Why So Many U.S. Dams Are Aging at the Same Time

Why so many U.S. dams are aging at the same time

The United States built most of its dams during a concentrated period in the twentieth century. The result is a national infrastructure portfolio where tens of thousands of structures are reaching the end of their design lives simultaneously. This is not a coincidence. It is the product of specific historical forces: economic policy during the Great Depression, postwar growth, Cold War-era federal investment, and the eventual slowdown in construction driven by environmental legislation and diminishing availability of suitable sites. Understanding why so many dams are aging at the same time is essential for understanding the infrastructure challenge the country faces today.

The National Inventory of Dams (NID), maintained by the U.S. Army Corps of Engineers, contains completion dates for the vast majority of its more than 90,000 records. This data provides a clear picture of when America built its dams, why construction eventually slowed, and what the consequences of that construction timeline mean for the decades ahead.

A history of dam construction in America

Dam building in the United States has followed a distinct arc, driven by federal policy, economic conditions, and changing social priorities. Each era of construction left behind a cohort of dams that now shares similar age-related challenges.

Early dams and the Industrial Revolution

The first dams in what would become the United States were built during the colonial period and the early republic. These were small structures, typically made of timber and stone, that powered water mills for grain, lumber, and textiles. New England, with its abundant streams and early industrial development, accumulated hundreds of these small dams during the eighteenth and nineteenth centuries. Many of them remain in the NID today, making them among the oldest engineered structures still standing in the country.

As the Industrial Revolution progressed, dams grew larger and more ambitious. The development of Portland cement in the mid-nineteenth century made concrete dams feasible, and the growing demand for municipal water supply drove the construction of large storage reservoirs near expanding cities. Some of these nineteenth-century dams experienced catastrophic failures. The Johnstown Flood of 1889, caused by the failure of South Fork Dam in Pennsylvania, killed more than 2,200 people and remains one of the deadliest dam failures in world history.

Depression-era construction

The Great Depression of the 1930s marked a turning point in American dam construction. The federal government, seeking to create jobs and stimulate economic recovery, launched massive public works programs that included dam construction on an unprecedented scale. The Civilian Conservation Corps, the Works Progress Administration, and the Bureau of Reclamation put thousands of people to work building dams across the country.

This era produced some of the most iconic dams in the world. Hoover Dam on the Colorado River was completed in 1936. Grand Coulee Dam on the Columbia River was completed in 1942. Fort Peck Dam on the Missouri River was completed in 1940. These monumental structures served multiple purposes: they generated hydroelectric power, stored water for irrigation, controlled floods, and provided employment during a period of severe economic hardship.

Beyond the headline projects, the Depression era also saw the construction of thousands of smaller dams for conservation, recreation, and water supply. The Tennessee Valley Authority, created in 1933, built a system of dams across the Tennessee River watershed that transformed the economy and landscape of the southeastern United States. Dams built during this era are now approaching or have exceeded 90 years of age.

Post-World War II boom

The decades following World War II saw the most intense period of dam construction in American history. Returning veterans, suburban expansion, agricultural intensification, and Cold War-era federal investment drove demand for water supply, flood control, and recreation facilities across the country.

The Natural Resources Conservation Service (then called the Soil Conservation Service) launched its small watershed program, which funded the construction of thousands of small flood-control and erosion-prevention dams on agricultural land. The Army Corps of Engineers expanded its flood control program, building large multipurpose dams on major river systems. The Bureau of Reclamation continued its western water development program, constructing large storage and irrigation dams across the arid West.

State and local governments built thousands of dams for municipal water supply, recreation, and flood protection. Private landowners built tens of thousands of small earth dams for farm ponds, stock watering, and recreational lakes. The combination of federal funding, state and local initiative, and private construction created a wave of dam building that peaked in the 1960s.

