What the National Dam Inventory Can Teach Us About Infrastructure

What the National Dam Inventory can teach us about infrastructure

The United States is home to more than 90,000 dams, and every one of them is tracked in a single federal database. The National Inventory of Dams (NID) is the most comprehensive catalog of dam infrastructure in the world, maintained by the U.S. Army Corps of Engineers in partnership with state dam safety agencies, federal dam-owning agencies, and the Federal Emergency Management Agency. What began as a safety-driven cataloging effort has become an indispensable resource for understanding the state of American infrastructure, the distribution of water resources, and the challenges of maintaining an aging portfolio of critical structures.

This guide explains what the NID contains, how to use it, and what its data reveals about the past, present, and future of dam infrastructure in the United States.

What the NID is and who maintains it

The National Inventory of Dams was established in the wake of several catastrophic dam failures in the 1970s, most notably the collapse of Teton Dam in Idaho in 1976 and the failure of Kelly Barnes Dam in Georgia in 1977. These disasters killed dozens of people and caused hundreds of millions of dollars in damage. Congress responded by directing the Army Corps of Engineers to create a national database of dams so that regulators could identify and prioritize safety concerns across the country.

The NID is updated on a regular cycle, with data submitted by state dam safety programs, federal agencies that own or regulate dams, and the territories. The Army Corps of Engineers serves as the central repository, maintaining the database, standardizing data formats, and making the information available to the public. As of the most recent update, the NID contains records for more than 92,000 dams that meet the minimum size criteria: dams that are at least 25 feet high, or that impound at least 50 acre-feet of water, or that are classified as high or significant hazard potential regardless of size.

The NID does not include every dam in the United States. Thousands of smaller structures, particularly farm ponds and minor impoundments, fall below the NID's inclusion thresholds. Some estimates suggest there are hundreds of thousands of these small dams scattered across the country, many of them unregulated and uninspected.

What data fields the NID contains

Each dam record in the NID contains dozens of data fields organized into several categories. Understanding these fields is essential for anyone who wants to use NID data for research, policy analysis, or community awareness.

Location fields

Every dam in the NID is geolocated with latitude and longitude coordinates, and its record includes the state, county, and nearest city or town. The NID also records the river or stream that the dam is built on, the hydrologic unit code (HUC) for the watershed, and the congressional district where the dam is located. These location fields make it possible to analyze dam infrastructure at every geographic scale, from individual watersheds to entire states or regions.

Physical characteristics

The NID records the physical dimensions and construction details of each dam, including its height (measured from the lowest point of the foundation to the crest), structural height, hydraulic height, length along the crest, and the volume of the dam body. It also records the structural type (earth, rockfill, concrete gravity, arch, buttress, or other), the year construction was completed, and the year the dam was last significantly modified.

On the reservoir side, the NID tracks the normal storage capacity (the volume of water at normal pool level), maximum storage capacity (the volume at the maximum designed water level), surface area, and drainage area upstream of the dam.

Safety and regulatory fields

The NID records the hazard potential classification (high, significant, or low), the condition assessment (satisfactory, fair, poor, or unsatisfactory), the date of the most recent inspection, and whether the dam has an emergency action plan (EAP). It also tracks the regulatory authority responsible for the dam, which may be a state dam safety program, the Federal Energy Regulatory Commission (FERC), or a federal agency that owns the dam.

Ownership and purpose fields

Each dam record includes the owner's name and type (federal, state, local government, public utility, or private) and the dam's purpose or purposes (recreation, flood control, water supply, irrigation, hydroelectric, navigation, fire protection, debris control, fish and wildlife, tailings, or other). Many dams serve multiple purposes, and the NID records all of them.

Time-related fields

The NID tracks several time-related fields that are critical for understanding the age and history of each dam. These include the year the dam was completed, the year it was last modified, and the date of the most recent safety inspection. These fields make it possible to analyze the age distribution of American dams and to identify structures that may not have been inspected recently.

For a practical walkthrough of how to read and interpret individual dam records, see our guide on how to read a dam record.

How to use NID data for research

The NID is one of the most valuable but underutilized datasets in American infrastructure research. Its combination of geographic, physical, safety, and ownership data makes it possible to answer questions that would be impossible to address with any other single source.

