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It's anyone's guess as to when the Minnesota River will stop growing.

Stretches of its eastern bend, not far from Mankato, have more than doubled in size since the 1940s. Several homes that were built more than 50 feet from its banks or its main tributaries have collapsed or been demolished before they could be washed away by the encroaching waters. The river, always prone to erosion, has been expanding much faster over the past 20 years than it ever has before.

As much of the state braces for the potential flooding of yet another spring, state lawmakers and pollution control managers are looking for new ways to stop or reduce erosion along the Minnesota.

While cities in the river's path are asking the state for tens of millions of dollars to raise roads, reinforce bluffs and move needed infrastructure farther away from its banks, all those efforts will likely prove to be temporary Band-Aids that don't get to the cause of the problem, said Karen Gran, professor of earth and environmental sciences at the University of Minnesota Duluth.

In the long term, the only way to address the erosion of the river is to increase the storage capabilities around it — to provide enough space for water to safely spill and hold back during flooding events in order to reduce the river's historically high flows, Gran said.

"There is just a lot more water going into the river right now," Gran said. "If we're serious about alleviating these issues, it means we need to start holding more water back on the landscape."

Gran, who has been studying erosion along the Minnesota's two main tributaries for more than a decade, was asked to speak before a bipartisan group of lawmakers in December. Even the creation of smaller temporary storage systems, such as drainage ditches and holding ponds, would go a long way in cutting down erosion, she said.

Those temporary systems are especially important in heavily farmed or fully developed areas where it might be next to impossible to bring back more permanent holding areas, such as a fully restored wetland, Gran said.

"A healthy balance of permanent and temporary storage gives people the flexibility to do what they can do on their land," she said. "The most important thing, though, is we actually have to start doing it."

River is higher, faster

While some erosion is natural along the Minnesota, the amount of sediment that has been choking the life out of the water is now 10 times higher than it was in the 1800s.

The reason for that is largely because the river is much stronger than it used to be. The Minnesota River's flow has more than doubled over the past decade, meaning that water is rushing through at twice the force that it did on average from 1950 to 2010, according to a 2017 study by researchers with Utah State University.

The soil the Minnesota cuts through has always been vulnerable. Now with the river's increased strength, the water is rapidly eating away its shoreline and carrying all that dirt, sediment and pollution downstream.

Several studies have shown that the river has gotten stronger because of two major changes: The state is getting more intense storms and more rainfall than it ever has before, including last year, which went down as the wettest in Minnesota's history. And over the past several decades, western and central Minnesota have become extensively drained by landowners who are converting more and more acres into row crops.

A changing climate has put more water into the system, while at the same time more and more farmers are shooting that water into the Minnesota River and its tributaries, Gran said.

It's unclear how much it might cost to add enough storage systems to actually reduce the river's flow.

The Minnesota has long been the state's most polluted river.

Hundreds of millions of dollars have been spent on the watershed since 1992, when Gov. Arne Carlson set the goal of making it "fishable and swimmable in 10 years."

The state never met that goal. Last summer, the Minnesota Pollution Control Agency (MPCA) found that the river was still so thick with erosion and runoff that sediment levels would need to be cut by at least 50% just to make the water safe for fish and vegetation, let alone for human use. The agency estimated cutting those sediment loads by 50%, along with other cleanup efforts, could cost about $360 million over the next 25 years.

But extensive costs are coming to taxpayers regardless of what happens to water quality.

In just the past 10 years, the Minnesota has washed away more than 50 feet of land in front of a city of Mankato drinking well. The well, which supplies about a third of the city's drinking water, is now teetering less than 8 feet from the river's edge. City officials, worried that they were one strong rainstorm or snowmelt from losing it entirely, have set aside several hundred thousand dollars to immediately reinforce that wall of the riverbank and could spend up to $7 million fighting erosion in the area.

The city of Henderson is requesting about $20 million from the state to raise Hwy. 93 about 8 feet to make sure the town always has a road open during floods. Last year three of the four routes in and out of town were flooded for weeks at a time. The U.S. Army Corps of Engineers said last week that it is studying a project that would cost up to $700,000 to stabilize just 2,000 feet of the river's shoreline in Yellow Medicine County.

Building water storage

What makes investing in temporary storage so attractive is that it can pay off in multiple ways, said state Rep. Paul Torkelson, R-Hanska.

The storage will help alleviate flooding, but it would also cut down erosion and get the river closer to its water quality goals, he said.

"We need to include water storage as part of our thought process," Torkelson said. "Whether it's a new township road or a new development, if it's a project on the land we need to be thinking about how we will incorporate water storage into it."

Scott Sparlin, who leads a grassroots group called the Minnesota River Congress, is in the early stages of working with lawmakers to support a bill that would prioritize water storage within the basin. It would seek to buy up easements from willing landowners.

"There is funding available to do some of these water storage projects, but it's so competitive to get that funding," Sparlin said. "There's no money that is just dedicated to this. That's what we need to seriously tackle this problem."

Greg Stanley • 612-673-4882