The peak decades: 1950s through 1970s

NID data shows that the 1950s, 1960s, and 1970s were the peak decades for dam construction in the United States. During this thirty-year period, more dams were built than in any comparable period before or since. The 1960s alone saw the completion of thousands of new dams across the country.

Several factors converged to drive this construction boom:

• Federal funding: Congress appropriated large sums for flood control, water supply, and conservation programs that included dam construction.
• Suburban growth: The rapid expansion of suburbs created demand for new water supply reservoirs and recreational lakes.
• Agricultural development: The intensification of American agriculture drove demand for irrigation water in the West and flood protection in the Midwest and South.
• Available sites: Many suitable dam sites across the country had not yet been developed, making new construction feasible.
• Engineering confidence: The dam engineering profession had developed standardized designs and construction methods that made dam building relatively routine.
• Cold War investment: Federal infrastructure spending was partly motivated by the desire to demonstrate American economic and technological prowess.

The dams built during these peak decades are now 50 to 75 years old. Many of them were designed for a service life of 50 years, which means they have reached or exceeded their original design horizons. This does not mean they are all in danger of failing, but it does mean they are entering a period where age-related deterioration becomes increasingly likely and where original design assumptions may no longer be adequate.

Why construction slowed

After the 1970s, the rate of new dam construction in the United States declined sharply and has remained low ever since. Several factors contributed to this slowdown.

Environmental legislation

The passage of major environmental laws in the late 1960s and 1970s fundamentally changed the regulatory landscape for dam construction. The National Environmental Policy Act (NEPA) of 1969 required environmental impact assessments for major federal projects, including dams. The Clean Water Act of 1972 imposed new restrictions on activities that affect waterways. The Endangered Species Act of 1973 protected habitat for threatened and endangered species, some of which depend on free-flowing rivers.

These laws did not prohibit dam construction, but they significantly increased the time, cost, and complexity of building new dams. Environmental review processes that might take months or years, combined with the possibility of legal challenges from environmental groups, made many proposed dam projects economically or politically unviable.

Fewer suitable sites

By the 1970s, many of the best dam sites in the United States had already been developed. The sites with the most favorable geology, the narrowest canyon profiles, and the best water supply potential had been claimed by earlier generations of dam builders. Remaining sites often had less favorable characteristics, higher construction costs, or greater environmental impacts.

Changing public attitudes

Public attitudes toward dams shifted during the 1970s and 1980s. The environmental movement raised awareness of the ecological costs of damming rivers, including the loss of free-flowing habitat, the disruption of fish migration, the alteration of downstream ecosystems, and the inundation of scenic valleys and cultural sites. Dam removal, once unthinkable, began to gain acceptance as a river restoration strategy.

Economic factors

The economics of dam construction also shifted. Construction costs rose faster than the perceived benefits of new dams, particularly as the highest-value sites were already developed. Alternative approaches to water supply, flood management, and energy production became more competitive, reducing the economic justification for new dam construction.

What aging means for dams

The simultaneous aging of tens of thousands of dams creates a set of interrelated challenges that affect dam safety, water management, and infrastructure policy across the country.

Material deterioration

All construction materials deteriorate over time, and dam materials are no exception. Concrete dams can experience alkali-silica reaction, freeze-thaw damage, and erosion of spillway surfaces. Earth dams can experience seepage, internal erosion (piping), slope instability, and burrowing by animals. Mechanical components such as gates, valves, and operating equipment can corrode, seize, or wear out. Aging dams require more frequent inspection and more extensive maintenance to detect and address these deterioration processes before they become safety concerns.

Outdated design standards

Many older dams were designed using standards and methods that have since been superseded. Hydrologic standards, in particular, have changed significantly. Many dams built in the 1950s and 1960s were designed for flood events that have since been revised upward based on better data and analysis. Some dams have spillways that are too small to pass the floods that current standards require, which means they could be overtopped during extreme storms. Seismic design standards have also evolved, and many older dams were not designed to withstand the earthquake loads that current standards specify.

