The most commonly misunderstood number in US dam data is the hazard classification. When a dam is labelled “high hazard,” it does not mean the dam is likely to fail. It means that if the dam failed, the consequences would be severe. This distinction matters because the same dam can be high hazard and in Satisfactory condition — rock-solid but consequential — or low hazard and in Unsatisfactory condition — structurally poor but in an area where failure wouldn’t hurt anyone.

This page explains how the US Army Corps of Engineers classifies dam hazard in the National Inventory of Dams, what each category means in practice, and how the 92,469 dams in the inventory actually break down. The map below pulls live from the USACE NID feature service — toggle hazard classes in the legend to see them on the ground.

Or jump straight to the full US Dams Interactive Map.

The Three Hazard Classifications

High Hazard

Failure would probably cause loss of human life. These dams sit upstream of homes, roads, schools, or businesses where a sudden release of stored water would reach occupied buildings before people could evacuate. High-hazard dams are subject to the strictest inspection, documentation, and emergency action plan requirements.

Count in the NID: 16,823 (18% of all US dams).

Significant Hazard

Failure would not probably cause loss of life, but would cause economic loss, environmental damage, or disruption of lifeline facilities. Examples include dams upstream of highways, power lines, municipal water supply infrastructure, or agricultural land. The threshold that separates “significant” from “low” is usually the presence of important infrastructure downstream, even in an area without residences.

Count in the NID: 11,345 (12% of all US dams).

Low Hazard

Failure would cause no probable loss of life and only minimal economic or environmental damage — usually limited to the dam owner’s property. Small farm ponds, stock-watering tanks, and agricultural irrigation dams on remote land make up the bulk of this category.

Count in the NID: 60,292 (65% of all US dams).

Undetermined

A small number of dams in the NID carry no classification yet — typically because the dam is new, newly transferred between owners, or awaiting reclassification following a downstream development change.

Count in the NID: 4,009 (4% of all US dams).

The Critical Point: Hazard ≠ Risk

A hazard classification measures consequence, not likelihood. Real-world risk requires both:

• Hazard potential (what happens if it fails) — tracked in the NID’s Hazard Potential Classification field
• Condition (how likely it is to fail) — tracked separately in the Condition Assessment field (Satisfactory, Fair, Poor, Unsatisfactory)

A dam classified as high hazard and rated Satisfactory is the safest high-hazard dam you can have: consequential if something goes wrong, but well-maintained and not close to failing. A dam classified as high hazard and rated Unsatisfactory is the combination that warrants action — and nationally, 2,645 US dams fall into the high-hazard + poor-or-unsatisfactory-condition bucket. We dig into that group in Most Dangerous Dams in the US.

High-Hazard Dams by State

High-hazard count is the single most useful state-level dam-safety metric: it tells you how many structures in that state would, if they failed, probably take lives downstream. The distribution is uneven — North Carolina, Texas, and Missouri each carry well over a thousand high-hazard dams, while several Western and New England states are under 100.

Who Assigns the Classification?

Hazard classifications are assigned by the state or federal agency with safety jurisdiction over a given dam. Each state runs its own dam safety program (48 states do; Alabama and Delaware have historically been the exceptions). The federal agencies — US Army Corps of Engineers, Bureau of Reclamation, Tennessee Valley Authority, International Boundary and Water Commission, FERC — classify the dams they own or license.

Classifications are reviewed periodically and can change. If downstream development increases — a new subdivision built below a previously low-hazard farm dam, for example — the dam is reclassified to significant or high, often triggering new inspection and EAP requirements for the owner.

How Hazard Drives Inspection Frequency

Inspection frequency typically tracks the hazard classification:

• High-hazard dams — annual or every 2-year inspections; formal Emergency Action Plans (EAPs) required in most jurisdictions; inundation maps maintained
• Significant-hazard dams — usually every 2–3 years; EAPs required in many states
• Low-hazard dams — every 3–5 years; no EAP required

Related Dam Resources

• US Dams Interactive Map — the full hub map for all 92,469 dams, filterable by hazard, condition, and purpose
• Most Dangerous Dams in the US — the 2,645 dams that combine high hazard with poor or unsatisfactory condition
• Tallest Dams in the US — the 30 tallest structures in the National Inventory of Dams

Frequently Asked Questions

Does “high hazard” mean the dam will fail?

No. High hazard means loss of life would be expected if the dam failed. It says nothing about how likely failure is. Condition Assessment is the field that tracks structural likelihood.

How many US dams are high-hazard?

16,823 dams in the US National Inventory of Dams are classified as high hazard — roughly 18% of the total 92,469 dams.

What’s the difference between high hazard and significant hazard?

High hazard means loss of life would probably occur on failure. Significant hazard means loss of life would probably not occur, but there would be economic, environmental, or infrastructure damage. The line between them is whether occupied structures exist in the expected inundation zone.

Can a dam’s hazard classification change over time?

Yes. If downstream development occurs — new homes, roads, or businesses built below the dam — the classification can be raised from low to significant or high. This commonly happens to older farm dams in the path of suburban growth.

Who decides the hazard classification?

State dam safety agencies for state-jurisdictional dams, and the owning federal agency (USACE, Bureau of Reclamation, TVA, FERC, etc.) for federal dams.

→ Filter all 92,469 US dams by hazard classification on the interactive US Dams Map