Water levels in Osoyoos Lake are rising as late season snow begins to melt in Canada, causing flood warnings along the Okanogan and Similkameen rivers in the United States.
Gates are wide open at Zosel Dam at Oroville, Wash., according to the Washington Department of Ecology, which operates the dam on the U.S. side of the border.
There is twice as much water coming into Lake Osoyoos than can exit, as the high flows in the Similkameen act as a dam at the confluence with the Okanogan.
“Once Zosel dam gates are wide open, as they have been since late March, the lake must seek its own level when runoff is high throughout the system,” said Al Josephy, with Ecology’s water resources program in Olympia. “This can mean people may see some flooding to property along the lake and down to the site of the dam itself.”
Ecology’s target is to maintain a level of between 911.5 and 912 feet from May 1 to Sept. 15.
Osoyoos is fed from Okanagan Lake and is governed by operations in British Columbia. Making room for snow runoff in the upper watershed puts pressure on smaller Lake Osoyoos.
“We are at 914.04 feet, up by a foot and a half since Friday,” said Josephy on Monday, May 7, 2018. “Last year, which was the highest we recorded in more than a decade, we reached 914.87 on June 2.”
Lake levels are mandated by the International Joint Commission (IJC), a board made up of representatives from the United States and Canada. Osoyoos Lake is a source of irrigation water and summer recreation in both the U.S. and Canada.