Sediment accumulation

Reservoirs trap sediment that would otherwise flow downstream. Over decades, this sediment accumulates behind the dam, reducing the reservoir's storage capacity and potentially affecting the dam's structural integrity. Sediment accumulation is a particular concern for flood control dams, which lose their effectiveness as their storage capacity diminishes, and for water supply dams, which may no longer be able to store the amount of water they were designed to hold.

Increased downstream development

As described earlier, downstream development has expanded dramatically since many dams were built. A dam that was in a rural area in 1960 may now have a suburb downstream. This development increases the consequences of potential failure, often leading to hazard potential reclassification from low to significant or from significant to high. The dam has not changed, but its importance to public safety has increased because more people are at risk.

The rehabilitation challenge

Addressing the aging of American dams requires a massive investment in rehabilitation, repair, and in some cases, removal. The scale of this challenge is daunting.

Costs

The American Society of Civil Engineers has estimated that the cost of rehabilitating the nation's dams runs into tens of billions of dollars. The cost of rehabilitating just the high hazard potential dams with identified safety deficiencies exceeds $75 billion. These estimates continue to grow as more dams age and as construction costs increase.

Funding gaps

Funding for dam rehabilitation comes from a patchwork of federal, state, local, and private sources, and it falls far short of what is needed. Federal programs such as the High Hazard Potential Dam Rehabilitation Grant Program, administered by FEMA, provide important but limited funding. State dam safety programs have varying levels of funding, and many are stretched thin. Local governments often lack the resources to rehabilitate their aging dams. And private dam owners may not have the financial capacity to undertake major repairs.

The Infrastructure Investment and Jobs Act of 2021 provided additional funding for dam safety and rehabilitation, but the amount allocated still represents only a fraction of the estimated need. Closing the funding gap will require sustained commitment from all levels of government and creative financing approaches.

Deferred maintenance

Deferred maintenance is a pervasive problem across the national dam portfolio. When dam owners lack the funds or the awareness to perform routine maintenance, small problems grow into large ones. Vegetation on dam embankments, clogged drains, inoperable gates, and deteriorating spillways can all escalate from maintenance issues to safety concerns if left unaddressed. The cumulative cost of deferred maintenance across tens of thousands of aging dams is enormous and growing.

Case examples

Several high-profile dam incidents in recent years have illustrated the consequences of aging infrastructure and the importance of maintenance and rehabilitation.

Oroville Dam, California (2017)

Oroville Dam is the tallest dam in the United States, a 770-foot earth-fill structure completed in 1968. In February 2017, heavy rains filled the reservoir to capacity, and water was released over the main spillway. A section of the concrete spillway eroded and collapsed, creating a massive crater that threatened to undermine the entire spillway. When the damaged main spillway could not handle the flow, water began to flow over the emergency spillway, an unlined hillside that had never been used. The erosion of the emergency spillway threatened an uncontrolled release of the reservoir, and nearly 200,000 people downstream were ordered to evacuate.

The crisis was contained without a dam failure, but the subsequent repair cost approximately $1.1 billion. Investigations revealed that the spillway deterioration had been developing for years and that maintenance and monitoring had been inadequate. The Oroville incident demonstrated how deferred maintenance on an aging dam can create a cascading emergency.

Edenville and Sanford Dams, Michigan (2020)

In May 2020, heavy rainfall caused the failure of Edenville Dam on the Tittabawassee River in central Michigan. The failure of Edenville Dam sent a wall of water downstream to Sanford Dam, which also failed. The resulting flooding devastated the communities of Sanford and Midland, damaging or destroying thousands of homes and causing hundreds of millions of dollars in damage.

Both dams were nearly 100 years old. Edenville Dam had been the subject of regulatory action by the Federal Energy Regulatory Commission, which had revoked its hydropower license in 2018 due to inadequate spillway capacity. The dam's private owner had been unable to fund the required upgrades. The failure illustrated how aging infrastructure, inadequate spillway capacity, and insufficient funding can converge to produce a catastrophe.

