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History of Water Regulation in the LOSLR Basin

Given the diverse interests and needs along Lake Ontario and the St. Lawrence River, regulating water levels is a complex task.

In 1909, as part of the Boundary Waters Treaty, the International Joint Commission was established to help manage the shared waters along the Canadian-U.S. border. During the 1950's, the IJC approved the construction and operation of a hydropower project in the international section of the St. Lawrence River for the purpose of producing hydroelectricity, enabling seaway navigation and providing some flood protection to Lake Ontario and the St. Lawrence River. Flows through the Moses-Saunders Dam would also be regulated so that the lower St. Lawrence River received no less protection than with unregulated flows. Under the treaty, the IJC is tasked with ensuring that all affected interests are considered in decisions that change the levels and flows of boundary waters.

The Lake Ontario St Lawrence River (LOSLR) Watershed

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In light of record floods in the early 1950's, the governments of Canada and the United States asked the IJC to investigate whether water levels could be regulated for the benefit of property owners on Lake Ontario, while "having regard to all other interests" in the basin and how they would be impacted by such regulation. The IJC found that Lake Ontario could be regulated between a low of 74.15 m in the navigation season and a high of 75.37 m (243.29 and 247.29 feet, respectively) based on water supplies recorded between 1860 and 1954. The governments approved this target range of water levels and the regulation plan recommended by the IJC, called Plan 12-A-9. While this particular regulation plan was never implemented, it was used to calculate river profiles and design channel excavations for safe navigation through the seaway. The target range of Lake Ontario levels, however, was retained in subsequent plan development.

In 1960, the IJC implemented a new water regulation plan known as Plan 1958-A. That plan was soon reevaluated due to problems with low water levels, particularly in the Port of Montreal. As part of that evaluation, the International St. Lawrence River Board of Control (Board) -- which the IJC created to help monitor and manage the basin's water levels -- was given authority to deviate from the regulation plan when relief could be provided to one or more interests without causing undue harm to other interests. In 1962, Plan 1958-C became operational, which provided more stable minimum water levels through reductions in summer flows and minimum winter flows.

In January 1963, the IJC asked the Board to proceed with further studies on ways to improve the regulation plan. The resulting plan, 1958-D, would improve water levels at Montreal Harbor without reducing the minimum winter flows of Plan 1958-C. Plan 1958-D was made operational in October 1963 and has remained the regulation plan for Lake Ontario and the St. Lawrence River ever since.

In the ensuing decades, shoreline development in the region continued to grow. New homes were built, many residents converted summer cottages to year-round residences, and recreational boating grew to become a significant economic activity. But greater development also meant that greater impacts were felt from fluctuating water levels, particularly on occasions when water supplies were more extreme than Plan 1958-D was designed to handle. By the 1990's, there was growing dissatisfaction with the current regulation plan.

In 2000, the IJC began a new International Lake Ontario-St. Lawrence River Study with financial support from the two federal governments. The five-year, $20 million study provided new insights into the shortcomings of the current water regulation plan, and outlined how system regulation might be improved.

After the study was released, the IJC proposed implementing a modified version of one of the water regulation plans recommended by the study. After holding extensive public hearings on the issue, the IJC elected to withdraw that proposal due to various concerns that were raised. Yet the underlying problems and challenges with the current regulation plan have remained.

  • Background

Learn more about the history of regulating water levels and flows on Lake Ontario

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