Aside from the paycheck, the now-legendary author had another motivation to take the Concord job: He despised the Talbot Mills Dam. A decade earlier, Thoreau had published the travelogue A Week on the Concord and Merrimack Rivers, in which he inveighed against the dam for blocking the passage of spawning fish, especially American shad. “Poor shad! Where is thy redress?” Thoreau wrote. Although Thoreau’s surveys supported the farmers’ case, on appeal, the state of Massachusetts permitted the dam to remain. This can’t have surprised Thoreau, who knew that dams made for intractable opponents. “Perchance, after a few thousands of years, if the fishes will be patient, and pass their summers elsewhere meanwhile, nature will have leveled the Billerica dam,” he wrote.

The Talbot Mills Dam in Billerica, Massachusetts—shown in this 1908 photograph—has vexed environmentalists, agrarians and native fish for centuries. Public domain via Billerica Public Library

For many fish-blocking dams, however, it hasn’t taken “a few thousands of years” to level them after all. Since 1912, more than 2,300 dams have been purged from waterways in the United States, most in the last quarter-century. The dam-removal movement hit its apogee in 2024, when, by the accounting of the nonprofit American Rivers, dam removals reconnected more than 2,500 miles of once-fragmented waterways. The year’s biggest and best-publicized dam obliteration occurred on the Klamath River, where the dismantling of four aging hydroelectric dams opened more than 400 miles of spawning habitat in California and Oregon to chinook salmon, which recolonized the watershed so rapidly that conservationists described the comeback as “magical.” Before the Klamath, the nation’s largest undamming took place on Washington State’s Elwha River, where two fish-blocking walls were obliterated on behalf of chinook, coho, steelhead and other salmonids.

After this and other dams were removed from the Elwha River in Washington State, long-absent salmonids came surging back. Trish Drury / Alamy Stock Photo

Yet it would be a mistake to presume that dam removals primarily benefit western salmon rivers. Though they’re typically smaller in scale and receive less fanfare, the majority occur in eastern states. In 2024, the Blake Higgins Dam came tumbling down in Vermont, the Edes Falls Dam departed the Crooked River in Maine, and the Kimball Brook Dam vanished from New Hampshire. Pennsylvania tore out 27 obsolete barriers that year, none taller than around 20 feet. Even the three-century-old Talbot Mills Dam that so vexed Thoreau may soon be taken down.

“We’re in this dam-removal renaissance, where we’re losing barriers faster than we’re putting them up for the first time in history,” says Nathan Furey, a fish biologist at the University of New Hampshire. Per one inventory, however, at least 35,000 dams still truncate northeastern fish runs, including more than 3,000 in Massachusetts, 5,000 in Connecticut and 9,000 in New York.

By the numbers: Klamath River dam removal

The $500 million project to remove four dams along the Klamath River in California and Oregon took place over a couple of years, with the river flowing freely by August 2024. The work opened 400 miles of salmon habitat, and thousands of the fish passed a former dam site in both 2024 and 2025.

Among the states destroying dams, albeit gradually, is New Hampshire, and one late-April morning I climbed into a car with Furey to tour a few of its newly free-flowing rivers. Furey had come to the Granite State in 2018 a neophyte to dam removal, having done most of his graduate research on relatively undeveloped Pacific salmon rivers in British Columbia. “But when you’re the fish guy at the University of New Hampshire, people want you to tell them what happens when a dam comes out,” he said as he navigated Durham’s leafy streets. “There’s a lot of people looking to you for answers.”

Furey’s domain is the Great Bay watershed, a tidal estuary fed by an arterial network of seven rivers. Historically, those waterways teemed with a roster of diadromous fishes: species that split their lives between freshwater and the ocean. That included Atlantic salmon, shad, sea lamprey, American eels, tomcod, sturgeon and, most numerous of all, alewives and bluebacks, a pair of silvery, foot-long fish collectively known as river herring. Sharks, striped bass, cod, seals, seabirds and other predators devoured river herring as they approached the estuary each spring, and juvenile Atlantic salmon may even have timed their departure from river to ocean to coincide with alewives’ return. While cormorants and otters gorged on alewives, young salmon slipped downstream undetected. There’s safety in numbers, and alewives supplied such prodigious numbers before they crashed that one historian deemed them “passenger pigeons of the sea.”

