06/23/2026
Lake Ontario once drained toward the Hudson River instead of the St. Lawrence River.
About 13,000 years ago, the Ontario basin held a much larger body of water known as Glacial Lake Iroquois. A wall of ice blocked the lake’s northeastern outlet, preventing water from escaping through the route it uses today.
As a result, the lake stood roughly 100 feet higher than modern Lake Ontario. Water flowed southeast through the Mohawk Valley, then down the Hudson River toward the Atlantic.
When the ice retreated from the St. Lawrence Valley, a new outlet opened to the northeast. The lake fell dramatically and began draining through the St. Lawrence system.
Evidence of that ancient lake is still easy to find. Ridge Road in New York follows an old beach ridge formed along the shore of Lake Iroquois. In Toronto, Davenport Ridge marks another section of that same ancient shoreline.
The next time you cross Ridge Road or Davenport Ridge, you’re crossing the shoreline of a lake that once drained through the Hudson River instead of the St. Lawrence.
Image: Tompkins / Vizettes (based on the work of H.L. Fairchild)
