May 29, 2020 - 2020 freshet update: Osoyoos Lake expected to rise again in early June
In compliance with the terms of the International Joint Commission (IJC) water management Order for Osoyoos Lake implemented by the International Osoyoos Lake Board of Control (https://www.ijc.org/en/olbc), the gates at Zosel Dam have been fully open since May 11 to allow for maximum outflow from Osoyoos Lake during melting of the snowpack in the Okanagan Basin. The IJC Order mandates the range of allowable Osoyoos Lake water levels, which should be maintained when possible between 911 and 912 feet from May 1 to September 15. During normal conditions, Osoyoos Lake levels are regulated at Zosel Dam by the Washington State Department of Ecology. During periods of high runoff the Order recognizes that lake levels will at times exceed this range, and directs that Zosel Dam operate to return to required levels as soon possible.
As of May 15, the Okanagan Basin snowpack index is 99% of normal and a significant snowpack remains at high elevations. Inflow to Osoyoos Lake from the Okanagan River is largely controlled by releases from Okanagan Lake Dam, which are anticipated to remain above normal due to high projected inflows to Okanagan Lake. Given current conditions, lake level in Osoyoos Lake is no longer controlled by Zosel Dam, but depends on both inflow to and outflow from Osoyoos Lake. At this point, a larger part of the lake level rise is attributable to a backwater effect from high flow on the Similkameen River, which restricts outflow from Osoyoos Lake.
As with the Okanagan Basin, the snowpack index in the Similkameen Basin is above normal this spring (currently 103% of normal as of May 15). The Similkameen River is not regulated by a dam and can therefore increase rapidly in response to warming temperatures and rainfall events. Rapid melting of the Similkameen Basin’s snowpack during warm weather resulted in increased discharge of the Similkameen River in mid-May, which peaked on May 19 at 17,100 cubic feet per second. High discharge of the Similkameen River contributed to reduced outflow from Osoyoos Lake and an increase in the level of Osoyoos Lake, which reached 913.01 feet on May 22.
As of May 29, warm weather and weekend precipitation is forecasted to contribute to additional snowmelt within the Similkameen and Okanagan Basin snowpacks, which may contribute to increased levels of Osoyoos Lake in early June.
The International Osoyoos Lake Board of Control will continue to monitor inflows to Osoyoos Lake and flows on the Similkameen River and ensure compliance of Zosel Dam operations with the IJC Order. Real-time lake levels are publicly available on the IOLBC’s website at https://www.ijc.org/en/olbc.