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Osoyoos Lake International Water Control Structure - Zosel Dam

Last Update May 25, 2017


Summer of high water expected along Okanogan River - Snowmelt runoff will impact lake and riverside communities

Communities along the Okanogan River from Oroville to Brewster are likely to experience high water along the shoreline through the summer and into September, due to greater-than-normal snowmelt from watersheds in Canada.

“Property owners with low-lying lawns and fields and folks who recreate in the river should take heed and prepare for the highest river levels we’ve seen in 20 years,” said Al Josephy with the Washington Department of Ecology’s water resources program. “Flows could be two or three times greater than what we usually experience in the summer months.”

Unusually high snowpack in British Columbia is resulting in volumes of runoff not seen since 1997. This is putting tremendous pressure on Canadian water managers to maintain optimal levels in the Okanagan system of lakes and reservoirs this summer.

Lake Osoyoos will likely remain above normal operating levels for much of the summer, Josephy said. This may mean periods of flooding along lake properties and further downstream because of extreme flows expected this summer. Gates at Zosel Dam will remain open as much as possible to relieve water backed up in the system.

Water managers will work all summer to try and steady water levels at Lake Osoyoos and bring them within normal parameters. The lake straddles the United States and Canada at Oroville and hosts summer homes, recreational activities, and supplies irrigation water on both sides of the border.

Lake Osoyoos water levels are mandated by the International Joint Commission, a board made up of representatives from the U.S. and Canada. More information is available on the International Osoyoos Lake Board of Control web page.

For hydraulic permitting information on work needed within water in response to flooding, please consult the Washington Department of Fish and Wildlife.

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.

For more information on the operation of Zosel Dam or Lake Osoyoos, contact:

Al Josephy
Department of Ecology - WR
Phone: 360 407-6456


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.

Project Description:

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.


Al Josephy
Department of Ecology - WR
Phone: 360 407-6456