Nobody Has Seen Mackinac Island’s 100-Foot Waterfall
A deep pool in Lake Huron shows that there was once a stunning cascade from the glacier-fed Mackinac River
If you could stand on today’s Mackinac Island shoreline near Mission Point Resort 10,000 years ago, you wouldn’t be looking across Lake Huron. You’d be looking down into a deep river valley, where a roaring glacial meltwater river would be approaching the edge of a 100-foot waterfall.
The land that makes up the island would be familiar. The same hard Mackinac Breccia that forms Arch Rock, Sugar Loaf and the island’s cliffs today would still be there. But beyond the shoreline, everything would be different. Instead of open water, a deep river valley would extend east from the island, carrying a roaring meltwater river toward the brink of a waterfall.
Today, that landscape is hidden beneath Lake Huron. Just offshore, the lake floor drops into a depression 269 feet deep. It isn’t a sinkhole. It’s the plunge pool left behind by the waterfall that once occupied this now drowned valley.
To understand how it formed, you have to picture a Great Lakes region in transition.
Around 10,000 years ago, glaciers had retreated northward across the Upper Great Lakes. Land was still depressed from the weight of ice more than a mile thick, which allowed melt water to drain quickly. This caused water levels to substantially drop, exposing landscapes that now lie far beneath the lake surfaces.
Instead of the connected Great Lakes we know today, separate bodies of water occupied the Michigan and Huron basins. Geologists call them glacial Lake Chippewa and glacial Lake Stanley.
Connecting them was the Mackinac River.
Fed by glacial meltwater, the river carried the outflow from Lake Chippewa eastward toward Lake Stanley. During much of this low-water period, the eastern outlet from the Lake Superior basin remained blocked by glacial ice.
Instead of flowing through the St. Marys River as it does today, meltwater from Lake Superior traveled south across the central upper peninsula, through what is now Green Bay, before entering Lake Chippewa. From there, it continued east through the Mackinac River and over the waterfall near present-day Mackinac Island.
Near the island, the river encountered a very hard and erosion resistant ledge of Mackinac Breccia. Unable to erode the hard rock as quickly as the softer layers downstream, it plunged about 100 feet over the escarpment. The force of the falling water carved a deep basin into the bedrock below, creating what geologists call a plunge pool.
As the glaciers continued melting and land continued rebounding, Great Lakes water levels rose. The Mackinac River Valley gradually flooded, drowning the waterfall and the river valley landscape.
For thousands of years, the ancient landscape was hidden beneath Lake Huron.
Modern sonar mapping eventually revealed the buried river valley, the escarpment where the waterfall once plunged and the deep basin it carved into the bedrock. Together, they tell the story of a landscape that disappeared as the Great Lakes assumed their modern form.
Today, visitors to Mackinac Island see an expanse of water from the shoreline. Few realize they’re standing above the rim of an ancient river valley, or that beneath the water just offshore lies the preserved footprint of a waterfall that vanished nearly 10,000 years ago.
