Spring Freshet Update: Okanagan/Okanogan and Similkameen basins
The spring freshet in the Okanagan/Okanogan and Similkameen basins has commenced as snowpacks have begun to melt due to above normal temperatures in the last week.
The snow pillow gauge at Mission Creek (elevation 1,780 meters or 5,840 feet) in the Okanagan basin was above normal for this time of year, but due to recent warming and precipitation has returned to normal conditions. The gauge at Blackwell Peak (elevation 1,940 meters or 6,365 feet) in the Similkameen basin is well below normal, around the 10th historical percentile (Figure 1).
Figure 1: 2023 Mission Creek and Blackwall Peak snow pillow station. Orange lines illustrate the Snow Water Equivalent (SWE) for the current year (BC River Forecast Center, 2023)
The snowpack at both stations has been decreasing rapidly, contributing to the increase in river discharge and lake levels.
As of May 8, Osoyoos Lake daily mean elevation is 912.09 feet (278.00 meters). The current level is above normal and remains above the range of allowable lake levels defined by the International Joint Commission (IJC) Orders of Approval for Osoyoos Lake (Figure 2). However, there has been a downward trend in lake level since May 8 and the Zosel Dam control gates are fully open to allow lake levels to return to allowable levels.
Figure 2: 2023 Osoyoos Lake levels (green) and allowable range under the normal rule curve defined by the IJC Order of Approval for Osoyoos Lake (solid black).
As of May 8, the April-July 2023 flow volume forecast for the Similkameen River near Nighthawk, Washington, is 1,055 thousand acre-feet, which is above the Condition 8(a) Drought Criteria of 1 million acre feet specified in the Order. Before May 3, the April-July volume was forecasted to be below the 1 million acre-feet threshold.
The forecasted May-July Okanagan Lake net inflow and June or July maximum elevation are both above the Condition 8(bi) and 8(Bii) Criteria. At least one of these criteria, in addition to Criteria 8(a), needs to be met before a drought can be declared under the 2013 Order. As it stands, the International Osoyoos Lake Board of Control will notify the Applicant to the Orders (Washington Department of Ecology) that Osoyoos Lake must be operated under the normal rule curve as defined by Condition 7 of the 2013 Order for the current year.
Please monitor the Osoyoos Lake Board of Control website for further hydrologic condition updates as the spring progresses. Real-time Osoyoos Lake levels from the US Geological Survey are available under “Watershed/Lake Level Status and Trends."