Posted on May 15, 2023
Beginning in June, the Portland District, U.S. Army Corps of Engineers will begin to gradually drawdown Lookout Point and Green Peter reservoirs to historically low levels.
The goal of these reservoir “drawdowns,” which are part of an injunction ordered by the U.S. District Court for the District of Oregon, is to increase juvenile spring Chinook and steelhead survival and passage through the reservoirs and past the dams.
As part of their lifecycle, Chinook and steelhead salmon hatch in the freshwater of the river and then migrate downriver to the ocean, where they stay several years before returning to the river from which they hatched. Lookout Point and Green Peter dams, which were constructed primarily for flood risk reduction, block the river routes salmonids would take during their lifecycle.
“Salmon are surface-oriented fish, so we’re hoping they’ll key into the increased surface outflows from the reservoir [during the drawdown] and swim downstream, where they need to go,” said Kathryn Tackley, a physical scientist at the Portland District who is working to implement injunction measures.
The reservoirs will be drawn down gradually but consistently throughout the summer to reach the levels outlined in the injunction.
Drawdown operations will reduce Green Peter’s reservoir to a surface elevation of 780 feet – about 120 feet lower than it’s been drawn down since construction. At Lookout Point, the Corps will take the reservoir down to an elevation of 750 feet, which is 80 feet lower than any previous drawdown since construction.
Both reservoirs are expected to reach their target elevations in early or mid-November, after which the Corps will hold lake levels at those elevations for one month. While the typical season to refill reservoirs begins on Feb. 1, the Corps will begin filling Green Peter and Lookout Point reservoirs to normal winter flood risk elevations on Dec. 16 to offset the deep drawdowns’ impact to reservoir levels.
Both reservoirs will refill according to rule curve – the measure which experts use to determine “flood storage” in the reservoirs – in the spring and will be held relatively full (just above spillway crest) until the following summer’s drawdown begins.
“It’s important to note that while this is the current 2023 plan, operations may be refined in future years as the Corps learns more about fish passage and survival, and the benefits from the drawdown operations,” said Tackley.
Officials at the Corps advise the public that these court-ordered measures will eliminate the use of local boat ramps from late August/early September until February. Portland District encourages the public to check its “teacup diagrams” before heading out to recreate: These diagrams show water elevations for Corps-managed reservoirs.
The Corps has also outlined several dangers for those recreating in the reservoirs or downriver of the dams:
- Anyone recreating in the reservoirs should always stay a safe distance (200 feet or more) from the upstream side of the dam where the regulating outlet draws in water from the reservoir and sends it downriver
- Downstream recreators should use caution during reservoir drawdowns as suspended sediment and woody debris are likely to be dislodged, causing low visibility and difficulties in seeing submerged obstacles
- The lower reservoir levels may uncover unknown hazards as the lakebed is exposed
“The lower exposed areas of the lakebed may be soft and muddy,” said Christie Johnson, a Corps park ranger. “I know of several incidents in the past where people and pets have gotten stuck in the mud and required rescue.”
During the deep drawdowns, the Corps will continue its mission to protect cultural and historical sites on federal land, and visitors are prohibited from collecting, excavating, removing, damaging or disturbing resources (e.g., artifacts, sites or resources of any kind, including rocks) from the lakebed.
BACKGROUND
On Sept. 1, 2021, the U.S. District Court for the District of Oregon issued an interim injunction that requires the Corps to undertake specified actions to improve fish passage and water quality at several Willamette Valley Project dams for the benefit of Upper Willamette River spring Chinook salmon and winter steelhead salmon, both of which are listed as threatened under the Endangered Species Act. The Corps is working in coordination with the National Marine Fisheries Service, U.S. Fish and Wildlife Service, Bonneville Power Administration, and U.S. Bureau of Reclamation to implement the injunction measures, some of which began in fall 2021.
The interim injunction describes the specific measures and is available for viewing here.
