The lake level is lowered to a winter level largely to protect shoreline facilities and property along the lake frontage from freezing and flooding.
As recreational activity on the lake begins to slow down in the fall, Ecology traditionally lowers the level of the lake from its summer (March 1-September 15) levels of 911.5 feet to its winter operational level of around 909.5 feet. These levels are mandated under orders of the International Joint Commission.
To track the progress of lake levels in “real-time” as well as find additional information go to the U.S. Geological Survey web page for Osoyoos Lake
Brief Chronology of Zosel Dam's History - 09/25/2001 International Joint Commission
Osoyoos Lake Water Quality Society - Dams and the Okanogan River
USGS Washington Water Science Center
Water Office Canada Real-time Hydrometric Data
National Weather Service
Zosel Dam, the dam in the U.S. that currently maintains Osoyoos Lake levels, was initially built in 1926 by William Zosel. Mr. Zosel owned a sawmill along the Okanogan River in the town of Oroville, WA, and needed a millpond for the storage of sawmill logs. Mr. Zosel received the necessary permissions to build the dam, but it appears that no consideration was given at the time to potential impacts of the dam on Osoyoos Lake levels in Canada. Operation of the dam at this time caused fluctuations in lake levels and lakeshore flooding.
By the early 1940s, local residents complained about lake conditions to the International Joint Commission (IJC), which had been created as a result of the Boundary Waters Treaty of 1909 to adjudicate transboundary water-body disputes. In 1946, the IJC issued its first Order of Approval for Zosel Dam, which established dam operating requirements. These Orders have been modified twice since, most recently in 2013. The Orders largely address lake levels as their main concern.
By the 1970’s this wooden structure had fallen into severe disrepair. As the Zosel operation had largely abandoned the dam, it fell to the State of Washington to propose a fix to the situation, in order to continue to maintain lake levels at Lake Osoyoos. Working cooperatively with the Province of British Columbia, a plan was put in place to build a new, all-concrete dam structure.
Construction of Zosel Dam was completed in April 1987 at Oroville. The site of the new dam is two miles downstream from the Osoyoos Lake outlet and nine miles south of the United States/Canadian border. The Department of Ecology owns this structure for the State, but the local irrigation district, Oroville Tonasket maintains the site and structure for the State.
To improve channel capacity, dredging was completed upstream from the new dam. Sand bars that partially blocked the channel at the Osoyoos Lake outlet were removed. Also, sedimentation buildup at the confluence of Tonasket Creek with the Okanogan River was dredged. Channel improvements were completed in early 1988. The State continues to monitor channel conditions on a regular basis as part of the Orders of Control.
Improvements were also made at the Osoyoos Lake State Park. A sheet pile wall was installed along 600 feet of the west bank of the state park to improve bank stability along the Osoyoos Lake outlet. A new boat launch, sidewalk, handrail, and rock fill-groin at the beach were included in the project.
Water level lowered at Osoyoos Lake for the winter - Ecology News Release - November 5, 2014
Help for thousands of salmon on incredible journey from Oregon to Canada - ECOconnect Blog July 28, 2014
Lake Osoyoos rises as runoff increases Washington operations help maintain lake levels for many uses - Ecology News Release May 29, 2014
Ecology makes seasonal water level changes at Osoyoos Lake - Ecology News Release September 19, 2013
Water levels rise at Lake Osoyoos as temps rise - Ecology News Release May 7, 2013
IJC issues Order for future management of Osoyoos Lake water levels International Joint Commission (IJC) January 31, 2013
U.S., Canada, Washington State agree to new cross-border lake operations - Ecology News Release January 14, 2013
Ecology makes seasonal water level changes at Osoyoos Lake - Ecology News Release November 8, 2012
Water Levels are rising at Lake Osoyoos - Ecology News Release May 24, 2011
Department of Ecology - WR
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