Why do Water Levels Change at Rattray Marsh?

Marsh with forest in background.

Spring is here, which often means higher water levels across the Credit River Watershed. Rattray Marsh in south Mississauga is no exception. As a coastal wetland on Lake Ontario its water levels are influenced by Lake Ontario water levels, precipitation, wave action and water flowing in from Sheridan Creek.

Water levels in the marsh are dynamic, meaning they change constantly. The connection point between Rattray Marsh and Lake Ontario is called a baymouth bar, where water movement is controlled naturally. The baymouth bar acts like a dam, slowly opening and closing.

How do water levels increase in Rattray Marsh?

Winds from the east create waves. Over time, the force of the waves pushes material like cobble stones and shale rocks from the lake onto the shore, depositing them to form a baymouth bar. This creates a partial water barrier between the lake and the marsh.

A partial barrier at the Rattray Marsh outlet has formed. This limits water flow between the marsh and the lake. The baymouth bar at Rattray Marsh is one of the last remaining on the western shores of Lake Ontario.

Eventually, enough material is deposited to form a complete barrier. This blocks water from exiting the marsh. As Sheridan Creek continues to drain into Rattray Marsh, water levels rise.

The baymouth bar forms a full barrier between the marsh and the lake. Water cannot exit the marsh.

How do water levels lower?

Water pressure builds inside the marsh as Sheridan Creek continues to drain into it. When it rains, it also builds pressure. As water levels rise, the pressure becomes too great and forces open the barrier once again. Water drains out of the marsh into the lake, lowering water levels in the marsh.

The baymouth bar is open. Water level is at or around the elevation of Lake Ontario.

How often does this cycle occur?

The baymouth bar can open and close many times every year, although it is highly variable and often unpredictable. In 2017 and 2019, the barrier stayed closed for the majority of the year. We saw record high water levels in Lake Ontario those years. As a result, Rattray Marsh experienced extremely high-water levels.

Rattray Marsh is naturally equipped to handle the changing water levels. Remember to always be careful around waterways. Stay on designated trails and recreational areas while you enjoy the beauty and wonders of the marsh.

Learn more about Rattray Marsh and its thriving ecosystem.

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