By Don Urquhart, Times Chronicle
The international organization overseeing the management of Osoyoos Lake levels has declared drought conditions for the lake which means water levels will be raised in a bid to capture as much spring runoff as possible for what is expected to be a long hot dry summer ahead.
The declaration was made by the International Osoyoos Lake Board of Control (the Board) under the authority granted it by the International Joint Commission’s (IJC).
As such the Washington State Department of Ecology (Ecology), which is responsible for managing lake levels within the ranges established in the IJC Orders, will reduce the outflow of water from the Zosel Dam near Oroville, Washington accordingly.
The Ecology deptartment has raised Osoyoos Lake to around 912.0 feet (277.98 metres), the Board noted, adding that In the coming days, “managers will likely continue to raise the lake to capture spring runoff.”
Water year 2026 (bright green) and water year 2025 (dark green) Osoyoos Lake Levels, along with allowable range under the normal rule curve defined by the International Joint Commission Orders of Approval for Osoyoos Lake (solid black). Expanded drought Conditions rule curve (red dashed). For the period of record from 1987-2025, the mean daily lake level is shown as a black line, and the historical gauged minimum, 10th, 25th, 75th, 90th, and historical gauged maximum lake levels are shaded grey.
USGS, IOLBC chart
The Board will continue to monitor lake and river conditions and if hydrologic conditions improve, it may rescind the drought declaration and direct Ecology to return lake levels to under the normal operating level of 912.0 feet (277.98 metres).
Earlier, on April 8, Washington State issued a statewide drought declaration due to low snowpack which it termed “dismal” and concerns about summer water supply. The BC River Forecast Centre’s May 1 Snow Survey and Water Supply Bulletin indicated elevated drought risk rather than flood risk for the Okanagan Valley.
According to the snow survey, the Okanagan basin had only 31 per cent of normal snowpack on May 1, which is the lowest on record since 1980, while the Similkameen basin had 61 per cent of normal snowpack on May 1.
Added to this is the fact that lower‑elevation snowpack has already melted, weeks earlier than typical. And upstream, Okanagan Lake is also being managed to store water and sustain basic environmental flows in preparation for a dry summer, the Board noted.
In dry years, the IJC allows Osoyoos Lake to be managed within a wider range of lake levels than normal. “The wider range affords greater flexibility in lake level management to adaptively meet the demands of ecological stream needs, irrigation and recreational users,” the Board said in a statement.
A drought declaration is triggered when multiple hydrologic indicators fall below specific thresholds, including:
- volume of flow in the Similkameen River from April 1 to July 31;
- net inflow into Okanagan Lake from April 1 to July 31;
- peak Okanagan Lake level in June or July.
Drought is declared when the Similkameen indicator as well as at least one of the two Okanagan Lake indicators fall below the thresholds.
The Board noted that current forecasts show that all three of the drought criteria are likely to be met, prompting it to authorize Ecology to operate the lake within the drought range this spring, summer, and fall.
Water supply in the Similkameen River for the period from April 1 to July 31 is currently forecast by the Amercian National Oceanic and Atmospheric Administration (NOAA) to be 1.029 million acre-feet (1.27 billion cubic metre), within the range of uncertainty for the 1-million-acre-foot drought threshold.
The Board also noted that the forecast has decreased over the last several days, as snow at lower elevations melts out entirely.
The net inflow to Okanagan Lake for the April to July period is currently forecast at 126,000 acre-feet, (155 million m³) which is below the 195,000 acre-feet (240 million m³) drought threshold.
Additionally, Okanagan Lake is expected to reach a peak elevation of 1,122.2 ft (342.05 m), which is below the 1,122.6 ft (342.17 m) drought threshold.
