Air mattresses have a reputation. You buy one for the guest room, it holds up for a couple of visits from your in-laws, and then one night somebody wakes up on the floor at 3 a.m. with the thing wrapped around them like a tortilla. So you can imagine my reaction when I found out that one of the oldest hydro dams in New England has been holding back the Connecticut River with five of them, more or less, for the last 25 years.<\/p>\n
The Holyoke Dam in western Massachusetts is a 30-foot granite wall, 985 feet long, finished in 1900. The top 3.5 feet of it, though, is rubber. Five rubber sections full of air sit bolted along the crest and raise the pool behind the dam by that much, and Holyoke Gas & Electric<\/a>, the city-owned utility that runs the place, is in the middle of swapping all five of them out. As of the utility’s August 27 update, two of the five sections were done and work was underway on the other three, with the whole job expected to wrap up this fall. The contractor doing the installing, a British outfit called Dyrhoff, said on August 17 that the first span was in and the second was going on.<\/p>\n Which, to be fair to the engineers, is roughly where the air mattress comparison stops.<\/p>\n So what are these things, exactly?<\/p>\n The basic idea’s pretty simple. You take a big sheet of rubber-coated fabric, clamp its edges to a concrete foundation on the crest of the dam with steel plates and anchor bolts, and pump air into it until it stands up to the height you want. Let the air out and it lies flat, and the river goes over the top of the dam as if the rubber weren’t there. Dyrhoff’s own product page makes the sales pitch for the design: no moving parts, no hydraulic oil, nothing to jam, and you can always “open” it by deflating. HG&E describes Holyoke’s version as a low-pressure bladder, which basically means it’s not inflated hard like a tire. It’s holding its shape with a modest amount of air and a lot of clamped-down rubber.<\/p>\n Holyoke’s five sections went in during the summer of 2001, the year HG&E bought the dam and the canal system from Northeast Utilities. The original bladders were made by Bridgestone, and the utility’s hydro page<\/a> lists what it took to fit them: nearly 23 tons of reinforcing steel, 600 cubic yards of concrete, more than a mile of air piping and about 1,500 steel tie-down bolts. None of that gets replaced. HG&E says the 2026 job reuses all of the 2001 upgrades, so what Dyrhoff’s doing is unbolting 25-year-old rubber and bolting new rubber onto that foundation.<\/p>\n Why does a 30-foot dam care about 3.5 feet? Because the whole plant lives off the difference in water level between one side of the wall and the other. The bladders back up a 2,290-acre pool, and that extra height is extra pressure pushing water through the turbines at Hadley Falls Station, the biggest of the stations in a hydro project HG&E rates at 42.9 megawatts. The utility’s own pages put Hadley Falls at either 30.6 megawatts or roughly 33, depending on which one you read, which I’ll chalk up to upgrades over the years. Either way, HG&E says the hydro system covers about two-thirds of the electricity the city of Holyoke uses in a year. Letting 3.5 feet of head go away for a season would be a noticeable bill.<\/p>\n Because the river doesn’t always want to be 3.5 feet higher. Flows at Holyoke average about 17,000 cubic feet per second, but HG&E says the spring melt can push past 80,000 and late summer can drop under 5,000. A fixed wall that’s the right height in August is a wall that’s in the way in April.<\/p>\n Before 2001, the answer was flashboards. If you’ve never run into the term, picture a row of wooden boards propped up on steel pins along the top of the dam, raising the pool a few feet. In a flood the pins bend and the boards wash away on purpose, and when the water drops somebody has to go out there and put them back. Holyoke’s flashboard system dated to 1900. It worked, in the sense that the dam stood for a century, but it’s about as precise as a system that relies on things breaking can be.