Research applications

• Infrastructure assessment: Researchers can use NID data to analyze the age, condition, and hazard potential of dams at any geographic scale, identifying areas where aging infrastructure poses the greatest risk.
• Water resources planning: The NID's storage capacity and purpose data can inform watershed-level water resources planning, helping communities understand how much water storage capacity exists in their region and how it is used.
• Emergency management: The NID's hazard potential classifications and emergency action plan data can help emergency managers identify dams that could affect their communities and assess whether adequate planning is in place.
• Environmental analysis: Researchers studying the ecological impacts of dams can use the NID to identify dams on specific rivers, analyze the density of dams in particular watersheds, and understand how dam purposes and sizes vary across ecosystems.
• Policy analysis: The NID's ownership and regulatory data make it possible to analyze how dam safety responsibilities are distributed across federal, state, local, and private entities, and to identify gaps in regulatory coverage.
• Climate adaptation: As climate change alters precipitation patterns and increases the frequency of extreme weather events, NID data can help identify dams that may face increased hydrologic loads or that may need to be modified to handle changed conditions.

Age distribution insights

One of the most striking patterns in the NID data is the age distribution of American dams. The data reveals a clear construction boom that peaked in the mid-twentieth century and has since declined sharply.

The 1950s, 1960s, and 1970s were the peak decades for dam construction in the United States. During this period, tens of thousands of dams were built for flood control, water supply, recreation, and agricultural purposes. Federal programs, including the Natural Resources Conservation Service's small watershed program and the Army Corps of Engineers' flood control program, funded thousands of these structures. State and local governments built thousands more, and private landowners constructed countless small dams for farm ponds and recreation.

Since the 1980s, new dam construction has slowed dramatically. Environmental regulations, particularly the National Environmental Policy Act and the Clean Water Act, have made it more difficult and expensive to build new dams. The best dam sites have already been developed. Public attitudes toward dams have shifted, with growing recognition of the ecological costs of impounding rivers. Some dams are even being removed to restore river ecosystems.

The result is a national dam portfolio that is heavily skewed toward middle age. The average age of a dam in the NID is now over 60 years, and thousands of dams are approaching or have exceeded their original design lives. This age distribution creates a concentrated maintenance and rehabilitation challenge that is one of the defining infrastructure issues of the coming decades.

Geographic patterns

The NID data reveals dramatic geographic variation in the distribution, size, and characteristics of American dams. Texas has more dams than any other state, with more than 7,000 structures in the NID, many of them small earth dams built for livestock watering, recreation, and flood control. Kansas, Missouri, and Oklahoma also have high dam counts, reflecting the prevalence of small watershed dams built under federal conservation programs.

The western states tend to have fewer dams in total but larger average dam sizes. The great federal dams of the West, built by the Bureau of Reclamation for irrigation and hydropower, are among the largest structures in the NID. States like Colorado, Montana, and California have portfolios dominated by larger dams serving water supply, irrigation, and energy production.

The eastern states have a mix of dam sizes and purposes. New England has a high density of small, old dams, many of them originally built to power mills during the eighteenth and nineteenth centuries. The Southeast has a large number of dams built for flood control and hydropower by the Tennessee Valley Authority and the Army Corps of Engineers.

These geographic patterns reflect the intersection of geology, climate, hydrology, settlement history, and federal investment across different regions. Understanding them is essential for developing policies and funding programs that address the specific dam safety needs of each part of the country.

Ownership trends

The NID data on dam ownership tells a story about the distribution of infrastructure responsibility in the United States. Private owners are responsible for the largest number of dams in the NID, but many of these are small structures with limited storage. Federal and state agencies own fewer dams but tend to own the largest and most significant ones.

The ownership distribution creates a tiered system of dam safety. Federal dams benefit from dedicated engineering staff, regular inspections, and federal funding for maintenance and rehabilitation. State-owned dams receive varying levels of support depending on the state's financial resources and political priorities. Local government dams compete for funding with roads, schools, and other municipal needs. And privately owned dams depend on individual owners who may or may not have the resources and expertise to maintain them properly.

This fragmented ownership landscape is a significant challenge for dam safety. A high hazard potential dam owned by a small rural county may pose the same downstream risk as a high hazard potential dam owned by the Army Corps of Engineers, but it will have a fraction of the resources available for maintenance and safety. For a detailed look at how ownership affects dam safety outcomes, read our analysis of dam ownership patterns across America.

Maintenance gaps

The NID's condition assessment data reveals concerning patterns in the maintenance and repair of American dams. While the majority of dams in the NID have satisfactory condition ratings, a significant number have been rated as poor or unsatisfactory. These dams have identified safety deficiencies that need to be corrected, and the backlog of needed repairs runs into billions of dollars.

