Why does Minnesota have so many lakes?

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Lakes are an inescapable part of Minnesota's identity. The Land of 10,000 Lakes boasts about its waters on license plates, butter packages and even a certain Los Angeles basketball team — originally known as the Minneapolis Lakers.

Legend has it that the state's abundant bodies of water were created by the massive footprints left behind by Paul Bunyan and Babe the Blue Ox. But there has to be another explanation, too, perhaps a little more scientific.

"So why DOES Minnesota have so many lakes?" a reader asked Curious Minnesota, the Star Tribune's community reporting project fueled by reader questions.

The answer, like so many things in Minnesota, starts with ice.

Towering glaciers periodically covered Minnesota over the last 2.5 million years, expanding and retracting over time. As they ultimately retreated about 14,000 years ago, the glaciers erased many of the rivers and other drainage systems on the land.

They spackled over those old river beds and left behind an entirely new landscape, filled with bumps, depressions, piles of rocks and other debris. In short, they created ideal conditions for water to pool.

A transformed landscape

The glaciers didn't fill in these old river beds and drainage systems evenly, said Carrie Jennings, a geologist and research and policy director for the nonprofit group Freshwater Society.

Over thousands of years, the ice gathered material and essentially deposited it in irregular piles around the state. With the ice long gone, that debris remains, with all of its peaks and valleys and random depressions that form many of the state's lakes.

The landscapes of states that weren't touched by the glaciers, on the other hand, did not experience the same smoothing process. Water has had more time in those places — eons — to find a way to drain off the land as quickly and efficiently as possible.

Not all of Minnesota's lakes were formed in the same way, however.

Some were created by the melting of the glaciers themselves. In the Brainerd area, there are some "kettle" lakes that were once house-sized, or even city-sized, chunks of ice. They broke off the warming glaciers and were buried in soft sandy soil that collapsed around the ice, creating the structure of a lake.

The lakes of the Boundary Waters Canoe Area Wilderness weren't created by the debris of the glaciers as much as by what the ice carried away. The glaciers removed all the loose material that had collected over hundreds of millions of years on top of the undulating bedrock of the area, Jennings said.

Some of the state's biggest lakes, which tend to be relatively shallow, were carved by glaciers moving through the area. The glaciers' massive weight and erosive power gouged out the bottoms of what are now Lake Mille Lacs and both Upper Red and Lower Red lakes, Jennings said.

Minnesota's longer glacial legacy

But glaciers aren't the whole story. After all, North Dakota was once covered in ice, too. So were large portions of Indiana, Ohio, Illinois and Iowa. Yet, all those places are practically water-starved compared with the North Star State.

The main difference, Jennings said, is simply time.

The glaciers left Iowa, Indiana and Illinois many thousands of years before they left Minnesota. The southwestern corner of Wisconsin was never touched by a glacier. The southeastern corner of Minnesota was glaciated in the very distant past, but avoided them in the most recent ice age. This resulted in the beautiful and unique topography of the Driftless Area.

"The goal of all lakes — if they have a goal — is to fill in," Jennings said. "Gravity just sees to it that holes don't exist on the landscape."

The glaciers erase rivers and drainage systems. But slowly, over time, they always return. Thousands of years ago, Indiana may have had enough lakes to rival Minnesota. And thousands of years from now, as the land continues to change and move and erode in a way that will only drain more water, Minnesota may look more like Indiana.

Aside from the return of glaciers, there are very few things that would naturally create more lakes in Minnesota. But the water table could change in a way that would create more.

Dry depressions today could turn into lakes if rainfall increased, causing groundwater to rise. Conversely, if rain drastically decreased in the coming centuries, many of today's lakes would more quickly become dry valleys. There's evidence that shallow Lake Winnibigoshish — one of Minnesota's biggest lakes by surface area — has fluctuated between water and land over the years.

"The east side of that lake was a dune field," she said. "But the water table has risen back up, so you see wetlands. We think of lakes as bowls that hold water, but really, they're just the visible expression of the groundwater."

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