International Osoyoos Lake Board of Control Spring Conditions Update

Osoyoos Lake levels remained low for much of the 2024-2025 winter, due to low precipitation in the Okanagan basin. Levels are beginning to rise as the lake operator targets summer operational levels to ensure sufficient water for both irrigation and ecological flow needs. Rain and snowfall in late-March brought much-needed water to the basin, but the snowpack remains low following a couple years of persistently dry conditions. Water managers are closely monitoring conditions in the basin and will keep the public informed of management decisions impacting Osoyoos Lake levels.

Osoyoos Lake Levels are Higher than Typical

Lake levels were very low through much of the winter, but now lake operators are intentionally raising lake levels in preparation for the summer. As of April 24, the water level in Osoyoos Lake sits at 911.75 ft (277.90 m), which is higher than is typical this time of year (Figure 1). Though water levels were low throughout the winter, they have remained within the allowable limits as specified by the International Joint Commission's Orders.

Figure 1: 2025 (bright green) and 2024 (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 1987-2024, the mean daily lake level is shown as a dotted black line, and the historical gauged minimum, 10th, 25th, 75th, 90th, and historical gauged maximum lake levels are shaded grey (USGS, IOLBC 2025).

Upstream of Osoyoos Lake, Okanagan Lake is at a higher-than-normal water level elevation. Okanagan Lake serves as a major water storage location, and operators are actively holding water in anticipation of persisting drought conditions. In typical, non-drought years, Okanagan Lake and Osoyoos Lake increase their outflows in the late winter to prepare for the inflow from melting snowpack. Both this year and last, the low snowpack observed in British Columbia led lake operators to hold more water than usual, meaning less water flowed through Osoyoos Lake in winter. Okanagan Lake levels are high because of proactive management decisions, not because there is more water in the Okanagan Basin. Water stored upstream may be released throughout the summer, impacting Osoyoos Lake levels.

Figure 2. Regulated streamflow from Okanagan Lake measured at Penticton (WSC, 2025) (Left) and Osoyoos Lake measured at Oroville (USGS, 2025) (Right). Blue lines indicate the daily average discharge in 2024.

Low Snowpack in the Similkameen and Okanagan Basins, Wet March

Winter 2024-2025 did not produce much snow. In early April, as reported in the BC Snow and Water Supply Bulletin, the Similkameen and Okanagan Basin Snow Indices were at 63 and 82 percent of normal, respectively. Two snow pillow gauges record the amount of water (snow water equivalent) is held in the snowpack. At the Mission Creek station (elevation 1,780 meters or 5,840 feet) in the Okanagan Basin, the April 24 snow water equivalent in the 25^th percentile (1969-2025). In the Similkameen Basin, the Blackwell Peak snow pillow (elevation 1,940 meters or 6,365 feet) is in the 8^th percentile (1967-2025) (Figure 3). March brought higher than normal precipitation to the Okanagan Basin, helping the basin partially rebound from the low winter snowpack.

Figure 3: 2025 Mission Creek (Left) and Blackwall Peak (Right) snow pillow station. Orange lines illustrate the Snow Water Equivalent for the current year (BC River Forecast Center, 2025)

Looking Ahead

The International Joint Commission Order of Approval allows Osoyoos Lake levels to be operated within two elevation ranges during summer months, depending on water availability in the Similkameen and Okanagan Basins. Three criteria are used to determine whether the normal or drought rule curve for Osoyoos Lake levels is enforced.

The first criterion considers the seasonal flow volume in the Similkameen River near Nighthawk, Washington. As of April 24, the April-July 2025 flow volume forecast is 745 thousand acre-feet. This is below the 1 million acre-feet threshold, meaning that the drought criteria are met.

The second and third criteria consider Okanagan Lake. Currently, the forecasted Okanagan Lake April-July net inflow and the June or July peak lake elevation are both below the criteria under which the board can make a drought declaration. The April-July net inflow forecast for Okanagan Lake is 267,000 to 285,000 acre-feet, which is above the 195,000 acre-feet threshold. The water level in the lake is forecast to reach 1,122.8 ft, which is above the 1,122.6 ft threshold. At least one of these criteria, in addition to the Similkameen flow forecast, needs to be met before a drought can be declared under the 2013 Order.

The International Osoyoos Lake Board of Control will closely monitor hydrologic conditions during the spring snowmelt period and communicate with lake operators and the public about drought conditions.

About the Board:

• The International Osoyoos Lake Board of Control oversees the operation of Zosel Dam to manage water levels in Osoyoos Lake, which crosses the British Columbia-Washington state border.  
• In 2024, Osoyoos Lake was operated under the drought rule curve as defined by the International Joint Commission's 2013 Order.
• To contact the board by email, use the Contact form on the board website.
• Subscribe to receive periodic updates on hydrologic conditions of Osoyoos Lake and Zosel Dam operations, annual public meeting announcements, and updates on special board projects.

Contacts

Sarah Dunn, US Secretary

Martin Suchy, Canadian Secretary

International Osoyoos Lake Board of Control