<\/p>\n The rubber crest replaced that, and HG&E’s main reason for it was finer control of the water level upstream. Operators can drop a section to pass a surge or lift it back once the river settles, without anybody walking the crest with a bundle of lumber. That matters on this river for more than the turbines, too. Holyoke is 86 river miles up from Long Island Sound and it’s the first barrier migrating fish hit on the way upstream, which is why there’s a fish lift on the Holyoke side. We covered a Maine dam that went dark for a herring run<\/a> this month, and the Connecticut River shad have their own version of that argument every spring.<\/p>\n I’d be doing you a disservice if I made rubber dams sound bulletproof, because about 200 miles north of Holyoke there’s a utility that would disagree. The Highgate Falls dam in Swanton, Vermont, carried one bladder across its crest instead of five, and it was a big one: 267 feet long, 57 feet wide, 47 tons, standing 15 feet tall on top of a 15-foot concrete dam. The village put that bladder in during 2018 to replace a Bridgestone original from the early 1990s. In June 2025 it ripped. Reports put the tear anywhere from 15 to 30 feet long depending on who was measuring, and the village ended up pulling the whole thing off the dam.<\/p>\n According to Vermont Public<\/a>, Swanton Electric told state regulators in May that the plant was making about 10,000 megawatt-hours a year less without its bladder and asked for a rate increase of around 30 percent. The village manager, Bill Sheets, has also said federal regulators signaled they won’t let the town put the same kind of rubber bladder back, with a smaller set of bladders or concrete gates among the alternatives on the table.<\/p>\n Now, I don’t think that’s an argument against what Holyoke is doing, and here’s why. Swanton’s problem was one 15-foot bag holding back 15 feet of water. Holyoke’s sections are 3.5 feet tall, there are five of them, and a failure in one would drop the pool by a few feet along a fifth of the crest, which is a different conversation from losing the entire top half of a dam at once. From the looks of things, Vermont is now being nudged toward something closer to the Holyoke setup, not away from it. I’d still rather be the utility with five small bags than the one with a single big one, and the 25-year service life Holyoke got out of its first set is, frankly, about what the industry promises.<\/p>\n It also explains the British contractor. Bridgestone told Swanton around 2010 that it was getting out of the rubber dam business, according to the village’s own meeting minutes, so anyone with 25-year-old Bridgestone bladders on a dam is shopping somewhere else for the replacements.<\/p>\n A few things. I couldn’t find a published price for the replacement or a note on who’s paying for it, and the utility hasn’t given a date more precise than “this fall.” It says the schedule is tied to river flows, which makes sense, since you can’t unbolt a rubber crest with the spring freshet pouring over it, and even a wet October could push things. The project also doesn’t show up on HG&E’s October 9 construction list, which is mostly gas mains and tree trimming, so the August 27 note is the freshest count I can give you. If three more sections have gone in since then, the utility hasn’t said so.<\/p>\n We do know the work didn’t come out of nowhere. Local TV reported last summer that HG&E lowered the river about three feet between the Montague Dam and Holyoke to inspect the old bladders ahead of this year’s replacement, which annoyed the canoe club and told the utility what it needed to know.<\/p>\n If you want the bigger pattern, it’s the same one we’ve been writing about all month. A Montana dam is bolting together a generator from parts that sat in a pit for 40 years<\/a> so it can take its 50-year-old machines offline one at a time. Holyoke’s dam is 126 years old and its newest major part dates to 2001. The rubber is just the first thing on the list to wear out.<\/p>\n HG&E’s August 27 update put the job at two of five sections complete, the remaining three underway, and completion anticipated this fall, with the timing tied to how much water the Connecticut River decides to send down.<\/p>\n","protected":false},"excerpt":{"rendered":" Air mattresses have a reputation. You buy one for the guest room, it holds up for a couple of visits … <\/p>\nA dam with a top you can let the air out of<\/h2>\n
So why not just build the wall 3.5 feet taller?<\/h2>\n
These things do fail, and Vermont found out the hard way<\/h2>\n
What HG&E hasn’t said yet<\/h2>\n