IOLBC News Release: Higher Lake Levels in Anticipation of Lower Spring Runoff

Osoyoos Lake levels are being maintained at a higher-than-normal level for this time of the year in anticipation of lower spring runoff due to lower-than-normal snowpacks. As of April 9, the Osoyoos Lake elevation is 911.81 feet (277.92 meters) and is at the historical maximum for the 1987-2023 period of record for this time of year (Figure 1). Lake levels have remained within the International Joint Commission (IJC) mandated rule curve in 2024 and the Zosel Dam owner/operator (Washington State Department of Ecology) has remained in compliance with the IJC orders.

Figure 1: 2024 Osoyoos Lake Levels (bright green) and allowable range under the normal rule curve defined by the International Joint Commission Orders of Approval for Osoyoos Lake (solid black).  For the period of record from 1987-2023, the mean daily lake level is shown as a dotted black line, and the historical gauged minimum, 10^th, 25^th, 75^th, 90^th, and historical gauged maximum lake levels are shaded grey (USGS, 2024). 

Upstream of Osoyoos Lake, Okanagan Lake is also at a higher-than-normal elevation. Discharge from both Okanagan Lake and Osoyoos Lake is typically increased in late winter to prepare for the inflow from melting snowpack (Figure 2). However, due to the low snowpack observed across British Columbia this winter, lake managers have decided to counteract the potential impacts of a low runoff year by holding more water than usual in the lakes.

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

In early March, as reported in the BC Snow and Water Supply Bulletin, the Similkameen and Okanagan Basin Snow Indices were at 62 and 80 percent of normal, respectively.  Snow Basin Indices are now 62 and 73 percent of normal, respectively. The snow pillow gauge at Mission Creek (elevation 1,780 meters or 5,840 feet) in the Okanagan Basin has recorded below-average snow water equivalent for all the 2024 water year. The water in the snowpack is currently around the 10^th percentile (Figure 3). The snow pillow station at Blackwell Peak (elevation 1,940 meters or 6,365 feet) in the Similkameen basin has experienced near historical recorded lows for snow water measurement since mid-November. The snow water equivalent is currently near the historical recorded minimum for April 9 (Figure 3).

Figure 3: 2024 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, 2024) 

As of April 9, the April-July 2024 flow volume forecast for the Similkameen River near Nighthawk, Washington is 685 thousand acre-feet, which is below the Condition 8(a) Drought Criteria of 1 million acre-feet specified in the IJC Order of Approval.  

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. 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 has notified the Washington Department of Ecology that the Order that Osoyoos Lake can be operated under the drought rule curve as defined by the 2013 Order. The drought rule curve will remain in place until November 1, 2024, or until the Board determines the conditions no longer meet the criteria of a drought year.