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Osoyoos Lake International Water Control Structure - Zosel Dam

Last Update August 29, 2016

Canada releases pulse flows to the Okanagan River

August 29, 2016 - Dam operators in Penticton, British Columbia are raising flows in the Okanagan River to assist migrating salmon on their trip down the Columbia River system to the ocean. Beginning Aug. 29 and continuing for approximately eight days, Canadian authorities will be ramping up flows from Lake Okanagan from current late summer levels to about 1,250 cubic feet per second.

For questions, please 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.

In the News

Ecology News Release - July 31, 2015

Water levels rising at Lake Osoyoos as spring snow melt proceeds  ECOconnect Blog, May 15, 2015

What do a salmon and a beloved dog have in common?  ECOconnect Blog, May 1, 2015


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