The torrent of water pushing the Rapidan Dam to the brink of failure this week came from one of southern Minnesota’s wettest stretches on record. For the second time in five years, near-record floods pounded the century-old structure and clogged it with trees that died during intervening years of drought.
And when the waters churned around the west side of the dam, its vulnerabilities already exposed in past floods, they scoured away so much earth, they carried away a utility substation and a home, and raised fears a bridge just upstream could be the next structure to falter.
Residents of downstream Mankato, Minn., where the Blue Earth River meets the Minnesota River, feared a wall of water was coming if the dam failed, despite authorities’ assurances the river would only rise modestly. Still, though this crisis didn’t live up to those worries, it underscored ways that extreme precipitation could overwhelm infrastructure not designed to endure it — and how other environmental, economic and social problems can cascade from there, experts said.
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More potential hazards loom even as waters slowly recede across the upper Plains: The pools of livestock manure that spilled waste and bacteria into the Mississippi and Missouri river basins; the threat that high waters made room for invasive carp to migrate north; the risk that nutrients washed from cropland will fuel larger algae blooms and contaminate drinking water wells.
And what if the floods had overpowered a bigger dam, upstream of more people? Some in Minnesota and across the country said more must be done to prepare for that growing likelihood.
“We’re not prepared for this new climate regime,” said Whitney Clark, executive director of Friends of the Mississippi River. “While we’re figuring it out, we’re at risk of lots more of these challenges.”
Floods shocked an already vulnerable dam
Even before waters began rising last week, the integrity of the Rapidan was in doubt. The hydroelectric dam built in 1910 hasn’t been generating power since floods that damaged it in 2019 and 2020. There has been debate over whether to repair or remove it ever since.
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The latest floods only complicate the matter.
They came amid widespread floods across the Midwest and upper Plains, the product of storms that dumped rainfall exceeding 10 inches, up to as much as 18 inches, across swaths of the Missouri River basin in parts of Minnesota, South Dakota and Iowa.
For much of southern Minnesota, it was about two months’ worth of rain within nine days, said Pete Boulay, a climatologist in the state Department of Natural Resources.
And it fell on soil already saturated from months of wet weather, sending it coursing into streams and rivers. Even before the most recent storms, researchers at the University of Minnesota’s Southwest Research & Outreach Center measured more moisture in the soil than ever recorded, since at least 1966. Since April, much of southern Minnesota has experienced one of its top 10 wettest stretches on record, Boulay said.
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“We have seen some wet spells before, but this one is a bigger shock because we had four dry years in a row,” Boulay said.
Even after dry spells and past floods, David Hruska told The Washington Post that he never imagined the river could overtake the only house he had ever lived in — one that crumbled from eroded banks into the Blue Earth River on Tuesday.
It should be a warning beyond Minnesota, experts said.
Intensifying rainfall is testing infrastructure nationwide
Around the country, infrastructure is being tested by new precipitation extremes. Even under the most optimistic forecasts, the number of extreme precipitation events is expected to rise dramatically in the upper Plains — and even more so in other parts of the country, said Eric Chu, an assistant professor of human ecology at the University of California at Davis.
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The number of extreme precipitation days — where totals are among the top 1 percent of all events — has grown by about 60 percent in the Northeast since the 1950s, and by 45 percent across the Midwest, according to the latest National Climate Assessment report.
Warming global temperatures, the result of a blanket of greenhouse gases from fossil fuel combustion, mean the air can hold more moisture. That is making downpours heavier and also droughts more intense because warmer air has a greater capacity to carry moisture, and to leach it out of the land.
Not only are existing dams and other infrastructure not built for such extremes, it’s even hard to keep guidelines for new projects in line with future climate projections, said Christine Kirchhoff, an associate professor of engineering design and innovation at Penn State University.
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“There’s got to be hundreds if not thousands of dams out there in similar situations,” Kirchhoff said.
Any trend in the frequency of dam failures is unclear in part because of incomplete historical data, said Martin McCann, director of the National Performance of Dams Program at Stanford University. But the rising intensity of precipitation is probably having an impact, with more recent examples of storm events elevating threats to infrastructure, he said.
In 2015, severe flooding across South Carolina caused dozens of dams to fail. In California, recent stormy years have damaged dams including the one impounding its largest reservoir, Lake Oroville, in 2017. Last year, flooding overwhelmed levees and inundated agricultural communities in the center of the state.
“We’re going to see more and more instances of these smaller dams with small spillway capacities overtopping and failing,” McCann said.
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Dams considered to be “high hazard” — meaning their failure could cause significant death and destruction of homes — are typically designed for as much as 25 inches of rainfall within a 48-hour period, said Bill McCormick, a past president of the Association of State Dam Safety Officials. In the Blue Earth River watershed upstream of the Rapidan — which is considered to be of a slightly lower risk category, “significant hazard” — rainfall came mercifully short of that intensity.
But those risk calculations don’t consider how often heavy rains are falling, and how much more frequently a dam may now be hit with major or near-record storms. With some $3 billion in outstanding costs to rehabilitate dams around the country and heavier rainfall becoming more common, the stress on infrastructure could compound to create more crises like the one in Minnesota, said Lori Cannon Spragens, the dam safety association’s executive director.
“That’s one of our biggest concerns nationwide,” she said.
Impacts can cascade far beyond floodwaters’ reach
The hazards go far beyond the floods and dam failures themselves, said Chu, the lead author of a chapter devoted to the vulnerability of socio-economic and natural systems to climate change in the National Climate Assessment, a major federal report on climate impacts and forecasts most recently updated last year.
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There are costs of disrupted supply chains when major roads are inundated or washed out, raising prices for consumers and perhaps lowering job security in transportation and shipping-related industries, he said. The damage is sending costs of infrastructure maintenance upward.
More frequent floods, plus rising heat, lead to greater incidence of mold and mildew, adding to scarcity and costs in housing markets, as well as exacerbating risks to public health, especially in historically Black neighborhoods that are more likely to sit in floodplains, Chu said.
Health problems and housing instability make it harder for people to participate in the labor market, he added.
“That’s the classic cascading effect,” Chu said.
In Minnesota, the situation on the Blue Earth may have spared the community of Mankato. But it still only adds to environmental concerns facing rivers.
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The Minnesota River, which the Blue Earth feeds, was already one of the most sediment-laden rivers in the state, Clark said, with about nine times as much dirt and silt clouding its waters as the natural background level. Sediment smothers underwater life and clouds waters, preventing sunlight from reaching plants and animals, destroying ecosystems.
The scouring on the Blue Earth will only add to that, and could send more debris downstream to the Mississippi, he said.
There was already a forecast of a larger-than-normal dead zone in the Gulf of Mexico this summer, the product of farm fertilizers, animal waste and other sources of nutrients that feed massive algae blooms. When those blooms die, they strip the water of dissolved oxygen and can suffocate fish and other aquatic creatures.
The upper Plains floods are likely to make that problem worse, too, Clark said.
Invasive carp that have been creeping their way up the Mississippi are known to spread their territory when water levels surge, he added. There are fears that breeding pairs could soon be discovered farther upriver, a threat to native fish populations and ecosystem balance.
Whether the impacts continue to cascade remains to be seen. But it’s nonetheless more proof that such consequences are becoming more possible, and probable, he said.
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