Is the fragile geology of St. Anthony Falls imperiled?

In 1992, Greg Brick secured permission to access a rarely visited place on the Mississippi River not far from downtown Minneapolis: the vestigial remains of the Eastman Tunnel.

Brick is a geologist with a passion for exploring underground spaces, so he knew the disastrous collapse of the unfinished tunnel in 1869 utterly transformed St. Anthony Falls. What was once a natural, 50-foot high waterfall with an irregular limestone ledge became the highly engineered, concrete-encased structure you see when you walk across the Stone Arch Bridge.

Brick’s interest in accessing the tunnel was driven in part by an obscure curiosity. Could he use it to catch a glimpse of a very old, well concealed but critical piece of riverine infrastructure known as “the cutoff wall?” 

The immense concrete dike, which runs deep under the riverbed, was constructed by the U.S. Army Corps of Engineers in the 1870s to prevent the Eastman Tunnel collapse from eroding St. Anthony Falls into oblivion — a potentially ruinous development for a young city dependent on water power.

Upon entering the Eastman Tunnel through a trap door at the St. Anthony Falls Laboratory, Brick immediately noticed the unusual nature of the muck he found himself slogging through. “All the sediments were very peculiar, like quick sand. It was unlike the other sediments I’ve seen in the mill district,” recalled Brick, who chronicled the expedition (and many others) in his 2009 book, “Subterranean Twin Cities.”

After trudging about 100 feet through the dark passage, Brick came to a dead end. The concrete bulkhead that blocked him, he realized, was an exposed portion of the cutoff wall.

Brick observed water steadily but noticeably dribbling out of tiny holes in the concrete. He hypothesized that the fine sediment he encountered in the tunnel came from soft sandstone on the other side of the wall, carried there by the flowing water in a process referred to as entrainment.

To Brick, the implications were clear. A void must be forming on the other side. And if a void was forming, he wondered, would that have consequences for the cutoff wall? Would the fragile geology of St. Anthony Falls be imperiled if such a void grew too large and enabled the river to carve a new, subterranean path downriver? What would that mean for all the infrastructure in the highly developed vicinity of the falls?

Brick said he shared these observations at the time with officials at Northern States Power (now Xcel Energy) because he knew the company’s nearby hydro plant could be impacted. But, he said, that only elicited shrugs. 

Concern spreads about a potential disaster

Thirty years later, Brick’s solitary inklings of concern about the state of the cutoff wall — and the damage its potential failure could cause — are no longer solitary.

Renewed interest in the forgotten infrastructure was spurred by the U.S. Army Corps of Engineers’ efforts to offload St. Anthony Falls dam and the adjacent navigational lock. (Owing to concerns about the upriver spread of invasive carp, Congress authorized the closure of the lock in 2015, which effectively eliminated the Corps’ navigation-driven mission on this stretch of the river.)

John Anfinson, a former historian for the Corps and, later, superintendent of the Mississippi National River & Recreation Area, said he began thinking more about the condition of the cutoff wall after he realized that the Corps, in soliciting proposals for the future ownership of the lock and dam, neglected to include the cutoff wall in the discussion.

Who is responsible for inspection and maintenance of the wall? Who owns it? 

The answers to those questions are not clear, which is what worries Anfinson.

“There is no default owner and no one accepts ownership because no one wants to get stuck with it,” Anfinson said. “Without knowing the owner, there is no one responsible for inspection, maintenance, or an emergency action plan if it fails. It’s unconscionable.”

Seeking help from the Legislature   

                                                                                                                         

Despite those uncertainties, Anfinson, along with non-profit groups such as Friends of the Mississippi (where he serves as on the board), launched a push this legislative session at the Capitol, where they are urging state lawmakers to fund a comprehensive look at the condition of the cutoff wall.

The three-pronged bill would provide $1 million to the University of Minnesota to conduct a geophysical survey of the wall and surrounding infrastructure, develop models of what would happen in the event of a failure, and devise an emergency action plan for worst case scenarios.

The Minnesota House included the measure in the major state and local government budget bill but the Senate did not. A conference committee will decide the fate of the legislation, likely as soon as this week.

“It’s got us worried,” said Whitney Clark, the executive director of the Friends of the Mississippi. “It’s not one of those issues where someone is gunning for it. It comes down to money. But I would say the stakes are high. If it doesn’t get this funded this year, it could be a while.”

Like Clark, Anfinson takes pains to note that he is not predicting the imminent collapse of the cutoff wall. 

“There may be nothing wrong. I don’t know if this will fail next year or in another 146 years,” Anfinson said. “No one does. That’s the problem.”

In pressing his case, Anfinson said, he has made formal presentations to several groups, including the Geological Society of Minnesota.  “I said, tell me if I’m off here,” Anfinson added. “And they said, ‘It’s probably worse than you think.’”

Carrie Jennings, a geologist with Freshwater, a Minnesota-based environmental nonprofit, points out that St. Anthony Falls has steadily retreated upriver for the past 12,000 years and, without human intervention, probably would have dissolved into a long series of rapids by now. Even though there is no evidence that the cutoff wall is likely to fail, she added, there is also no evidence to the contrary.

“We need to get a good look at what’s going on down there,” said Jennings.

Eric Waage, director of Hennepin County Emergency Management, concurs. 

“It could be that this is really low risk and we don’t need to worry. But not asking the question is a risk,” Waage said.

A failure of the cutoff wall, he added, has the potential to create havoc both above and below the falls. 

Nearby infrastructure, such as the Third Avenue Bridge, would be damaged if a new water course were to erode the sandstone that lies underneath the limestone and shale upon which the bridge’s piers sit. 

Upriver, the potential disruptions could be worse, conceivably impacting Minneapolis’ water supply. The chief author of the bill in the House, Rep. Sydney Jordan, DFL-Minneapolis, said in a hearing in March that a disruption of the drinking water supply is the prospect that keeps her up at night.

Waage pointed to a 2021 report from the Corps of Engineers that addressed what would happen in the event the dam at St. Anthony Falls failed.

Without extensive stabilization, the study states, the falls would be transformed into a series of rapids and a “head cutting erosion would extend far upstream, affecting roads, bridges, homes and other infrastructure.”

“The 19th century architects of the falls recognized that the loss of the falls would be catastrophic,” the study continues. “With the upstream and downstream development on the river, the same conclusion applies today.”

For his part, Greg Brick has zero ambivalence about the need for a comprehensive survey of the cutoff wall. Given his long ago observations in the Eastman Tunnel, he finds it difficult to believe that there are no substantial voids forming in the sandstone above the falls.

 But he is less certain about whether policy makers will meet the moment.

“I suppose it’s like back before the I-35 Bridge collapse, when the now-infamous ‘rusty gusset plates’ were pointed out,” Brick ventured. “Easy to dismiss as ‘nothing.’”