Which dams in America actually pose a risk? According to the US Army Corps of Engineers National Inventory of Dams, 2,645 US dams are classified as both high-hazard AND in poor or unsatisfactory condition. That combination — failure would likely cause loss of life, and the structure is not currently in good shape — is the clearest signal in the dataset for which dams warrant attention.
—dams tracked
—high hazard
—poor / unsat condition
—high hazard & poor
Loading dams…
Hazard
Condition
Purpose
Use the legend above to filter to high-hazard + poor/unsatisfactory dams, or jump to the full US Dams Interactive Map.
What Makes a Dam “Dangerous”?
The NID tracks two separate dimensions that together determine real-world risk:
Hazard Potential Classification — what would happen if the dam failed? High = loss of human life expected. This is not a measure of failure likelihood; it’s a measure of downstream consequence. Full breakdown in Dam Hazard Classifications Explained.
Condition Assessment — how is the dam currently doing? Satisfactory, Fair, Poor, or Unsatisfactory. This is a measure of how close the structure is to failure.
A high-hazard dam in Satisfactory condition isn’t dangerous — just consequential if something goes wrong. A low-hazard dam in Poor condition is structurally concerning but won’t kill anyone if it fails. The combination that matters: high-hazard AND poor/unsatisfactory.
The Numbers
92,469 total dams in the National Inventory of Dams
16,823 classified as high hazard
7,705 rated in Poor condition
1,072 rated in Unsatisfactory condition
2,645 are both high-hazard AND in poor or unsatisfactory condition
States with the Most High-Hazard + Poor-Condition Dams
The geography of dangerous dams looks nothing like the map of famous dams. The states with the highest counts are concentrated in the South and Mid-Atlantic, not the West — a function of older infrastructure, denser downstream development, and more aggressive state condition assessments.
Top 15 states by count of dams that are simultaneously high-hazard AND in Poor or Unsatisfactory condition. Source: USACE National Inventory of Dams.
One pattern worth noting: California — usually associated with the country’s largest and most famous dams — isn’t in the top 15 here. That’s because California’s largest structures are also among its best-instrumented and most-actively-monitored. The state’s dangerous-dam count is far smaller than its raw infrastructure size would suggest. The opposite is true for Eastern states: more older, smaller, locally-owned dams in the path of suburban downstream growth.
The 30 Tallest High-Hazard Dams in Poor Condition
Ranking by NID height — because a taller dam generally means larger stored volume and greater downstream reach.
Source: USACE National Inventory of Dams. Filtered by HAZARD_POTENTIAL = High AND CONDITION_ASSESSMENT in (Poor, Unsatisfactory), ordered by NID Height.
What Stands Out
Age matters. Of the 30 tallest high-hazard + poor-condition dams, 23 were completed before 1970, and several predate 1930. Many are at or past their original design life.
California dominates the height list, not the count list. The state holds 11 of the top 30 tallest dangerous dams — reflecting its large, old water-supply network — but isn’t in the top 15 by raw count, because most of its dams are well-rated. The top-30-by-height view skews toward big federal/utility-owned structures; the by-count view skews toward smaller state-jurisdiction dams in the South.
Water supply dominates the height list. 13 of the top 30 are municipal water supply dams. Unlike flood-control structures that operate episodically, these are full year-round — a different risk profile.
Hydroelectric follows close behind with 11 of the top 30, many owned by utilities licensed under FERC.
Important Caveats
“Dangerous” here is a shorthand for a specific NID data combination. A few things the data doesn’t capture:
Active rehabilitation. A dam can be listed as Poor condition while a multi-year repair is already underway. Condition assessment reflects current state, not future plans.
Inspection recency. A “Satisfactory” rating from 2018 is not the same as one from last month. Check the condition assessment date on individual dams.
Not all states rate condition. Several states leave Condition Assessment blank or “Not Rated,” which means they’re invisible to this list even if structurally concerning. Nationally, 36,710 dams have a “Not Rated” condition.
Failure likelihood isn’t here. This ranking orders dams by size, not by probability of failure. A small dam in Unsatisfactory condition can be more likely to fail than a well-maintained taller one.
Related Dam Resources
US Dams Interactive Map — the full hub map for all 92,469 dams, filterable by hazard, condition, and purpose
Tallest Dams in the US — the 30 tallest structures in the National Inventory of Dams, ranked by NID height
Frequently Asked Questions
What is the most dangerous dam in America?
By the NID’s combined hazard + condition metric, Mossyrock Dam in Washington state (606 ft, Unsatisfactory condition, high hazard) is the tallest dam on the high-hazard + poor-condition list. “Dangerous” depends on definition — likelihood × consequence — but Mossyrock scores poorly on both dimensions.
Which state has the most dangerous dams?
North Carolina leads with 236 dams that are simultaneously high-hazard and in Poor or Unsatisfactory condition, followed by Georgia (190) and Texas (182). The pattern reflects older Southern dam infrastructure, denser downstream development, and aggressive state condition assessments.
How many US dams are unsafe?
8,777 US dams are rated Poor (7,705) or Unsatisfactory (1,072) by the National Inventory of Dams. 2,645 of those are also classified as high hazard.
Does high hazard mean the dam is likely to fail?
No. “High hazard” refers only to the consequence of failure — loss of human life would be expected downstream. It says nothing about likelihood. A well-maintained high-hazard dam may be perfectly safe; a poorly maintained low-hazard dam may be more likely to fail.
When was the last major US dam failure?
The May 2020 failures of the Edenville and Sanford dams in Michigan remain the most recent high-profile US dam failures, triggering evacuations of 10,000+ residents. Earlier incidents include the near-failure of Oroville Dam’s spillways in 2017.
I'm Daniel O'Donohue, the voice and creator behind The MapScaping Podcast ( A podcast for the geospatial community ). With a professional background as a geospatial specialist, I've spent years harnessing the power of spatial to unravel the complexities of our world, one layer at a time.
