Frequently Asked Questions

The Board was appointed by the International Joint Commission (IJC) under condition 14 of the Commission’s 2013 Supplementary Order of Approval for Zosel Dam and Osoyoos Lake. Its duties are to ensure compliance with the Order, and keep the Commission informed of all matters related to the Order. These duties include invoking and terminating drought condition operations as appropriate. Board members serve at the discretion of the Commission.

The State of Washington is the Applicant. The State owns and operates Zosel Dam, which is located about 1.6 miles (2.6 kilometres) downstream of the outlet of Osoyoos Lake. Under most conditions, the dam can be used to control the lake’s water level.

The 2013 Supplementary Order states that the Order shall be subject to review no later than 25 years following the date of its adoption (29 January 2013), or as otherwise determined by the Commission.

The original dam was built by Zosel Lumber to create a pond in which to sort and move logs. Over time, people became accustomed to the controlled water level in the lake. The original dilapidated dam was rebuilt in 1988. Water stored in Osoyoos Lake is used primarily for irrigation, recreation, fisheries and ecosystem interests.

The IJC’s Order calls for the lake to be held between 909.0 and 911.5 feet in the winter and between 911.0 and 912.0 feet from 1 June to 15 September in a normal summer. In a drought summer, the water level must be held between 910.5 and 912.5 feet from 1 June to 15 September. Lake users and residents have a variety of opinions on optimum lake levels. The levels were chosen based on the public and government input that the IJC received prior to writing the 2013 Order.

A drought summer is declared during a year of drought as determined by the Board of Control in accordance with the criteria (a) and either (b i) or (b ii) below.  The requirement in the 2013 Supplemental Order that both criteria a) and either criteria b i) or b ii) be met is different than the previous Order that only required that one of the criteria be met.

The criteria are:

a) the volume of flow in the Similkameen River at Nighthawk, Washington for the period April through July as calculated or forecasted by United States authorities is less than 1.0 million acre-feet


b i) the net inflow to Okanagan Lake for the period April through July as calculated or forecasted by Canadian authorities is less than 195,000 acre-feet


b ii)  the level of Okanagan Lake fails to or is forecasted by Canadian authorities to fail to reach during June or July elevation 1122.6  feet Canadian Geodetic Survey Datum.

The Okanagan River system experiences a natural spring freshet that may occasionally overwhelm the discharge capacity of Zosel Dam, by high flows in either the Okanogan or Similkameen rivers (or both). Large freshets can sometimes lead to lake levels that may significantly exceed 913 feet. During winter, lake levels are drawn down to protect shoreline structures from ice damage.

Historically, the lake level has been known to range from 908.8 feet (1929) to 919 feet (1894).  Hydrological analyses indicate that the level of Osoyoos Lake has, and probably will again, exceed elevation 913 feet every other year for a duration varying from two days to two months. The probable recurrence interval of the lake level exceeding elevation 915 feet is, on average 10 years. In 1972 Osoyoos Lake level peaked at elevation 917.1 feet.

Find out more information on natural factors that affect Osoyoos Lake levels.

Condition 13 of the 2013 Supplementary Order states that lake levels are to be measured at the international gauging station known as Osoyoos Lake near Oroville and expressed in terms of the NGVD 1929 datum which is equivalent to the US Coast and Geodetic Survey (USCGS) datum that was used in previous Orders. This datum is 0.26 feet lower than the Geodetic Survey of Canada (GSC) datum north of the border. For example, lake elevation 911.00 NGVD 1929 near Oroville equals elevation 910.74 feet CGS near Osoyoos.

The Osoyoos Lake near Oroville gauge is operated by the US Geological Survey. Access real-time water levels.

Condition 10 of the 2013 Supplementary Order provides that the Commission may allow a temporary deviation from the prescribed lake levels upon written advice and recommendation from the Board of Control. The Board has been provided information indicating that lowering the lake level is unlikely to eradicate milfoil. To be successful, the lake would need to be lowered significantly below 909 feet during a prolonged freeze. Lowering the lake level below 909 feet would have associated risks. Water lines into the lake could freeze. Important riparian habitat could be adversely affected. And importantly, it could be difficult to refill the lake to 911 feet in the spring during dry years. Based on the information at hand, the Board is not convinced that the benefits (or likelihood of success) would outweigh the risks.

Climate change may result in more winter precipitation falling as rain rather than snow. This would result in losing the ability to store water in the snowpack and higher streamflows in the winter. Additionally, climate change could lead to a two to three week advance in the spring freshet. However, runoff forecasts for individual years are based on observations of snowpack, weather and antecedent conditions for that particular year. Forecasts can vary significantly from year to year even in the absence of climate change. The 2013 Supplementary Order permits additional storage of water after April 1st during drought years.

No. However, the Province of British Columbia attempts to keep cross-border flows above 100 cubic feet per second (cfs) when possible. In particular, this assists the Washington State Department of Ecology with releasing at least 100 cfs from Zosel Dam for enhancing fish migration in the short distance between the dam and mouth of the Similkameen River.

The channel capacity is of concern in the 1.6-mile (2.6-kilometre) reach between the lake’s outlet and Zosel Dam. In this reach, Tonasket Creek enters the Okanogan River. A bar occasionally develops at the creek outlet that can constrict the river. If the bar is interfering with the flow at the outlet of the lake, Zosel Dam can no longer control lake levels efficiently. In such a circumstance, the channel may require dredging. The Applicant confirms the channel capacity through timely channel surveys and observed flows.

No, the Orders are directed to the State of Washington’s maintenance and operation of Zosel Dam. The most appropriate way to ensure good water quality is to control the quality of the runoff that enters the lake.