The Only Waterfall on the Mississippi is Broken

Walls and dams have been built out onto the falls, the water being so low the Mississippi could not carry away the load of sawdust, chips, odds and end of board and plank, and logs dumped in upstream. This industrial waste was stuck everywhere in big jumbled heaps in the falls attractive little niches and in the rocky clefts intended by nature for the joyous downward passage of crystalline waters. These are the words of Johann Kohl, a German scientist who visited the Falls of St. Anthony here in downtown Minneapolis in 1856. They don't paint a very pretty picture and that's because at the time the city was in the process of breaking this waterfall. They just didn't know it yet. And as a result of their actions there is now a long concrete wall submerged beneath the falls that if it were to fail could destroy the city of Minneapolis. I'm Knowles, I'm here to explore and explain. St. Anthony Falls, the only falls on the Mississippi River, was first used in an industrial capacity by Franklin Steele in 1847 when he placed a dam across the eastern channel of the river running from the eastern shore to Hennepin Island and then to Nicollet Island in order to catch logs and power sawmills. The dam was 700 feet long and 16 feet high. Crews began logging along the Crow Wing River and the lumber was used to build the burgeoning town of St. Anthony on the east bank of the river. By 1852 there were four sawmills producing 50,000 board feet of lumber each day. From time immemorial the Falls, or Owamini-Yomini, were a holy place to the local Dakota people. Since the Falls were a site of portage and trade with other native peoples they were seen as the home of spirits, particularly Unctehi, the spirit of water and evil. European settlers saw the Falls as a holy place too, it's just that they worshipped different things, namely the Almighty Falling water meant energy and energy meant production of goods to sell to consumers. Soon dams and miles of mill races, canals, and tunnels were built to power the first lumber mills and later flour mills as Minneapolis, on the west bank of the river, became the milling capital of the world and then the largest city between Chicago and the Rocky Mountains. In 1856 millers in Minneapolis built a canal to power their mills and a dam to supply it with water. The next year millers in St. Anthony built their own dam and the two met in a v-shape above the Falls, a shape which remains prominent in the Falls today. By 1869 there were 18 lumber mills and 11 flour mills, Pillsbury and General Mills among them, operating along the Falls and so much water was being diverted from the river into the system of tunnels and mill races that on some days in summer and winter the Falls ran dry. In 1865 a group of industrialists led by William Eastman and John Merriam bought Nicollet Island just upstream of the Falls and the purchase came with water power rights. After a short legal fight with the millers downstream they secured the right to build a 2,500 foot long tail race tunnel from the tip of Nicollet Island to Hennepin Island. Whereas a head race leads water from the river to the mill, a tail race takes it back to the river after it has been used. And while other tunnels had been dug in the area this was the first to actually dig beneath the river itself and this is where the problem started. You see St. Anthony Falls was formed 12,000 years ago at the end of the last Ice Age as water from a glacial lake poured over a limestone cliff in present-day St. Paul. The Falls were 2,700 feet across and 175 feet high. Beneath the limestone was a thin layer of shale and then a ton of sandstone, some of which was the very pure St. Peter sandstone. As the water cascaded over the relatively hard limestone layer it eroded the very soft sandstone layer. As the sandstone gave way the limestone could no longer support itself and it too eventually gave way, the chunks then being slowly washed downstream creating small rapids. The Falls moved upstream quite quickly reaching the confluence of the Mississippi and the Minnesota Rivers 2,000 years later. In the past 10,000 years they've moved eight miles upstream to Minneapolis. In those times when the Falls would run dry it exposed the limestone to the elements making it susceptible to the freeze-thaw cycle and cracking it. Water seeped through the cracks and hastened the Falls retreat. Logs going over the Falls and slamming up against them also contributed. In the 1850s the Falls were retreating about 26 feet per year. Minnehaha Falls, south of Minneapolis and formed by the same processes, provides a glimpse at what the pre-industrial Falls might have looked like. They're 53 feet tall and have eroded their sandstone layer quite a bit forming the concave area behind them. But because the volume of water going over them is so much smaller than in the Mississippi, Minnehaha Falls are eroding at a much slower pace. While St. Anthony Falls has lived a whirlwind life, Minnehaha Falls is something of a homebody. Father Louis Hennepin, a Catholic missionary, was the first European to write about St. Anthony Falls or Les Chutes de St. Antoine which he came across in 1680. In his day the Falls were over 50 feet high and could be heard up to 15 miles away. Their beauty was often compared to Niagara and into the 1850s an overland trip from St. Paul