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The Rise and Fall of our Wetlands

March 29, 2021

By Tys Theysmeyer, Head of Natural Lands, Royal Botanical Gardens.

Lots of people are wondering what happened to all the water in Cootes Paradise Marsh this spring. Cootes Paradise Marsh is the western tip of Lake Ontario and so has the same water level as the lake. Lake levels have declined from the recent record high in June 2019, to an extraordinary 1.5m (~5 ft) lower in just 20 months (though these levels are still about 1m higher than record lows of the 1930s).

Is this climate change? Well it’s a little bit of lake management and climate change together, as Lake Ontario’s level is controlled by a dam near Cornwall. But it is also a result of a dramatic shift in precipitation patterns over the past year, with all the Great Lakes coming down noticeably from record highs (though none more dramatically than Lake Ontario).

Is this normal? It is somewhat normal, but the pattern of lake water level fluctuations is shifting overall. A scan of the past century of water level patterns on the Great Lakes shows that all the past highest years are followed by a dramatic decline in the following years, and this up-and-down cycle was thought to reflect a 30-year weather pattern cycle with dramatic natural shifts. However, the ups and downs in the past decade suggest this cycle is now changed.

What does this mean for Cootes Paradise in 2021? Two dramatic outcomes are expected. First, marsh plants, particularly reeds and cattails will have an extraordinary year of regeneration, and second, fish and wildlife will be puzzling about how to go about nesting and spawning.   How often is it this this low at the end of March? Only twice in the past 60 years, including 1999 and 1964/65, so this year will be one for the history books.  Keep in mind that water levels are still expected to rise through the spring – likely about 40cm, peaking in June before dropping in the summer heat.

Graph of Lake Ontario Water Level Forecast for Spring Summer 2021
Lake Ontario Water Level Forecast for Spring Summer 2021

If you’d like to learn more about the ups and downs of the Great Lakes,  the US Army Corps of Engineers summarizes it here.

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