Great Lakes coastal wetlands depend primarily on one physical feature to maintain their high levels of biodiversity – that is, naturally fluctuating water levels. When lake levels are high, some plant species will be eliminated and often more open water areas become established in the wetland – this is called interspersion. These pockets of open water mixed in among the vegetation are thought to provide important breeding habitat for several marsh-nesting bird species such as American Coots, Common Moorhens and Pied-billed Grebes.

Water levels in the Great Lakes have been recorded since 1860. These data show that levels in Lake Ontario have varied by up to two metres since that time. In 1958, however, lake level regulation was implemented which resulted in a moderating of high and low levels. While lake levels still fluctuate they do not do so to the extent that occurred prior to regulation.

Water level regulation has many benefits for human-related activities, such as commercial shipping, recreational boating, electrical power production, etc. Unfortunately, the decreased fluctuations that have occurred since 1958 appear to have impacted many coastal wetlands that depend on greater water level fluctuations to maintain high levels of biodiversity.

The Lake Ontario - St. Lawrence River Study Board is currently undertaking a five-year study for the International Joint Commission (IJC) to evaluate the criteria used for regulating water levels on Lake Ontario and in the St. Lawrence River. Both human and natural interests in water level regulation are being considered as part of this study. Extensive public consultation has been and will continue to be part of this process prior to final recommendations being made.

Lake Ontario - St. Lawrence River Study Board
Water Level Links - Canadian Hydrographic Service

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