Alewives suffered as settlers expanded their presence in North America. Schuchart / Shutterstock

Like their avian analogues, alewives fared dismally during North America’s colonization. Beginning in the 1600s, eastern colonists launched a dam-building spree, impounding and harnessing streams to spin waterwheels and power mills—gristmills for grinding corn, sawmills for hewing logs, paper mills for pulping wood, textile mills for spinning fabric. By the mid-1800s, more than 65,000 mills stapled eastern creeks, with catastrophic consequences. Over the past four centuries, scientists have calculated that New England’s dams have deprived its ecosystems of nearly 20 billion pounds of alewives—roughly the weight of 300,000 humpback whales.

To show me what happens when one of those fish-blocking walls comes down, Furey took me to the Bellamy River, its frothing course previously straddled by a sprawling brick compound, former headquarters of Sawyer Woolen Mills. The textile mill, long defunct, had been converted to apartments in the 1980s, but its two dams had lingered, funneling the Bellamy through a narrow, fish-stopping pipe. As they aged, the dams also became flood hazards, and in 2019 and 2020 a coalition of agencies, nonprofits and the apartment complex’s owner demolished them. Now the unfettered river churns beneath the brick colossus, ripping over a recontoured bottom of bouldery pools and pockets.

The dams are long gone, but can the herring navigate the obstacle course they left behind? “Alewives are not salmon—they can’t jump,” Furey explained over the Bellamy’s roar. “They’re not big, powerful swimmers like sturgeon; they’re not eels that can climb and crawl.” After the dams came out, the state attempted to count returning alewives with visual surveys—in other words, by staring at the stream and tallying fish, an obviously imprecise method. “We have no idea how many fish come back here,” Furey said.

Furey had been tasked with answering that question. He and his students would capture alewives as they entered the Bellamy and implant them with trackers, including rice-grain-size microchips known as PIT tags, or passive integrated transponders, to figure out what percentage of returning herring managed to skitter through the rapids. We strolled past the apartment complex and upstream to watch Furey’s lab members set up the PIT tag reader that would register each fish. A metal box, containing batteries and a recorder, was chained to a tree near the bank; from it, a cable ran to a riffle, ready to log the serial number of any tagged alewife that wriggled past: a modern technology to surveil an ancient fish.

Nathan Furey, a fish biologist at the University of New Hampshire, attempts to quantify how many alewives and other fish are returning to New England rivers after dams come down. Ben Goldfarb

As Furey’s colleagues fiddled with the PIT tag array, they were observed with curiosity by the residents of the Sawyer Mills apartments, many of whom had taken their lunches outside to savor the balmy spring day. Whereas other wildlife migrations, like the trans-hemispheric flights of Arctic terns, unfold largely beyond human ken, New England’s diadromous fish often pass through neighborhoods and backyards. “They don’t need pristine environments—they can be right here,” Furey said.

Many West Coast salmon runs collapsed within the span of human memory, but New England’s rivers have been plugged for so long that few residents know that they once ran silver with bluebacks, alewives and shad. “It’s a challenge to talk to people about these fish, because they don’t have any cultural memory,” Furey said. Fish depletion, therefore, is often a self-reinforcing cycle. The fewer fish there are, the less the public cares about them, a feedback loop the biologists John Waldman and Karin Limburg dubbed eco-social anomie.

Dam removal is often described as rewilding rivers, but it also liberates the human imagination. In a 2014 paper, the ecologist Loren McClenachan and colleagues published interviews with Mainers living along rivers where alewife runs had been restored, among them the Sebasticook, up which nearly two million herring swam immediately after a dam came out in 2008. McClenachan’s subjects were jubilant: The return of herring had instilled pride, generated revenue from fishing and tourism, and solidified social bonds as kids and lobstermen gathered to watch fish ascend. If eco-social anomie describes a societal forgetting, dam removal achieves the opposite—a “collective remembering,” as McClenachan put it, that cures our amnesia and unshifts our baselines.

A migrating herring Cami Johnson / Shutterstock

While knocking out major dams on main stems achieves dramatic results, plenty of smaller obstacles require redress, too. In 2025, 12 years after the demolition of two dams, an astonishing six million river herring returned to Maine’s Penobscot River—double the figure from just two years prior. But although the river’s lower main stem runs free for 40 miles, its capillary networks of tributaries remain “somewhat overlooked,” according to Joshua Royte, senior conservation scientist at the Nature Conservancy. While the Penobscot Nation and others have removed many barriers, dozens of mill dams and undersized culverts still choke the watershed’s dendritic headwaters, thwarting migratory fish from accessing some of the basin’s most ecologically important areas—spawning grounds for sea lamprey and salmon, chilly climate refugia for brook trout, grottos overhung with leaves that feed aquatic invertebrates.

