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Corps bracing for another challenging water year in Willamette Basin

PORTLAND DISTRICT
Published April 21, 2022
A boat travels on a lake on a sunny spring day.

Fall Creek Dam's reservoir is currently 8% full, as of April 21 (photo from earlier in the month). The U.S. Army Corps of Engineers, Portland District (Corps) began refill at Fall Creek in mid-April (versus Feb. 1) to improve downstream passage conditions for juvenile salmon and comply with a court order. System-wide reservoir storage is 33% below the rule curve. Year-to-date precipitation in the valley is 96% of normal.

PORTLAND, Ore. – Despite substantial help from recent rain and snow events, Army water managers are bracing for another challenging year as they work to refill 13 Willamette Valley reservoirs for the upcoming conservation season. U.S. Army Corps of Engineers (Corps) officials report the system is 53% full, which is 33% below the rule curve, as of April 21.

“Extended drought conditions, variability of snowpack in the Cascades, and modified dam operations to support downstream passage of juvenile salmon are impacting refill of the system,” said Erik Petersen, Willamette Valley Project operations project manager. “This will be a tough water year with less overall system storage, limiting our supplies to augment for water quality and fisheries habitat downstream during the summer and fall.”

The Willamette Valley Project reservoirs depend on spring and early summer rainfall to refill, and minimal precipitation is making it challenging to fill multiple reservoirs. In addition, the Corps did not begin to refill Fall Creek Reservoir until the middle of April, and it won’t begin refill of Cougar and Foster reservoirs until May to improve downstream passage conditions for juvenile salmon and comply with a court order. 

“We expect that delayed refill will impact recreational access to both Cougar and Fall Creek reservoirs,” said Petersen. “Foster Reservoir refills from Green Peter storage late every spring, so it should look normal from late May until after Labor Day. Detroit and Fern Ridge are a little behind right now, but we see more rain in the near-term forecast, which could help out if it is realized.” 

Diminished water storage during the summer months means that reservoirs with more water may drop in elevation faster than normal as the Corps draws from them to meet downstream needs.

To help explain the situation, Corps staff will host a virtual public information session April 28, 4-5 p.m.

The Corps invites the public to attend the session to learn more about current operations, future forecasts, and potential impacts to the Willamette Valley System.

Date: Thursday, April 28, 4-5 p.m.

Link: https://usace1.webex.com/meet/erik.s.petersen

Call: 1-844-800-2712 (US) (Call-in toll-free number)

Access Code: 1999-18-2318 #

The Corps encourages questions but asks participants to send questions using the chat function in WebEx during the call.

Year-to-date precipitation across the Willamette was 96% of normal, as of April 21. The snowpack is 128% of median for the Willamette. Snowmelt helps keep reservoir elevations up in the summer if it lasts and matches outflows, but it only accounts for less than 10% of the system’s storage.

The Corps manages reservoir inflows based on a water control diagram, more commonly called the “rule curve,” which is the authorized maximum elevation on a given day to balance flood risk management and storage for other authorized purposes, such as recreation, hydropower, and irrigation supply. The Willamette Valley System’s reservoirs are kept lower in the winter to reduce downstream flooding and are refilled in the spring to prepare for recreation and adequate flows for fish and water quality.

Portland District encourages the public to check its “teacup diagrams” before heading out to recreate. These diagrams show water elevations for Corps-managed reservoirs. Willamette River Basin teacup diagram: https://www.nwd-wc.usace.army.mil/nwp/teacup/willamette/ or http://pweb.crohms.org/nwp/teacup/willamette/.