Wet month improves conditions at mid-valley reservoirs

Scott Swanson

A wet April has definitely brightened the outlook for Linn County’s two reservoirs, Erik Petersen, Willamette Valley Project operations project manager for the U.S. Army Corps of Engineers, said last week in a virtual presentation on the area’s water situation.

Despite substantial help from recent rain and snow events, Corps water managers are bracing for another challenging year with some of the 13 reservoirs it manages between Detroit and Hills Creek, south of Oakridge.

“Precipitation in the basin has not been great, at least in the south,” Petersen said during the virtual public information session held Thursday, April 28.

He noted that late snowfalls in April have provided “a little more base.”

As of Monday, precipitation in the Willamette basin for this water year is at 97%, but the snowpack is 145% of median. 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.

“It doesn’t influence the system in an incredible way, like help it fill fast, but it helps in the summer months,” he said of the precipitation.

As of April 27, Green Peter was 70% full and Foster was 5%, according to the Corps.

“Foster Reservoir refills from Green Peter storage late every spring, so it should look normal from late May until after Labor Day,” Petersen said.

He acknowledged that the system has been impacted not only by drought conditions but by an injunction handed down by a federal judge that has required the Corps to manage water levels in an effort to optimize passage of young salmon and steelhead from the reservoirs.

“This is a unique year because of the fish operations,” Petersen said.

However, he predicted that recreation at Green Peter and Foster should “look OK” this summer.

“The April rains have made a big difference for us.”

Brian Carroll, Linn County Parks director, said that was encouraging news compared to a couple of weeks earlier, when it looked like Green Peter would be dropping below boat ramp levels during the summer.

“I thought it was actually really good news compared to what we had been anticipating,” he said. “A month ago, we thought we’d be lucky to keep the Whitcomb Creek Boat Ramp until the Fourth for July. Now it looks like it will be up all summer long, which is great news.”

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 the court order.

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.

According to the Corps’ predictions, the water levels in Green Peter are expected to be roughly at 1,000 feet above sea level, the gauge the agency uses to measure those, by mid-May and should stay there until early July, when the lake starts coming down – as it does every year as it fills Foster and provides for downstream communities’ and irrigation needs. Water levels should stay above the 972-foot mark, which is the level at which Whitcomb Creek Boat Ramp can no longer be used, by early September, and should stay above the 922 limit for Thistle Creek Boat Ramp, farther to the west.

At Foster, according to current estimates, water levels should stay high enough all summer to keep the boat ramps at Calkins Park (631 feet), Gedney Creek (619 feet) and Sunnyside Campground (613) usable.

Another issue for Foster is how fast the lake is filled prior to Memorial Day. If the water level goes up quickly, debris in the lake does not tend to go to the shores as easily as it does when the increase is gradual, which makes boating more dangerous.

“I did actually hear some good news on Foster,” Carroll said. “They think they can get Foster up a week before Memorial Day, a week sooner, which might help with some of the debris.”

Also, he said, it doesn’t appear that a lot of large trees have drifted into the lake, unlike winters in which ice storms cause trees to fall into the rivers that feed Foster.

Other area reservoirs don’t look to be in such good shape, Petersen said. Boat access to Hill Creek, Fall Creek and Cougar reservoirs will be limited or non-existent this summer, he said. At Detroit Lake, boat access at Kane’s Marina and the South Shore/Cove Creek ramps should be possible through Labor Day, according to Corps estimates.

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.

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