Boone Dam, Tennessee (2014-present)

Boone Dam, a 160-foot concrete and earth-fill dam completed in 1952 by the Tennessee Valley Authority, was found to have a significant seepage problem in 2014 when muddy water was observed downstream of the dam. Investigation revealed that internal erosion was occurring through the dam's foundation, a condition that could have led to failure if left unaddressed.

TVA lowered the reservoir and undertook a massive rehabilitation project that has cost hundreds of millions of dollars and taken years to complete. The repair involved constructing an underground barrier wall through the dam's foundation to stop the seepage. Boone Dam illustrates both the dangers of age-related deterioration in dam foundations and the enormous cost of addressing safety problems that have been developing unseen for decades.

For a broader historical perspective on how American dam construction evolved over the centuries, see our article on the history of dams in America.

What the NID age data tells us

The NID's completion-date data provides a clear quantitative picture of the aging challenge. Analysis of the data reveals several key findings:

• The median age of dams in the NID has now surpassed 60 years, with many dams in the 50-to-75-year age range.
• Thousands of dams have exceeded their original 50-year design lives without major rehabilitation.
• The rate of new dam construction has been far below the rate of dam aging for decades, meaning the average age of the portfolio continues to increase.
• The age distribution varies significantly by state. New England states have some of the oldest dam portfolios, with many dams dating to the nineteenth century. Midwestern and Southern states have large cohorts of mid-twentieth-century dams. Western states have a mix of older federal dams and newer private structures.
• Many of the dams nearing the end of their design lives are owned by private individuals, local governments, or small utilities that may lack the resources for major rehabilitation.

The path forward

Addressing the aging of American dams requires action on multiple fronts simultaneously.

Increased funding

The most fundamental need is more money. Federal, state, and local governments need to increase their investment in dam rehabilitation to begin closing the estimated tens of billions of dollars in unfunded repair needs. New financing mechanisms, such as infrastructure banks, public-private partnerships, and revolving loan funds, could supplement traditional appropriations.

Prioritized investment

Because the total cost of addressing all deficient dams is so large, investment must be prioritized. High hazard potential dams in poor condition should receive the highest priority because they represent the greatest risk to public safety. NID data provides the foundation for this prioritization by combining hazard potential classifications with condition assessments and age data.

Updated standards

Engineering standards for dam design and rehabilitation need to continue evolving to reflect current knowledge about hydrology, seismicity, climate change, and dam performance. Standards should be practical and achievable, recognizing that perfect safety is not possible and that resources must be allocated where they will do the most good.

Dam removal

For some aging dams, particularly those that no longer serve their original purpose, removal may be a better option than rehabilitation. Dam removal can restore river ecosystems, eliminate safety risks, and avoid the ongoing costs of maintaining structures that provide little or no benefit. The number of dam removals in the United States has increased significantly in recent years, and this trend is likely to continue as more dams age out of useful service.

Better data

Improving the quality and coverage of NID data will help policymakers, regulators, and communities make better decisions about aging dams. This includes updating condition assessments more frequently, improving the accuracy of hazard potential classifications, and expanding the NID to include smaller dams that may pose safety risks but are currently below the inclusion thresholds.

For a broader look at how infrastructure data like the NID can support better policy decisions, see our article on the value of open infrastructure data.

Community engagement

Communities downstream of aging dams need to be informed and involved in decisions about dam safety. This includes access to NID data, participation in emergency action plan development, and engagement in planning processes that affect dam rehabilitation and removal decisions. An informed public is a powerful ally in the effort to address the aging dam challenge.

The simultaneous aging of tens of thousands of American dams is not a crisis that appeared overnight. It is the predictable result of a concentrated construction boom followed by decades of insufficient investment in maintenance and rehabilitation. The data in the NID tells the story clearly. The question now is whether the country will respond with the sustained investment and attention that the challenge demands.