Historic low flows turned the Mississippi River into a construction area in 2023, as the U.S. Army Corps of Engineers dredged huge quantities of sand to keep the channel open for barge traffic. Massive machines like the Dredge Goetz, a 225-foot-long vessel with a suction pipe nearly 2 feet wide, were moving through the river constantly to keep it clear.

From May to July, "day in and day out, we were digging," said Tom Heinold, chief of operations for the Rock Island District of the Army Corps of Engineers.

Alternating extremes of heavy rainfall and drought are resulting in wildly varying river levels. For the Corps, which is required by federal law to maintain the Mississippi River for commerce, that makes the multimillion-dollar practice of constant dredging more difficult to predict and plan.

In the upper reaches of the navigable Mississippi River, it's a challenge to place dredged sand in the narrow river landscape — and old agreements for where to put it aren't keeping up with the continuous flow of sediment.

"We've got a mission to accomplish and commercial vessels need to pass," Heinold said. "That's the backbone of our economy out there, and we can't let it fail."

The ability to navigate the river as a whole has deteriorated since 1963, according to a study published in the journal Geophysical Research Letters this year.

A quick switch from flood to drought is one of the most challenging dredging scenarios, said Heinold, whose territory runs from Dubuque, Iowa, to Saverton, Mo. A sudden drop in flow means the water in the river loses velocity, and all the sand flowing with it drops to the bottom.

That scenario unfolded this year, as a springtime slug of snowmelt swelled the upper river, and then almost as quickly, a drought set in. That led sand to pile up in usual choke points — and in some less-expected places, too.

The three Corps districts that maintain the Mississippi north of its confluence with the Ohio River in Cairo, Ill., have to move mountains of sand every year. They shifted almost 8.8 million cubic yards in 2022 alone. That's enough to fill the Great Pyramid of Giza roughly two and a half times.

It's also expensive work for the Corps and the taxpayers, between surveying potential dredging areas, sucking the sand up and moving it into storage areas. The dredging program on the Upper Mississippi cost an average of $45.4 million a year between 2014 and 2023, according to data provided by three Corps offices after a Freedom of Information Act request.

Where the river runs freely, dredge sand is often sprayed back under the water. But farther upstream, it's more complicated to find a place to stash the sand.

Sand surplus

The St. Paul Corps District maintains the river from the head of navigation in the Twin Cities to Dubuque. The district has spent an average of $15.7 million a year dredging its section of the river for the past decade. The cost peaked in 2018 at $27.1 million — and more than half that cost came from moving sand between temporary and permanent sites.

The costs reflect in part a hard-fought compromise between the Corps and the state of Wisconsin. The state sued the Corps after the Clean Water Act was passed in the early 1970s for indiscriminate disposal of dredged sand. Before that suit, "they'd put the sand wherever it was most convenient," said Jeff Janvrin, who has spent his career working on river management at the Wisconsin Department of Natural Resources.

What resulted, in the 1980s, were plans for dozens of sites to place dredge spoils. In some cases, the Corps is placing the sand on islands that can tower hundreds of feet above the water. Sometimes the sand is later scooped off these islands, and used for habitat-building projects, or shuttled to a final resting place on high ground.

But those carefully brokered plans have largely reached the end of their 40-year timelines.

As the Corps seeks new places to dump sand in the future, Sabrina Chandler, manager of the Upper Mississippi River National Wildlife and Fish Refuge, said, "I'm not sure that it's going to be a very simple fix."

Finding new sites can trigger intense opposition. In 2017, the St. Paul District proposed buying a fourth-generation family farm just south of Wabasha, Minn., and slowly burying it under 15 feet of dredge spoils. But that created an uproar from the Drysdale family, which owned the farm, and the local community.

This section of the river has always presented a dredging challenge. Just upstream of Wabasha, the Chippewa River pours in from Wisconsin, carrying a fountain of sand. The Corps uses four locations in the river nearby as temporary sand stashes, but there's a limit to what they can hold.

The Corps ultimately abandoned the Drysdale farm, and this summer struck a novel agreement to pay the city of Wabasha to handle some of the sediment instead. The agency reached into an obscure part of a 1996 funding bill to find the legal language to support the new agreement.

Bob Edstrom, a project manager for the St. Paul office of the Corps, said Wabasha gets the tipping fee, and the Corps solves its capacity problem. "So it kind of becomes a win-win for everybody, which is really what we were charged with," after the first plan fell apart, he said.

