How the Great Lakes formed

The Great Lakes exist because glaciers the size of mountains carved massive valleys across North America during the last Ice Age, then melted and filled them with water.

During the last glacial period, ice sheets over a mile thick scraped the bedrock like a giant file, creating interconnected basins that couldn't drain naturally afterward.

The glaciers didn't just carve downward—they pushed sediment ahead of them, creating natural dams that trapped meltwater and shaped the lakes' exact boundaries today.

Lake Superior's basin formed when glaciers exploited a weak zone in ancient bedrock, making it Earth's largest freshwater lake by surface area.

The lakes' water levels still rise and fall because the land beneath them is slowly rebounding upward after the glacier's immense weight finally lifted.

Isostatic rebound means the northern lake shores are rising while southern shores sink, gradually tilting the entire Great Lakes system over millennia.

Lake Michigan is sinking while Lake Superior's north shore rises, eventually forcing water to flow differently between the lakes in thousands of years.

The Great Lakes' outlet shifted dramatically when glaciers retreated, causing ancient water to suddenly drain northward into Hudson Bay instead of southward.

The lakes' drainage reversal happened so fast it created Niagara Falls—a waterfall that's actively eroding backward toward Lake Erie at measurable rates today.

Niagara Falls erodes backward roughly one meter per year, meaning it'll reach Lake Erie in about 50,000 years, fundamentally reshaping North America's hydrology.