Why Does Minnesota Have So Many Lakes? The Glacier Did It

Minnesota has so many lakes because, in geological terms, the glaciers just left. Ice covered most of the state until roughly 10,000 to 14,000 years ago, and it retreated across a landscape too young and too flat for rivers to have drained yet. The ice left behind buried chunks of itself, piles of debris that work like natural dams, a giant meltwater lake bed in the northwest and scraped bedrock in the northeast. Each of those produced a different kind of lake. The Minnesota DNR sums it up in the title of one of its own articles on the subject: the glacier did it.

Ice a mile thick

Picture Minnesota 14,000 years ago and you are looking at an ice sheet as much as a mile thick, flowing slowly south out of Canada. Glaciers do not just sit. They quarry rock, grind it and carry it, and when they melt they drop the whole load. That load, a mix of clay, sand, gravel and boulders, is called till, and it covers most of the state. Near Crosby in Crow Wing County, for example, the DNR puts the glacial material at 100 to 200 feet thick.

The ice did not advance once and leave once. Separate lobes pushed in from the northeast and northwest at different times, each rearranging what the last one left. The lakes sit in the wreckage.

Kettle lakes: holes where ice used to be

The DNR says most of Minnesota's 11,842 lakes are kettle lakes, also called ice-block lakes. The recipe:

1. A chunk of ice breaks off the melting glacier front.
2. Meltwater streams bury it in sand and gravel.
3. Insulated by the sediment, the ice takes a long time to melt.
4. When it finally does, the ground above it sags into a pit.
5. The pit reaches below the water table or collects runoff, and you have a lake.

Kettles explain a lot of what you see from a boat in central Minnesota. They tend to be compact and irregular rather than long. Many have no real inlet or outlet and are fed mostly by groundwater. Because the ice blocks were big, kettles can be deep for their size, with a sharp drop not far from shore. Where several blocks sat close together you get chains and clusters separated by narrow necks of sand, which is the classic look of the Brainerd Lakes area, the Park Rapids area and the lake country around Alexandria.

Moraines: where the lakes pile up

Lakes are not sprinkled evenly across the state. They bunch up along moraines, the ridges of debris that built up wherever the edge of the ice paused. A glacier front that stalls works like the end of a conveyor belt, piling up sand, gravel and boulders in a belt of hills. That hummocky ground is full of buried ice blocks and closed depressions.

The DNR describes moraines along the North Shore of Lake Superior, a horseshoe of them through the middle of the state, and others marking the farthest reach of the ice in the southwest and southeast. Their names tell you where they are: the Itasca moraine, the Alexandria moraine, the St. Croix moraine. Lay a lake map over a moraine map and they match. Otter Tail County, with more lakes than any other county, lies in Alexandria moraine country, and Lake Itasca and its neighbors sit in the Itasca moraine.

The glaciers left other signatures too. Eskers are winding gravel ridges laid down by rivers that ran in tunnels under the ice; at Scenic State Park near Bigfork, a trail follows an esker between the basins of Coon and Sandwick lakes. Drumlins, streamlined hills shaped by moving ice, give some Otter Tail County roads their roller-coaster profile.

The ghost of Lake Agassiz

Northwestern Minnesota is the opposite of moraine country: so flat that the horizon looks ruled. That is the bed of Glacial Lake Agassiz. As the ice retreated north it blocked the natural drainage toward Hudson Bay, and meltwater ponded against it into a lake that, by the DNR's description, covered more than 120,000 square miles of Minnesota, North Dakota, Manitoba, Saskatchewan and Ontario. That is larger than all of today's Great Lakes combined.

When the ice dam finally gave way and Agassiz drained, it left:

• The Red River Valley, whose flat, rich farmland is old lake bottom.
• Beach ridges, low sandy rises that mark former shorelines. Lake Bronson in Kittson County was created by a dam built in a notch through one.
• The big bog country of Beltrami, Koochiching and Lake of the Woods counties.
• Upper and Lower Red Lake, water still sitting in the lowest part of the old basin. The Red Lakes total 288,800 acres, the largest lake within the state, yet Upper Red is only about 15 to 18 feet deep at most. Lake of the Woods, farther north, is generally described the same way: a remnant in a low corner of the Agassiz plain, though its northern reaches in Ontario are rocky shield country.

That shared origin is why these lakes behave alike. They are huge, shallow and wind-driven, with long sand shorelines on their Minnesota sides. They stir easily and warm quickly, which makes them enormously productive walleye water.

Bedrock lakes of the Arrowhead

Northeastern Minnesota is different again. In the Arrowhead the ice did more scraping than dumping. It stripped soil off some of the oldest rock on the continent and gouged out the weaker layers, leaving long troughs between ridges of harder rock. The lakes of the Boundary Waters and the Gunflint Trail fill those troughs.

You can read it on a map. Many border lakes are long and narrow and line up in the same direction, following the grain of the rock. Shorelines are ledge and boulder rather than sand. Depths are serious: Lake Saganaga in Cook County, at 280 feet, is the deepest natural lake in the state, and many smaller lakes nearby top 100 feet. Cold, deep, low-nutrient basins like these are why the Arrowhead has native lake trout while most of the state does not. Scratches left by rock-studded ice are still visible on bare ledges along the North Shore.

Lake Superior itself occupies a much older and larger basin in the bedrock, deepened by the ice and filled by its meltwater.

Where the lakes are not

The DNR lists four counties with no natural lakes: Mower and Olmsted in the southeast, Pipestone and Rock in the southwest. The pattern fits the glacial story. The DNR notes that parts of Minnesota, notably the southeast, escaped at least the most recent glaciers. Without fresh till and buried ice there were no basins to fill, and streams have had far longer to carve valleys and drain the land. The result is bluff and trout-stream country rather than lake country. The far southwest corner is similarly old, well-drained ground.

South-central and southwestern Minnesota outside those corners were glaciated, but the till plain there is gentler, so lakes are shallower and fewer. Lake Shetek in Murray County covers about 3,460 acres and is only 10 feet deep. Many prairie basins were also drained for farming over the last 150 years.

Lakes are temporary

On a geological clock, lakes are short-lived. Streams carry in sediment, plants fill the shallows, and outlets cut down. Shallow kettles become marshes, then bogs, then meadows; some of Minnesota's peatland began as open water. The state has so many lakes today because so little time has passed since the ice left. Runoff from farms and lawns speeds the filling-in, which is one reason the MPCA and lake associations work to keep phosphorus out of the water.

Bottom line

• Most Minnesota lakes are kettles: pits left by buried ice in the debris the glaciers dropped.
• The big shallow lakes of the northwest are leftovers of Glacial Lake Agassiz, and the deep narrow lakes of the Arrowhead are carved in bedrock.
• The lakeless corners of the state are the places the last glaciers missed. For the full count, see how many lakes Minnesota has.