Zosel Dam History

On January 31, 2013, The International Joint Commission announced that it had issued a Supplementary Order of Approval renewing the State of Washington’s authority to operate Zosel Dam and making minor modifications in how water levels on Osoyoos Lake will be managed.

But how did the Zosel Dam exist in the first place?

Scenic picture of Zosel Dam on the Okanogan River


Zosel dam constructed to provide a millpond for delivery of logs to the Zosel Mill. No reference made to Canada or the International Joint Commission (IJC). 


Complaints about high Osoyoos Lake levels result in Washington State (WA) asking the IJC to hold hearings. This done, they appoint a Board of Engineers (B of E) to investigate the hydraulics of the outlet of Osoyoos Lake.


The B of E determines that under some conditions, Zosel Dam is causing high lake levels. The IJC order Zosel to increase the capacity of the dam so that it can pass 2500 ft3 /sec with the millpond level not exceeding elevation 911 ft. No mention is made of Osoyoos Lake levels. An International Osoyoos Lake Board of Control is established to monitor the operation.


Modifications to Zosel Dam completed to the IJC's satisfaction. 1960s Zosel sawmill stops using the millpond to deliver logs.

Zosel Dam circa 1970

Partial collapse of Zosel dam (due to age/maintenance) necessitates emergency repairs by Washington and the US Army Corps of Engineers (USACE).


Another failure of the dam occurs, again repaired.


The IJC ask the USACE to report on the condition of Zosel Dam. Their report expresses grave concern about the dam's structural integrity and indicates that, were it to collapse, Osoyoos Lake could fall as low as 905 ft. Although there has been no reference from either British Columbia (BC) or WA, the IJC calls hearings to discuss the condition of Zosel Dam. During these hearings, BC and WA agree that there is a basis for a cooperative approach and tell the Commission that they will return within a year with a joint reference. The IJC asks the USACE to draw up plans for a replacement structure.


The USACE produce a plan for a replacement structure almost a kilometre upstream of the original dam. The cost estimate is for US$6.2 million. 1980 BC and WA exchange a document entitled British ColumbiaWashington Cooperation Plan for Osoyoos Lake Levels and Trans-Border Flows. This document which is non-binding lays out target lake evelations and trans-border flows during both normal and drought conditions once a new dam is built. WA applies to the IJC for permission to rebuild Zosel Dam much as outlined in the USACE report with the BC-WA Cooperation Plan attached for reference only.


The IJC holds public hearings in both Oroville and Osoyoos to gather local input to the proposed dam.


The IJC issues an Order of Approval for the construction of the dam. This details the required capacity of the dam as 2500 cfs at an Osoyoos Lake elevation not exceeding 913 ft and the range of allowable lake levels under various conditions. (This capacity is very similar to that required under the 1946 Order of Approval as a millpond elevation of 911ft. at 2500 cfs implies an Osoyoos Lake level of about 913ft.)


WA and BC agree on a Memorandum of Understanding regarding the cost sharing of the design and construction of the new facility. (The delay was due to fiscal problems in both jurisdictions.) Acres International (Vancouver) is appointed as the consulting engineer. After detailed site investigation and design they submit plans which show the dam moved downstream (close to the original dam location), with a different gate design and proposing a new solution to the Tonasket Creek fan problem from the proposed by the USACE. Because the dam location is changed and because the original time limit for construction of the dam would be exceeded, the IJC decides to hold further public hearings in Oroville and Osoyoos. After these a Supplementary Order of Approval is issued, approving the new design and location and extending the completion date.


Construction of the new dam begins.


Construction essentially complete. Drought conditions are forecast on the Similkameen. After the water has been stored and the lake raised to elevation 913, drought does not materialize and the stored water is dumped in early August. This leads to discussions regarding how best to lower water levels if drought declaration is rescinded. 1988 Drought forecast and occurs. Osoyoos Lake held above elevation 911.5 from May 6 through August 25, maximum 912.63 ft.


Drought forecast and occurs. Osoyoos Lake above 911.5 from June 18 until September 30th, with a peak of 912.87 in midAugust. This is achieved by importing water from the Similkameen through the old Okanagan-Tonasket Irrigation District flume. Many complaints about high lake levels are received.


Drought forecast and occurs. BC and WA agree on a maximum regulated Osoyoos Lake level of 912.5 with BC supplying the volume equivalent of 6 inches on Osoyoos Lake out of Okanagan Lake when asked to do so by WA. Not to be considered a precedent.


Drought forecast and occurs. Same agreement as '93 re 912.5 is agreed by BC and WA.


Drought forecast but does not occur.


Drought forecast and occurs. Similar agreement between WA and BC to keep max regulated level to 912.5.


Drought forecast and occurs. Same agreement as '93 re 912.5 is agreed by BC and WA. Process on renewal of Orders discussed at public September meeting.


Drought forecast but does not occur and drought declaration rescinded. 2005 Drought forecast and occurs. Similar agreement between WA and BC to keep max regulated level to 912.5 during drought declaration in return for 2,850 acre-feet of water in the spring to assist sockeye smolts passage of Zosel Dam. Draft Plan of Study for Order renewal prepared by Glenfir Resources.


Draft Plan of Study for Order renewal approved by IJC and sent to governments.


Osoyoos Lake Water Science Forum on strategies to promote future sustainability of the Lake.


Drought forecast and occurs. Work commenced on several studies in the Plan of Study for Order renewal.

Zosel Dam over Okanogan river circa 2019

Drought forecast but revised forecasts do not meet criteria and the drought declaration was rescinded. Work continued on studies for Order renewal.


Work on the Plan of Study for Order renewal was completed and the results of the studies were presented at the Osoyoos Lake Water Science Forum held September 18-20 in Osoyoos.