The Giant Ancient Lake That Once Covered South Dakota

If you've ever found yourself driving through the James River Valley, you’ve likely noticed one thing above all else: it is impossibly, perfectly flat. But why? To the casual observer, it’s just the natural landscape of South Dakota agriculture. Except that's not it.  If you could peel back the layers of time by about 10,000 years, you'd no longer be driving through farm fields; instead, your car would be at the bottom of a giant ancient inland sea.

This was Lake Dakota, a prehistoric giant that once dominated the northeastern part of our state. Stretching nearly 100 miles long and around 25 miles wide, this glacial wonder was more than just a body of water; it was the reason the land in this part of the state is so incredibly flat and fertile to this very day.

While the lake itself is a "bygone" relic of the Ice Age, its ghost still haunts the horizon every time you look out over the perfectly even plains of the "Jim" River valley. This is the story of how a frozen dam created a South Dakota landmark that disappeared before the first map was ever drawn.

Get our free mobile app

The Ancient Inland Sea That Stretched From Oakes, ND to Redfield, SD

Roughly 14,000 years ago, an enormous wall of ice (the James Lobe) carved out the valley. As the ice began to melt, it had nowhere to go. Near the town of modern-day Mitchell, a giant ridge of debris from the glacier acted as a "plug" in a bathtub. This created a giant pool of melted water that we now call Lake Dakota, stretching all the way from Oakes, North Dakota, down past Redfield, South Dakota.

The lake flourished in the area for close to 3,000 years; however, around 11,000 years ago, the southern "plug" finally gave out. It must have been an amazing sight to see; the water rushed out, leaving behind the path for the James River we know today.

Even though this took place thousands of years ago, the lake is still providing us with benefits to this day. When the water departed, it left behind what is now known as the Lake Dakota Plain. This is why the land around Aberdeen and the surrounding area is so incredibly flat and fertile.

So, every time a farmer in the valley heads out to harvest corn or soybeans, they are still harvesting the long-lasting legacy of the ancient inland sea of South Dakota and North Dakota.