The partial failure of Minnesota's 114-year-old Rapidan Dam highlights the risks many communities face as the number of dams in disrepair increases and rainfall becomes more frequent and intense due to climate change.
Before the breach, which caused floodwaters to erode the Minnesota River bank and swallow a waterfront home, local leaders in Blue Earth County, Minnesota, had been weighing whether to raze the aging structure or make costly long-term repairs. Regulators deemed the dam a "significant" hazard, and it was classified as in "poor" condition in April 2023, according to the National Inventory of Dams.
Nearly 4,100 dams are categorized as at the same risk level and condition - or worse - according to an NBC News analysis of the inventory data. Every state has at least one such dam. Ohio has the most with 373.
Overall, America's dams-more than 91,000 in all-are aging. Many need expensive rehabilitation. Few are designed for today's climate, with a warmer atmosphere that can hold and release more water. The number of people living in the floodplains below these dams continues to grow.
The average American dam is 57 years old, and dam safety experts say the pace of investment has been too slow to bring the infrastructure up to the standards it was built for, much less increase climate risks.
"It's falling behind," said Sharon Tapia, president of the Association of State Dam Safety Officials. "We're in a situation where we're seeing more and more dams needing to be rehabilitated or repaired to meet current standards."
The association estimated in a report last year that it would cost $157.5 billion to bring non-federal U.S. dams up to standard. That only addresses current safety standards - it doesn't take into account improvements to address additional, future risks from climate change, she said.
President Joe Biden's 2021 infrastructure bill included $3 billion for dam safety projects, a fraction of the total need.
It's still too early for scientists to say whether global warming played a role in the Rapidan Dam's partial failure, but it has made the conditions that led to it more likely.
For every degree Fahrenheit of warming, the atmosphere can hold and release about 3% to 4% more moisture, making storms pack a stronger punch and making precipitation totals once considered rare much more common. That in turn increases the risk of devastating floods.
The Rapidan Dam, built in 1910 and operated by Blue Earth County, was described on the county's website as being in a "state of disrepair." After floods in 2019 and 2020, a power company stopped leasing the dam, leaving it without a hydroelectric plant operator.
But the Federal Energy Regulatory Commission, which regulates the dam, was not overly concerned last month about the integrity of the structure. According to FERC documents, the dam was inspected on May 21 and found "in generally satisfactory condition."
"No major safety deficiencies were identified at the dam that require immediate remedial action," the letter said, although it did note serious concrete deterioration, exposed rebar and cracks that required close monitoring.
The inspection, an analysis of the safety and stability of the structure, was separate from the assessment by the National Dam Inventory, which considers the full performance of the project, a FERC spokesperson said.
Blue Earth County officials had weighed whether to remove the dam, which would cost more than $82 million, or repair it for about $15 million. Both options had drawbacks: Repairs might last only 40 years, while it could take five years to plan for removal and obtain permits, according to 2021 engineering documents. A county spokesman said officials were unable to answer questions beyond providing updates at news conferences.
The partial failure of the dam came after three days of heavy rainfall that raised the Minnesota River to its third-highest flood stage since at least 1881, according to Brennan Dettmann, a National Weather Service meteorologist based in the Twin Cities.
In the Mankato area, where the dam is located, 3 to 8 inches of rain fell in three days, based on an analysis by Kenny Blumenfeld, a senior climatologist with the Minnesota State Climate Office.
"That elbow of the Minnesota River got hit pretty hard," he said, adding that such heavy rainfall could occur in southern Minnesota at between 0.5% and 2% of the rate each year.
Bill McCormick, who led the state of Colorado's dam safety program from 2011 to 2021, said extreme rainfall has increased pressure across the country.
"We are increasingly experiencing violent storms that put the aging infrastructure to the test. Spillways and dams that may not have seen as many storms in a given year are now seeing more storms," he said. "All these aging systems are now being tested more and more often."
Residential development also increases risk levels for some dams, McCormick added, as people settle in once-rural areas, where dams built for farmland now guard subdivisions.
Hiba Baroud, an assistant professor of civil and environmental engineering at Vanderbilt University, said the Rapidan's partial failure should prompt lawmakers to take a hard look at how to strengthen dam infrastructure and triage repairs.
"We really need to be proactively thinking about possible scenarios for all the dams in the U.S. and start prioritizing which dams need to be rehabilitated or upgraded to prevent a situation like this from happening," she said, "rather than witnessing a major event and using it as a wake-up call about this particular dam."
According to data from the Association of State Dam Safety Officials and analyzed by NBC News, 283 dams in the U.S. have experienced some form of failure between 2013 and 2023. Some caused minor problems, but others had serious consequences. In 2019, a snowstorm caused a dam to break, washing away a Nebraska man's home and causing him to drown.
According to Tapia, dam renovation is too often hampered by insufficient financing and lengthy environmental permitting procedures.
"They're just taking too long to resolve because of the funding and permitting issues," she said. "The engineering is usually the easiest part."
This article was originally published on NBCNews.com