There Are 823 Dams in Western Mass and Nobody Has Classified Half of Them

Somewhere upstream of a great many houses in this region there is an earthen embankment holding back a pond. It was probably built to power a mill that stopped running a century ago. Nobody who lives near it thinks about it, and in most cases nobody has ever formally assessed what would happen if it let go.

The Commonwealth's inventory records 823 dams across the 101 towns of Western Massachusetts. Every single one of the 101 towns contains at least one. Of that total, 99 carry a high hazard potential classification and 132 carry a significant one. And 404, the largest group by a distance, carry no classification at all.

What "High Hazard" Actually Means, Because It Is Not What It Sounds Like

This is the single most misread term in dam safety, so here it is plainly. Hazard potential describes what would happen if the dam failed. It says nothing whatever about how likely the dam is to fail, and nothing about its condition.

A high hazard potential dam is one where a failure would likely cause loss of life downstream. That classification is driven by what is below it, houses, a road, a school, not by the state of the structure. Build a housing development below a perfectly maintained dam and it becomes a high hazard dam without a shovel touching the embankment.

The corollary matters just as much. A dam in genuinely poor repair with nothing below it but woodland is classified low hazard. Condition and hazard are two independent axes, and the federal inventory rates them separately for exactly that reason. So "there is a high hazard dam in my town" is not a statement about danger. It is a statement about what sits downstream, and in a region settled along its rivers the answer is usually houses.

The Biggest Group Is The One Nobody Has Looked At

404 dams here have no classification. The main reason is unglamorous: Massachusetts regulates dams above a size threshold, and most of these sit below it. Nobody was ever required to assess them, so nobody did.

That is not the same as a clean bill of health, and it is worth being precise about why. A small mill dam that was never assessed can still put a serious volume of water into a narrow valley. What the absence tells you is that no engineer has been asked the question, which is a different and in some ways less comfortable fact than a low rating.

A second group inside that number is structures that no longer hold water back at all. The inventory keeps breached and removed dams on the books, so a town's headline count includes some structures that are, functionally, a gap in an old wall. A dam count is a count of inventoried structures, not of ponds.

Who Is Responsible For The One Above Your House

Ownership is where this stops being trivia and starts being a property question.

• Privately owned: 423 dams
• Municipally owned: 221 dams
• State owned: 96 dams
• Ownership recorded as unknown: 67 dams

The largest category is private, which in practice frequently means a person who bought a house with a pond and discovered afterwards that the embankment at the end of it is a regulated structure they are liable to maintain and inspect. Dam repair is engineering work and it is priced accordingly. If a property you are considering has a pond with a built outlet structure, establish who owns the dam before you buy, in writing, from the deed.

The counts above overlap slightly, because some dams record shared responsibility between a town and a private owner. That is a real answer about liability rather than a data error, and it is the arrangement most likely to end in an argument.

What This Data Cannot Tell You, And Will Not Pretend To

You will find websites offering to tell you whether your house is "downstream of a dam". Be careful with them, because the underlying information to answer that properly is not public in Massachusetts.

Dam breach inundation maps, the modelling that shows where water would actually go, are withheld for security reasons. Without them, the only way to guess is a radius drawn around a dam, and a radius is not a breach zone. Water follows the valley. A house a mile downstream in the floodplain can be at risk while a house four hundred yards away up the side of the hill is not, and no circle drawn on a map captures that.

So the honest limit of this article, and of the table below, is the town. This town contains this many dams, classified this way. If you want the question answered for a specific property, the Office of Dam Safety is the body that holds the file, and the town's emergency management officer will know whether an emergency action plan exists for a particular structure.

Every Town

Sortable and filterable. 101 of the 101 towns contain at least one inventoried dam, and 49 contain at least one classified high hazard potential. Monson holds the most at 25, and Washington holds the most high hazard classifications at 7.

The per-town column adds to 826, slightly more than the 823 the region actually contains, because a handful of dams sit on a town line and are counted for both. We publish both figures rather than quietly picking one.

What To Do With This

For most buyers, nothing. A dam in your town is not a fact about your house, and the region has been living alongside these structures for two centuries.

It becomes worth a question in three situations: if the property includes a pond with a built outlet, in which case find out whether you would own the dam; if the property sits in a narrow valley directly below an impoundment, in which case ask the town whether an emergency action plan exists; and if you are buying in a floodplain anyway, in which case what the flood claims record actually shows is the more useful read, since ordinary flooding has put water in far more houses here than any dam ever has.