Dangerous dams: Where are the six most at risk of failing in the US?
The number of dams in the United States at risk of overtopping is increasing, "threatening their structural integrity and downstream communities," according to a new study from Florida researchers.
About 250,000 people in three states live downstream of those dams, the researchers found.
Published July 21, the study analyzed five decades of water-level data across 33 strategically selected dam sites to quantify how overtopping probability has evolved since 1973.
"We identified six dams having the greatest overtopping probability, with several being located near large population centers, posing potential risks to the downstream communities," warned the study, published in the peer-reviewed British journal Nature Communications. "All six dams are classified as large and high-hazard potential."
Where are the most at-risk dams located?
The six dams with the highest probability of overtopping and the closest downstream cities were all located in Texas, Kansas and California:
According to Florida State University scientists, this geographic distribution shows that certain regions are particularly vulnerable to changing weather patterns, such as more-frequent extreme rainfall.
Aging infrastructure a concern
Many U.S. dams were built nearly a century ago. Aging construction and changing hazards add to the risk for this crucial infrastructure.
"In its 2025 infrastructure report card, the American Society of Civil Engineers gave U.S. dams a D+ rating, underscoring the urgency of reassessment and modernization," according to a statement from Florida State University.
'Decades ago, dam builders used the best available technology knowledge, but things have been changing, so aging infrastructure is something that is a concern,' said study co-author Ebrahim Ahmadisharaf, an assistant professor in the Florida A&M/Florida State University College of Engineering.
'In this study, we showed where hazards from overtopping are greatest, both because of risk of occurrence and possible consequences. It is a guide to where infrastructure spending could have the greatest impact,' he said.
31 of 33 dams studied were 'high-hazard'
Of the 33 dams studied, 31 dams were classified as 'high hazard' by the Federal Emergency Management Agency (FEMA), meaning their failure could result in loss of life.
'We have to plan upfront for this potential risk,' Ahmadisharaf said. 'This information can help dam managers to consider whether they need to revisit their emergency action plans and strategy for operating dams.'
What is 'overtopping?'
In the study, "we defined dam overtopping as water levels rising uncontrollably above the dam crest, which may or may not lead to catastrophic failure."
Overtopping occurs when stored water exceeds the capacity of a reservoir and spills over the top. This does not necessarily result in immediate damage or failure, but it weakens the dam structure and can increase the risk of catastrophic failure if it continues.
Overtopping because of inadequate spillway design, debris blockage of spillways or settlement of the dam crest accounts for about 34% of all dam failures in the United States, according to the Association of State Dam Safety Officials.
Dams have an important role in mitigating flood damages by regulating flow and storing excess water during periods of heavy precipitation. "However, they also pose a substantial risk of catastrophic failure when they become subject to hydrologic extreme events, such as floods that are exacerbated by climate change," the study reports.
'Uniquely severe' risk
"Dam failures are uniquely severe compared to other infrastructure failures due to their potentially extensive damages to the economy, society, and ecosystem," the study said.
"For example, in 2017, the spillway failure of Oroville Dam in California, the tallest dam in the United States, led to over 180,000 evacuations while the 2020 failure of Edenville and Sanford Dams in Michigan forced about 11,000 residents to evacuate, damaged over 3,000 homes, and incurred substantial recovery costs," the study said.
"In 2024, Hurricane Helene caused flooding in North Carolina, where Lake Lure Dam partially overtopped, leading to evacuations in Rutherford County and over 25 swift water rescues."
Study co-author Eunsaem Cho, a former FSU postdoctoral researcher who is now a research associate at NASA Goddard Space Flight Center, said, 'this study represents the first step toward a comprehensive assessment of dam overtopping probabilities in the U.S. Understanding this potential risk is crucial for protecting communities and prioritizing dam rehabilitation before catastrophic failures occur.'
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