Portland, Ore. --
After a historically dry start to the refill season, water managers with the Portland District, U.S. Army Corps of Engineers (Corps) are now optimistic about refilling most Willamette Valley reservoirs after increased rainfall and snowmelt in April.
To help explain the nuances of this water year, including refill and the conservation forecast, Corps staff will host a virtual public information session May 10, noon to 1 p.m., and May 11, 5-6 p.m.
“The last few years have been a hydrologic roller coaster, and this year is no exception,” said Salina Hart, the Portland District chief of the Reservoir Regulation and Water Quality Section. “Spring rains are essential for refilling reservoirs and fortunately, we saw a substantial improvement in April.”
The Corps reports the Willamette Valley system of dams and reservoirs is 62% full as of May 2.
Weather is one critical factor among a host of other criteria that influence the way the Corps approaches water management in the Willamette Valley system.
In February, water managers begin the “delicate dance” of balancing the many Congressional purposes of both the reservoirs and the dams that impound them.
One primary purpose of the system’s dams and reservoirs is flood risk management: Water managers must keep reservoir elevations low to maintain storage space in the reservoirs through the spring to capture seasonal rains and prevent flooding downriver. This must be balanced with what sometimes seems to be conflicting purposes: refilling the reservoirs for irrigation, hydropower generation, water quality improvement and recreation.
The balancing act doesn’t end there: The Corps must also meet minimum water releases according to the requirements of the 2008 Biological Opinions, which define minimum releases, or “beneficial flows,” from the reservoirs to improve downstream habitat for endangered species.
Additionally, this year, water managers will continue to operate under the constraints of a 2021 court injunction that aims to improve conditions for endangered species. The injunction orders have called on the Corps to delay reservoir refills this spring at Cougar and Fall Creek and begin early fall drawdowns of the reservoirs at Lookout Point and Green Peter, which will impact recreational access at all those reservoirs as the reservoirs release water sooner to provide fish passage.
Water managers forecast that Foster Lake will remain full for Memorial Day through Labor Day. Several smaller reservoirs are currently full or nearing full levels. The remaining larger reservoirs, which are seeing improvement given the April rainfall and some snowmelt, are expected to fill, although later than typical. Reservoir refill levels for the Willamette Valley can be found at https://www.nwd-wc.usace.army.mil/nwp/teacup/willamette/
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: Wednesday, May 10, noon to 1 p.m., and Thursday, May 11, 5-6 p.m.
Call: 1-844-800-2712 (US) (Call-in toll-free number)
Access Code: 1999-18-2318 #
The Corps encourages questions during the meeting and asks participants to either send questions using the chat function in WebEx during the virtual meeting or email the Portland District Public Affairs Office ahead of the meeting at firstname.lastname@example.org.
Willamette Valley Project water year overview:
Year-to-date precipitation across the Willamette was 92% of median, as of May 2. The snowpack is 236% of median (24 inches) for the Willamette. Snowmelt accounts for less than 10% of refill in the system and helps keep reservoir elevations up in the summer – but only if the inflow of the snowmelt to the reservoir matches outflows from the reservoir. This year, given the late season melt, reservoirs that do have heavier snowmelt influence, like Detroit, will see additional refill benefit.
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 Endangered Species Act compliance 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: https://www.nwd-wc.usace.army.mil/nwp/teacup/willamette/ or http://pweb.crohms.org/nwp/teacup/willamette/.