But even the new agreement comes with tradeoffs. Wabasha is now trying to build a new port for barges that would ship the sand to shore, where it would then be trucked to an old gravel pit. The city would own the $4.6 million facility, which has been partly funded already by a Minnesota Department of Transportation grant. Remaining funding would come from additional grants and, potentially, city-issued bonds.

Final design for the project is happening now. In a comment letter, the Fish and Wildlife Service said that the project designers had "not sufficiently consulted on threatened and endangered species," and that project designers needed to include more information on how increased barge traffic would affect the refuge.

A changed river

It's a challenge to tease out all the impacts of dredging and sand placement. The upper Mississippi has already been chopped into 29 pools, each ending in a lock and dam to keep water high enough for shipping navigation. Since Congress required the 9-foot shipping channel in 1930, that navigation mission remains first and foremost for the Corps on the river.

Nearly a century of engineering has fundamentally transformed the characteristics of the river system, said John Anfinson, a historian who wrote a book about the upper Mississippi.

He said that under natural conditions, this part of the river was 2 feet deep or shallower during low water. One report described mussel beds a mile and a half long.

"That mussel habitat that would have been in a natural river has just been wiped out in many places," he said.

Long-term monitoring by the U.S. Geological Survey has indicated that deep-water habitat on the backwaters along the river is filling in.

Wildlife managers who bargain with the Corps on sand placement are left looking for the least harmful scenario in a river system that has already been massively changed by human intervention in 145 years.

With the way the river is managed, Chandler said, "we also are realistic enough to know that [dredging is] really kind of unavoidable at this point."

All of those environmental considerations, and the Corps' need to be cost-efficient, collided this summer in the near-closure of the river at Cassville, Wis.

As water levels fluctuated, sand started dropping into the river bed. Suddenly, so much had fallen out near Cassville that the Corps was faced with shutting down the river to boat traffic.

"We were all just kind of frustrated," said Crystal vonHoldt, a Mississippi River regulatory expert at the Wisconsin Department of Natural Resources. "We were trying to get a placement site and find a location, get all the plans ironed out and our Ts crossed and Is dotted. And now we have the imminent closure."

The Corps had been working on a plan that involved buying land to dump the sand that was gradually piling up there, said Kirk Hansen, Mississippi River regional supervisor at the Iowa Department of Natural resources. But it had simply taken too long.

Initially, the Corps suggested simply putting sand in the thalweg of the river — a term for the deepest and fastest-moving section of water. The state of Wisconsin objected, worried about the potential for the sand to wash away and pile up somewhere else.

The stretch of Mississippi that Wisconsin borders is mostly maintained by the St. Paul District of the Corps, which almost never puts sand back in the water. But it's a regular practice for the Rock Island District, as the river grows bigger and faster farther south, where more than a quarter of dredged sand was placed in the thalweg last year, according to Corps data.

The Corps operates under a difficult cost-benefit equation: the farther sand moves from where it is extracted, the more expensive the work becomes.

But each possible site poses other potential problems. Laying sand in the wrong place could smother the freshwater mussels left in the river, or the sediment could drift into ecologically sensitive backwaters.

"You've got one side that wants to [do] everything as cheap as possible," Janvrin said. "And then you have … the other side that might be wanting to protect everything they can. And what is usually chosen is someplace in between."

Patrick Moes, a spokesman for the St. Paul district of the Corps, wrote in an email that "As stewards of tax dollars, we need to ensure we work within the federal standard, which simply means that we find the least costly option that is environmentally acceptable."

Ultimately, the sand from Cassville landed on an Iowa river bank. Many of the officials who are responsible for managing the river gathered in early November to plant willows there to stabilize the sand, Hansen said. He added that of several options, it had the fewest freshwater mussels, and no endangered species that might be harmed.

Heinold said the site was cleared "just moments before we were ready to put a dredge in the water," making for a solution that everyone agreed upon.

Ultimately, he said, the Corps had to get traffic moving again.

Madeline Heim of the Milwaukee Journal Sentinel contributed reporting. This story is a product of the Mississippi River Basin Ag & Water Desk, an independent reporting network based at the University of Missouri in partnership with Report for America, with major funding from the Walton Family Foundation. Sign up to republish stories like this one for free.