The American Society of Civil Engineers has consistently given the nation's dams a D grade in its Infrastructure Report Card, citing the large number of high hazard potential dams in poor or unsatisfactory condition and the growing backlog of needed repairs. The estimated cost of rehabilitating all high hazard potential dams with identified deficiencies exceeds $75 billion, a figure that continues to grow as more dams age and more downstream areas develop.

Maintenance gaps are not evenly distributed. Federal dams generally have smaller maintenance backlogs relative to their size and importance. State and local government dams have larger backlogs, and privately owned dams have the largest backlogs of all. In some cases, private dam owners have simply abandoned their dams, leaving the responsibility for aging, deteriorating structures to state and local governments.

How NID data supports policy

The NID serves as the factual foundation for dam safety policy at every level of government. Federal funding programs, including the High Hazard Potential Dam Rehabilitation Grant Program administered by FEMA, use NID data to identify eligible dams and prioritize investments. State dam safety programs use NID data to plan their inspection schedules, allocate staff resources, and report on the condition of their dam portfolios.

The NID also supports policy analysis by making it possible to answer questions about the national scope of dam safety challenges. How many high hazard potential dams lack emergency action plans? How many dams have not been inspected within the required timeframes? How does the age and condition of dams vary across states? These questions can all be answered using NID data, and the answers inform legislative proposals, budget requests, and regulatory decisions.

For a broader discussion of how infrastructure data like the NID supports better policy outcomes, see our article on the benefits of open infrastructure data.

Limitations of the NID

While the NID is an invaluable resource, it has several important limitations that users should understand.

Incomplete coverage

The NID only includes dams that meet its minimum size criteria. Thousands of smaller dams, some of which may pose significant safety risks, are not included. The number of unregulated, uninventoried small dams in the United States is unknown, but some estimates place it in the hundreds of thousands.

Data currency

The NID is updated periodically, but not all records are updated at the same time. Some dam records may contain data that is several years old. Condition assessments, in particular, reflect the state of the dam at the time of the most recent inspection, which may have been years ago. Changes in downstream development, which can affect hazard potential classifications, may not be reflected in the NID until the next update cycle.

Data quality variation

The quality and completeness of NID data varies from state to state. Some states have robust dam safety programs that submit detailed, up-to-date information for every dam. Others have limited programs with fewer staff and less complete data. Federal agencies generally provide high-quality data for their own dams, but the overall quality of the NID depends on the collective efforts of all data providers.

Missing context

The NID provides a standardized set of data fields, but these fields cannot capture the full context of every dam's situation. Local knowledge about seepage, erosion, vegetation, instrumentation readings, and other indicators of dam health is often more detailed than what appears in the NID. Users who need site-specific information should consult with the dam owner, the state dam safety program, or the relevant regulatory authority in addition to reviewing NID data.

Evolving standards

Dam safety standards and practices have evolved significantly over the decades. Some data fields in the NID reflect older standards or definitions that may not align perfectly with current practice. The Army Corps of Engineers works to update the NID's data standards over time, but the process of reconciling historical data with current standards is ongoing.

Using NID data responsibly

The NID is a powerful tool, but it should be used with an understanding of its limitations. Researchers should verify NID data against state and local sources when possible. Policymakers should consider the age and source of NID data when drawing conclusions. Community members who want to learn about specific dams should use the NID as a starting point and follow up with their state dam safety program for the most current information.

The NID is not a substitute for professional engineering judgment, site-specific analysis, or regulatory oversight. It is a screening tool and a planning resource that helps identify where further investigation is needed. Used appropriately, it is one of the most valuable infrastructure datasets available to the public.

The future of the NID

The Army Corps of Engineers continues to improve the NID by expanding its data fields, improving data quality, enhancing its public access tools, and integrating new technologies such as geospatial analysis and remote sensing. Future versions of the NID may include more detailed information about dam instrumentation, real-time monitoring data, climate vulnerability assessments, and links to state regulatory databases.

As the nation faces growing challenges from aging infrastructure, climate change, and competing demands for water resources, the NID will become even more important as a tool for understanding and managing the country's dam portfolio. The inventory's value depends on continued investment in data collection, quality assurance, and public access. Every dollar spent on improving the NID is a dollar invested in better decisions about the more than 90,000 dams that shape the nation's water landscape.