specialist for the U.S. Army Corps of Engineers Northwestern Division.

“The agreement contemplates the potential of impacts to adult salmon and steelhead and has provisions to temporarily decrease spill in response,” he said.

Increased spill could also pose health risks for fish in the system due to the production of TDG in the water.

“We will monitor the impacts and make adjustments with regional input to mitigate those impacts,” Conning said.

The increased spill does not change the volume of water moving through the system, only the route through which water is passed, i.e. turbine vs. spillway, Conning said.

Any pool restrictions under the agreement fall within normal operating ranges and are adequate to provide continued irrigation opportunities.

The agreement does not pose any additional risk for barge operators.

“We are committed to maintaining safe navigation conditions to allow passage of barges and other vessels on the Columbia River System,” he said. “As has been done in the past, we modify dam operations, as needed, to provide safe conditions as vessels approach our projects and enter or exit our navigation locks.”