Osoyoos Lake Level Rising During Spring Snowmelt


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 (http://www.ijc.org/en_/iolbc), the gates at Zosel Dam have been fully open since March 26 to allow for maximum outflow from Osoyoos Lake during melting of the snowpack in the Okanagan Basin.  As of May 1, the Okanagan Basin snowpack reached 206% percent of normal this Spring. 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 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. 

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, but during periods of high runoff the Order recognizes that lake levels will at times exceed this range, but directs that Zosel Dam operate to return to required levels as soon possible.

As with the Okanagan Basin, snowpack in the Similkameen Basin is also above normal this year. Flow on 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 the fourth week of April, which reduced outflows from and increased the level of Osoyoos Lake. For reference, in Spring 2017, Osoyoos Lake level also exceeded mandated lake levels due to high rates of discharge and reached a maximum level of 914.87 feet on June 2, 2017. The lake level exceeded this elevation on May 9, 2018 and localized flooding of low elevation lakeshore areas is occurring. The maximum level of Osoyoos Lake recorded since regulation was 917.11 feet on June 2 and 3, 1972. The maximum water level that will be reached on Osoyoos Lake during the 2018 freshet is difficult to predict due to its dependence on changing weather conditions and resulting rates of snowmelt.

At this time, lake-shore residents, property owners and others who might typically be impacted by higher spring lake levels should take precautions to reduce the risk of impacts due to high water levels on Osoyoos Lake (contact local municipality, town hall, or regional district/country office for emergency preparedness information).  The Town of Osoyoos declared a local state of emergency on May 9.

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 http://www.ijc.org/en_/iolbc



Bruno Tassone
International Osoyoos Lake Board of Control
Chair, Canadian Section

Cindi Barton
International Osoyoos Lake Board of Control
Chair, U.S. Section