Runoff will impact lake and riverside communities
By Ann McCreary
The Okanogan River will run higher this summer than it has in two decades, regional water managers predict.
Communities along the Okanogan River from Oroville to Brewster are likely to experience high water along the shoreline throughout the summer and into September, due to greater-than-normal snowmelt from watersheds in Canada.
“Property owners with low-lying lawns and fields and folks who recreate in the river should take heed and prepare for the highest river levels we’ve seen in 20 years,” said Al Josephy with the Washington Department of Ecology’s water resources program. “Flows could be two or three times greater than what we usually experience in the summer months.”
An unusually high snowpack in British Columbia is resulting in volumes of runoff not seen since 1997. That is putting tremendous pressure on Canadian water managers to maintain optimal levels in the Okanogan system of lakes and reservoirs this summer, Josephy said.
Osoyoos Lake will likely remain above normal operating levels for much of the summer, he said. This may mean periods of flooding along lake properties and further downstream because of extreme flows expected this summer.
The lake straddles the United States and Canada at Oroville and hosts summer homes, recreational activities, and supplies irrigation water on both sides of the border.
Gates at Zosel Dam, which regulates Osoyoos Lake levels, will remain open as much as possible to relieve water backed up in the system
The level of Osoyoos Lake has been rising since late April when increasingly warm temperatures began to melt the Okanogan River basin’s deep snow pack, which exceeded 150 percent of normal
Snowmelt has also contributed to high water levels in reservoirs upstream of Osoyoos Lake including Okanogan Lake, which is at its highest level since construction of the current Okanogan Lake Regulation System in the early 1950s.
Working on water levels
Water managers will work all summer to try and steady water levels at Osoyoos Lake and bring them within normal parameters, Josephy said. Water levels are mandated by an International Joint Commission (IJC) made up of representatives of the United States and Canada.
Allowable levels are between 911 and 912 feet from May 1 through Sept. 15. During normal conditions, Osoyoos Lake levels are regulated at Zosel Dam by the Washington State Department of Ecology, but during periods of high runoff lake levels are allowed to exceed this range, according the IJC.
Since Zosel Dam’s construction in 1987, Osoyoos Lake levels have exceeded 912 feet six times, including reaching as high as 915.09 feet in May 1997.
Given that this year’s snow index level for the Okanogan and Similkameen watersheds are 30-40 percent higher than 1997, water managers said it is possible that water levels on Osoyoos Lake could exceed the high water levels experienced in 1997, depending on the persistence of current high temperatures.
The gates at Zosel Dam have been fully open since late April to allow for maximum outflow from Osoyoos Lake. With Zosel Dam no longer restricting outflow from the lake, the level of Osoyoos Lake depends on inflow from the Okanogan River upstream and the level of the Similkameen River which, at high flows, can back up water flowing out of Osoyoos Lake, IJC officials said.
Inflow to Osoyoos Lake from the Okanogan River is largely controlled by releases from the Okanogan Lake Dam, which are anticipated to remain above normal through the summer due to current inflows to Okanogan Lake, an above normal snowpack in the contributing basin, and the resultant high level of Okanogan Lake.
Snowpack in the Similkameen basin is similarly above normal, but flow on the Similkameen River is not regulated by a dam and can increase rapidly in response to warm temperatures and rainfall events.