to these majestic Falls was a popular sightseeing excursion. By 1868, of course, as the Eastman Tunnel was being dug through the sandstone beneath them, the Falls had become commodified. Work began on the Eastman Tunnel in September 1868 and went well for a year until the tunnel began to leak in October 1869. On October 5th the river broke through the limestone layer. The tunnel had been six feet square but within a few hours water carved out an area sixteen and a half feet deep and 90 feet wide. The entire Falls were suddenly in danger of eating themselves into rapids, removing the water power at the site and thus ruining the economic prospects of the fledgling city. Hundreds of local men volunteered to build wooden rafts filled with rocks, logs, soil, gravel, anything they could think of and drive them into the whirlpool in an attempt to fill the hole. Nothing worked. The Falls ate everything. Finally, after weeks of work, three different cofferdams were constructed to divert the water away from the area but new sinkholes would pop up and pull chunks of the Falls into them. In 1870, the plugged tunnel collapsed again, taking a couple mills down with it. Each year there were new dams and new collapses. The Falls were now retreating 30 feet each year. Finally, in 1876, the Army Corps of Engineers completed what's called the Cut-Off Wall, a concrete wall six feet thick which extends 40 feet beneath the limestone layer and runs 1,800 feet across under the river. This wall stops the water coming downstream from scouring out the sandstone. In 1880, they completed the concrete apron along the face of the Falls, which gives the water a smooth angled alternative to the vertical limestone drop and pushes the water away from it, thus preventing erosion in the upstream direction as well. The Corps also raised the level of the V-shaped dams, provided a sluice way for logs, and filled in the gaps in the sandstone with over 22,000 cubic yards of gravel. Stopping the geologic clock by encasing the Falls in concrete did nothing to stop their use by millers. In 1881, Pillsbury built their A-Mill, the largest flour mill in the world, which contained the largest water wheels ever built, and that same year the Falls were used to produce a new product, hydroelectricity. The streetcar system and many streetlights in the city were powered by two large hydroelectric plants. In 1948, the Army Corps of Engineers again changed the Falls when they dredged the river and began construction of the upper lock and dam. This lock and dam, with the highest lift of any lock on the Mississippi, obliterated two islands which had sat below the Falls, one of which was a nesting ground for bald eagles. The lower lock and dam, completed in 1956, turned the natural rapids into a slack water pool. In 1952, the concrete apron was rebuilt. The upper lock and dam were closed by Congress in 2015 due to the invasion of Asian carp, and that effectively eliminated the role of the Army Corps of Engineers at the Falls. In 2025, five acres of federal property were transferred to a Dakota nonprofit, which is demolishing some structures and will be restoring about a mile of the western riverbank to its pre-industrial appearance. The Army Corps does not claim responsibility for the cutoff wall, even though they built it, and neither does anyone else, for that matter. Xcel Energy, which owns the hydroelectric plant, inspects the cutoff wall once a decade. Well, only the little bit of it they can see on the downstream side of the Eastman Tunnel. Leaking has been reported, and the safety reports are classified because they pertain to critical energy infrastructure, but trust us, it's fine. If the cutoff wall were to collapse, the remaining limestone layer would collapse, and the city's drinking water supply would quickly disappear as the intake pipes would be above water level. The Third Avenue bridge upstream, whose piers rest on the limestone, might collapse, and the rush of water and rock and concrete could severely damage the hydro plant, the lock and dam, and bridges and developments downstream. In 1987, this happened on a much smaller scale as water eroded away the foundation of the lower hydro plant, leading to its collapse and subsequent demolition. If the cutoff wall were to go, the city would not just lose its infrastructure, but would lose the very thing which made it all possible. But here's the geologic punchline. Even if humans had never intervened in the falls in the first place, never utilized them for their energy, nor concreted them in place, they would have destroyed themselves in the next couple centuries anyway. Prior to the mid-19th century, the falls were moving upstream somewhere between four to six feet each year, relying on a cycle of sandstone erosion and limestone collapse. But that layer of limestone actually ends about a thousand feet north of the falls' present-day location, at the southern tip of Nicolet Island. In the end, it almost reads like something out of an ancient myth. The falls were saved by the same people who had sought to destroy them not too long ago. But in order to be saved, they had to be frozen in time forever, never allowed to age, never allowed to meet their geologic fate. The city would not exist without the falls, nor could the falls exist without the city. Thank you so much for watching. I'll see you in the next one.