How the dam inventory figures are made

What the National Inventory of Dams is

The National Inventory of Dams (NID) is the U.S. Army Corps of Engineers’ inventory of dams, compiled with the states, territories and federal agencies that submit their dams to it. These pages read the NID’s public national file, as it stood on October 30, 2026, and nothing from the parts of the NID that require a login. The NID sets its own inclusion criteria for which dams it holds; this page does not quote them, so the pages do not say that the NID holds every dam in the country, and it does not.

What a record is

The pages count records, never “dams”. A record is one row of the NID’s national file, identified by its Federal ID. The NID’s Federal ID is, in its Data Dictionary’s words, “the unique identifier for each dam record”, and for a saddle dam, dike or other separate structure associated with a dam project it joins the primary dam’s NID ID to the structure’s own. So a record can be a separate structure that belongs to a dam, and a count of records is not a count of dam projects. The NID’s own state totals count records the same way. The file this page reads holds 92,692 records.

A record’s primary owner type and primary purpose are the single category the NID names for it when it lists several, so a table by either sums to the records. A record lists every purpose it serves, and a count of the records that name a purpose at all would sum to more, so the pages do not print one.

Hazard potential: a consequence, never a likelihood

The NID’s Data Dictionary defines the hazard potential classification as a “category to indicate the potential hazard to the downstream area resulting from failure or mis-operation of the dam or facilities. It reflects probable loss of human life and impacts on economic, environmental, and lifeline interests. The hazard potential does not speak to the condition of the dam or the risk of the dam failing.” It names four classes. The low, significant and high definitions open alike, “Dams assigned the … hazard potential classification are those where failure or mis-operation” (the significant one reads “those dams where”), and differ in what follows:

• Low: “…results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner’s property.”
• Significant: “…results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.”
• High: “…will probably cause loss of human life.”
• Undetermined: “Dams for which a downstream hazard potential has not been designated or is not provided.”

A hazard class is therefore what would follow a failure, as the responsible agency assigned it. It says nothing about how likely a failure is or what shape a dam is in, and no page calls a high-hazard record dangerous, unsafe or at risk. Of the 92,692 records counted here, 17,036 are high, 11,350 significant, 60,242 low and 4,064 undetermined.

Condition, and a condition that is not published

Condition is a separate field with its own definition. The NID defines the condition assessment as an “assessment that best describes the condition of the dam based on available information.” Its categories are Satisfactory, Fair, Poor, Unsatisfactory, Not Rated and Not Available. Two of them, in the NID’s words:

• Not Rated: “The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.”
• Not Available: “Dams for which the condition assessment is not provided to the NID.”

The pages treat a condition as published when the NID prints one of its categories, Not Rated included, because Not Rated is the NID’s own category. Not Available, and an empty field, are not published, and neither is ever counted under a rating, drawn as a blank or shown as zero. Every table that counts by condition carries the not-published count in the same row, and a share is taken only over records whose condition is published, with that denominator printed. Of the 92,692 records here, 71,237 have a condition published, of which 36,627 are Not Rated and 9,023 Poor or Unsatisfactory; the NID marks 3,532 Not Available and prints nothing for 17,923.

States submit their own ratings, and how many of their records they rate differs, so a state with more records rated Poor may rate more of its records. No page ranks states by condition for that reason.

Dates

The file carries its own date, which these pages print as the date the figures are as of: the NID file of October 30, 2026. Each record also carries the date it was last updated, which can be years earlier, so a record is as current as its own date and not as current as the file. The NID says of itself: “This is not an emergency response site, and the information provided is not real-time.” Some dates in the file are impossible, such as an inspection date later than the file’s own, and they are kept as the NID printed them and never corrected.

The inventory is one snapshot with no earlier edition kept by the publisher, so these pages draw no trend, and no page says that a rating improved or worsened.

What the NID does not make public

The NID says that “some dam information is publicly available, while some dam regulatory and oversight programs may restrict access to some of the information on the dams within their jurisdiction to a more limited set of users.” These pages read the public national file only. They never request the part of the NID that requires an account, and load no field the NID marks as controlled unclassified information. Where the public file is silent, the pages say so, and a missing value is never filled from anywhere else.

People

A dam’s owner can be a company, an association, a public body or a family, and the file does not always say which. These pages show an owner by type only, and print no owner’s name. If several individuals own one dam, the NID lists its owner type as Private. The pages hold no name of any individual.

The check against an independent value

The first question for every figure here is what independently produced value it is checked against. Today there is none.

The NID publishes its own summary for each state. It is a second output of the same system, so agreement with it is not independent. Each load holds the file to those summaries: a state’s records and its primary and associated structures must match exactly, or nothing is loaded. Its records in each hazard class may differ when the differences cancel out within the state. Those are the same records, some classified differently, because the summary is read live and the file is the day’s export. So a state’s hazard counts here are the file’s, as of its date, and the NID’s own summary may show reclassifications made since. An independent count would come from a state dam safety program’s own list, and none has been read. These figures are from the NID’s file as retrieved on November 5, 2026.

The license, and the NID as the source

The NID’s website states no license and has no terms page. Its footer says:

This site is intended for informational purposes only. … This is not an emergency response site, and the information provided is not real-time.

The Bureau of Transportation Statistics distributes the inventory in its National Transportation Atlas Database, and its metadata record for that copy says the work is “not protected by any U.S. copyrights” and “available for unrestricted public use”, asking for the acknowledgment of the U.S. Army Corps of Engineers. That statement describes the Bureau’s copy, not the Corps’ own file.

Every page names the U.S. Army Corps of Engineers’ National Inventory of Dams as the publisher and links the NID. None carries an Army Corps insignia or claims to be official, and none says that the Corps endorses the site.

What this data cannot show

• A hazard class is the consequence if a dam failed, as the responsible agency assigned it. The inventory says it does not speak to the condition of the dam or the risk of the dam failing.
• Condition is not published for every record, and states rate their dams by their own practice, so a state with more records rated poor may rate more of its dams.
• The inventory is not real-time, counts records (which include separate structures recorded with a dam) and holds no failures, no incidents and no owner who is a person.

Sources

• National Inventory of Dams — U.S. Army Corps of Engineers, National Inventory of Dams · The NID states no license of its own. Cleared on October 24, 2026 on the condition that no individual owner’s name is printed, owner type and organizations only. Its footer, verbatim: “This is not an emergency response site, and the information provided is not real-time.” · Updated: Regenerated as agencies update the inventory