The Secret Tunnel That Nearly Swallowed St. Anthony Falls

Dave Kowalski — Columbia Heights
Retired Fridley history teacher, third-generation Northeast, obsessed with the Minneapolis streetcar system and working-class neighborhood history.

Hey there, folks. It's a beautiful September morning up here in Columbia Heights, leaves just starting to flirt with gold along the boulevards. Down by the river, the air is crisp, and if you're like me, you probably love a good stroll along the Stone Arch Bridge or a coffee at a spot overlooking St. Anthony Falls. It's a truly iconic view, right? The rushing water, the skyline in the background, the history practically humming in the air.

But here's the thing: what most people don't know, as they gaze at that magnificent cascade, is that the falls you see today are a miracle of human ingenuity, born from an almost catastrophic blunder that nearly erased them from the map. And with early October bringing the anniversary of a really wild moment in our city's past, it feels like a good time to dig into a story that literally runs deep beneath our feet.

The Great Mill Race: A Thirsty Business

Back in the mid-1800s, St. Anthony Falls wasn't just pretty scenery; it was the engine of Minneapolis. The sheer power of the Mississippi crashing over those limestone ledges was what drew the millers here, folks like Cadwallader C. Washburn and the Pillsbury brothers, who were building their flour and lumber empires. They needed water power, lots of it, to turn their giant turbines and grind grain into flour for a growing nation.

Now, the falls themselves, before all the modern engineering, were a bit... temperamental. They used to be called the 'Spirit Island Falls' by the Dakota, and they were gradually eroding upstream, retreating about seven feet a year! The mill owners knew this, and they also knew they needed more efficient ways to harness all that power. Existing canals and flumes were getting crowded, and the competition was fierce. So, in 1868, the powerful St. Anthony Falls Water Power Company, which essentially controlled the falls, came up with a truly audacious plan.

They decided to dig a massive tunnel, a 'horizontal shaft' as they called it, directly under the bed of the Mississippi. The idea was to create a gigantic, underground tailrace – basically, a big drainage channel – that would allow them to build new mills on the east bank and drop their spent water directly into the river below the falls, getting maximum power without interfering with anyone else. On paper, it sounded brilliant.

"Like a Mole Blinded by the Sun": The Dangerous Dig

What most people don't realize is the sheer, backbreaking human effort that went into this project. This wasn't done with modern tunnel boring machines. Oh no. This was a crew of mostly Irish and Scandinavian immigrant laborers, working in the pitch black, deep underground, by the flickering light of candles and coal oil lamps. They dug with picks, shovels, and black powder.

Imagine it: the constant damp, the claustrophobia, the fear of a cave-in, the dust and smoke from the blasting. It was brutally hard, dangerous work for low wages. These working-class men literally chipped away at the bedrock, carving out a tunnel that was 75 feet wide, 20 feet high, and eventually stretched over 2,000 feet upstream, right under the heart of the falls. Their goal, of course, was to connect to the river above the falls, creating a powerful, controlled flow.

They called the project the 'Great Tunnel.' And for a while, it seemed like an engineering marvel, a testament to Minneapolis's unstoppable industrial ambition.

The Day the River Roared (Underground)

But here's the kicker: the geology wasn't on their side. The falls are essentially a hard limestone cap over softer sandstone. The tunnel was mostly through that softer sandstone. On October 4, 1869, just as they were getting agonizingly close to breaking through to the river above, the unthinkable happened. The limestone roof of the tunnel, weakened by all the blasting and the persistent seepage, gave way.

Suddenly, the full force of the Mississippi River, carrying tons of logs and debris, started pouring into the tunnel. It was like pulling the plug on a bathtub, but the bathtub was the entire upper river. Eyewitnesses described the ground shaking, a roar emanating from underground, and then, a massive whirlpool appearing in the river above the falls, sucking down timber and chunks of earth. The river was literally trying to swallow itself, right under the city.

Panic ensued. The mill owners, realizing their folly, scrambled. This wasn't just about a lost investment; it was about the very existence of St. Anthony Falls. If the river kept eroding the soft sandstone, it could retreat so far upstream it would become just a series of rapids, destroying the power source that fueled Minneapolis. The city's future hung by a thread.

From Disaster to Design: Saving the Falls

The immediate aftermath was a frantic, desperate effort to plug the gaping hole. Barges loaded with clay, rocks, and even tree branches were deliberately sunk over the whirlpool, trying to stem the flow. It was a Herculean task, costing a fortune and taking months. But it worked, just barely.

What followed was a decades-long, monumental project led by the U.S. Army Corps of Engineers. They realized the falls were simply too unstable in their natural state. They essentially put the falls in a concrete corset. They built a massive, underwater concrete 'apron' (which is what you see the water flowing over today) and a series of dams, like the Upper and Lower St. Anthony Falls Lock and Dam, to stabilize the bedrock and control the water flow. They injected concrete into every crack and crevice, creating an artificial, unyielding foundation.

So, the falls, which used to be called a 'rapidly retreating cascade,' were saved. But the cost was immense, and the original, raw natural wonder of the falls was forever changed, tamed by human hands.

Standing on History: A Modern Ramble

Next time you're enjoying the riverfront, maybe watching the leaves drift down the Mississippi this fall, take a moment to really look at St. Anthony Falls. What you're seeing isn't just a natural landmark; it's a testament to the wild ambition of early Minneapolis, the incredible grit and danger faced by working-class immigrants, and the hard-won lessons learned from a near-disaster.

That concrete apron and the locks? They're the direct legacy of that secret tunnel and the day the river tried to disappear. It's a reminder that even our most beloved natural features often have a surprising, human-made story flowing beneath their surface.

Enjoying this? Get Minnesorta in your inbox every week — no fluff, just good Twin Cities stuff.

Photo by Christopher Osten on Unsplash