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By Farshid Vahedifard, professor of civil and environmental engineering at Tufts University. Originally published on The Conversation.
In recent weeks, powerful atmospheric river storms have battered Washington, Oregon, and California, dumping significant amounts of rain. As the river emerged, it overtopped or breached multiple levees, long, unnoticed embankments that held back floodwaters from homes and towns.
In most cases, contempt does not require attention. They quietly do their job every year. But when the storm intensifies, calm events suddenly become significant in very personal ways. They can determine whether a neighborhood will stay dry or become flooded.
The damage in the West reflects a nationwide problem that has been building for decades. Mild populations are aging across the United States, and weather is becoming more extreme. Many of these structures were not designed with the large responsibilities they now assume.
As a civil engineer at Tufts University, I study water infrastructure, including levee fragility and strategies to improve levee resilience. My research also shows that small failures do not impact the population equally.
It was almost by chance that the levee became critical infrastructure.
Many believe that the levees were built as part of a modern, carefully designed flood control system. In fact, many of the levees still in use today had much more rustic beginnings.
Decades ago, farmers built simple earthen embankments to protect their fields and livestock from seasonal flooding. These early levees were practical solutions shaped by experience rather than formal engineering. They were not built using rigorous design standards and did not follow consistent construction or maintenance guidelines.
Over time, the landscape around these embankments changed. Farmland was replaced by neighboring areas. Roads, railroads, factories, and ports were extended into the floodplain. The population has increased. Once modest local structures protecting farms have gradually become the first line of defense in the homes and workplaces of millions of people.
During the Mississippi Flood of 1927, the river overflowed and overflowed, inundating thousands of square miles of land. This photo shows both the overflow and the collapse. National Weather Service archive photo, Steve Nicklaus, NOS, NGS
Without much public discussion or planning, these semi-artificial levees have taken on an unintended and significant role. The question that remains is whether they were ready for it.
Vast, aging levee system protects millions of people
The National Levee Database currently lists more than 24,000 miles (38,600 kilometers) of levees in the United States, with an average age of about 61 years, and many of them are much older. Together, they will protect more than 23 million people, approximately 7 million buildings, and approximately US$2 trillion in property value.
This is an extraordinary level of responsibility for a system that is unevenly maintained under different oversight. Some minor issues are inspected regularly. Others are owned by small local agencies or private entities with limited funding. In some cases, responsibilities may be unclear or fragmented.
One of the levees that collapsed along the Green River in Washington state during a mid-December storm has been due for repairs for several years, but disagreements between governments have recently delayed necessary work, the Seattle Times reported. The invasion forced thousands of people to evacuate.
Many states are at slight risk. This map shows all levees in the U.S. National Levee Database (red) and 478 levee segments (blue) that are known to have overtopped in the past 15 years. S. Flynn et al., 2025
The American Society of Civil Engineers’ 2025 Report Card on U.S. Infrastructure, to which I contributed, gave the nation a D-plus grade, citing aging infrastructure, inconsistent monitoring, and a long-term lack of investment. A new dataset on levee damage that my colleagues and I created includes 487 instances in the past 15 years where rivers overtopped their levees and overflowed. That doesn’t mean contempt fails everywhere. That means many companies operate with little margin for error.
Mechanism of levee collapse
Mild disorders rarely collapse suddenly. It often starts quietly.
The most common cause of levee failure is overtopping, where water from rivers, streams, or lakes behind the levee flows over the top of the levee. When that happens, erosion can begin on the land side, weakening the structure from the back side. Initially slow droplets can quickly grow into cracks, creating large gaps in the embankment through which water can flow.
Here’s a diagram that shows the difference between overflowing and intrusion, and some of the reasons why overflowing can fail. S. Flynn et al., 2025
Atmospheric river storms greatly increase the risk of overtopping and breaching. These storms bring huge amounts of rain over large areas in a matter of hours, often combined with snowmelt. Rivers rise faster and stay higher for longer. Many levees were not designed for such sustained pressures.
Even small breaches can cause floods to flow quickly and deeply, leaving little time for evacuation and causing damage far beyond the floodplain.
Who relies on levees today?
Millions of Americans live and work in areas protected by levees, often without realizing it. Homes, schools, highways, railroad lines, ports, and power facilities depend on the integrity of these structures.
A recent national study found that across the contiguous United States, urban expansion into floodplains occurred more than twice as fast after levee construction compared to surrounding counties, highlighting how levees can impact community perceptions of risk.
In fact, when levees break, flooding can be worse than in areas without levees because water flows in quickly and drains more slowly.
Risks are also uneven and shaped by history, economics, and political decisions.
That reality was made painfully clear in March 2023 during an atmospheric river storm that breached levees along California’s Pajaro River and flooded the town of Pajaro. Pajaro is home to many low-income farm workers. The flooding forced hundreds of residents to evacuate, with some trapped by rising water levels.
What made this disaster particularly troubling was what came after. Officials and engineers have known for decades that the Pajaro River’s levees are vulnerable. The report noted its weaknesses, but repairs were repeatedly delayed.
Interviews with the Los Angeles Times and public records showed that part of the reason was economic. Decision makers did not prioritize investment in modest institutions that protect low-income areas. The risks were known, but protection was deferred.
Birds are not a special case. Nationally, disadvantaged communities and communities of color are more likely to rely on minors who are older or not participating in major federal programs. Rural towns often rely on neglect of agriculture. Urban neighborhoods may rely on structures built for a much smaller population.
When a minor failure occurs, it has a cascading effect that closes roads, causes power outages, contaminates water supplies, and disrupts life for years.
Disparity refers to the percentage of residents in each state protected by levees who are considered disadvantaged based on the U.S. Council on Environmental Quality’s Climate and Economic Justice Screening Tool. All levees in the national levee database are counted. F. Vahedifard et al., 2023
Why is this moment important?
Advances in engineering, monitoring, and risk assessment have improved the way risks are assessed and designed.
Hurricane Katrina was a turning point in 2005, when storm surge broke through the milder protecting New Orleans. Hundreds of people died in the floods. This disaster revealed the consequences of neglect and fragmented responsibility for meager maintenance.
At the same time, real progress has been made. Over the past two decades, significant investments by the federal government have strengthened the condition and control of many nations’ military forces, particularly through the efforts of federal agencies such as the U.S. Army Corps of Engineers.
Still, the legacy of decisions made decades ago remains, and climate change is increasing the risks. Heavy rains, rapid snowmelt, and rising sea levels are pushing water management systems beyond the limits for which many levees were designed. Events once considered rare have become more frequent.
With atmospheric river conditions becoming calmer in the West and flooding risk increasing across the country, the challenges are no longer just technical. It’s about how societies value protection, communicate risks, and decide whose safety to prioritize.
Levees will continue to play an important role in protecting communities. As future storms arrive, it is essential to understand their history and their limitations.