“If you protect and connect massive networks of these headwaters, you can maintain productivity all the way downstream,” Royte says. “A cold-water tributary just a quarter-mile long could be really important.”

Consider Sedgeunkedunk Stream, a Penobscot tributary where dam removal reopened around 2.8 miles of creek and boosted populations of native sea lamprey. Sea lamprey, like Pacific salmon, accumulate nitrogen and phosphorous during their time at sea, then die after migrating into rivers to breed. Their carcasses feed animals such as snapping turtles and raccoons, and the marine-derived nutrients in their flesh fertilize algae and forests. Ecologist Steve Coghlan, who studied lamprey’s recovery in Sedgeunkedunk, once described to me watching the eel-shaped fish spawn, seizing rock after rock with their mouths and positioning them with utmost care in their redds, or nests. Behind the amorous lamprey hovered shoals of shiners, dace and fallfish, waiting as though on a buffet line for eggs and the drifting aquatic insects the lamprey dislodged—a delicate symbiosis that rose when a wall fell.

Despite the virtues of free-flowing rivers, however, many New Englanders remain as attached to their dams as they were during Thoreau’s day. After stopping at the Bellamy, Furey took me to the Oyster River, where a century-old wall called Mill Pond Dam spread and slowed the river’s flow—just one in a series of dams that had stoppered the river since the 1600s. Water sheeted gracefully over the dam’s lip and frothed at its base, a scene I had to admit was picturesque. We inspected the fishway, a sloping ramp via which river herring and other species surmounted the dam—though, Furey pointed out, large fish such as sturgeon likely couldn’t navigate the ladder’s tight corners or even fit into its narrow channel. Nor did the fishway prevent the pond from becoming a warm, low-oxygen puddle in summer, or stop herons and other predators from picking off out-migrating juvenile herring as they struggled through the pond’s slack water.

The dam hadn’t served any industrial purpose in decades. But not everyone wanted to tear it down. On a lawn across the Oyster’s mirrored surface, an all-caps sign insisted that we “HELP SAVE MILL POND DAM.” Dozens of removals have been thwarted or delayed by local dam enthusiasts, who are sometimes described as NOOMBYs—Not Out of My Backyard. Many NOOMBYs consider local dams indispensable ties to their community’s past; in Billerica, some town officials have objected to the demolition of the Talbot Mills Dam on the grounds that “it has long served as a symbol of the Industrial Revolution” that remains “unquestionably one of the major focal points in the area.” Dams and reservoirs also accrue recreational value: They’re the scenic, still-water backdrops against which people paddle kayaks, feed ducks and teach their children to catch sunfish.

Some residents and history buffs argued to keep and repair the faltering Mill Pond Dam on New Hampshire’s Oyster River, but the ecological and financial logic for destroying the dam won out in the end. Ben Goldfarb

These days, the sentimental attachment to defunct dams appears to be waning—and Mill Pond Dam is Exhibit A. In 2002, the state of New Hampshire declared the dam structurally deficient and ordered the city of Durham to repair it, a process that engineers estimated would cost far more than simply tearing it down. After residents protested the possibility of removal, the city temporarily dropped the idea. But the ecological and financial logic was too overwhelming to ignore, and, in 2022, the issue was put before a ballot initiative, in which Durham voters elected to destroy the dam by nearly a three-to-one margin. The $3.1 million demolition project began this summer and will conclude in 2027 with the planting of native vegetation around the now-drained reservoir.

“Historically, the Oyster [River] would’ve boiled with river herring,” Furey said as we gazed at the doomed dam. “That’s the history I’m interested in.”

Yet history is only one factor preserving old dams—current events play a role, too. Although many northeastern dams are obsolete granite-block affairs, hundreds are of 20th-century vintage and still feed hydroelectricity to regional grids. Some are owned by towns, some by utilities; still others are the property of corporations, private landowners or investors who acquire them within renewable energy portfolios. Though they’re hardly major suppliers of energy—some only power a handful of homes—it can be difficult to justify taking any electrons offline in a world of spiking energy prices and explosive data-center construction. Still, as small-scale hydroelectric dams age, the cost of repairing them, and the flood risk they pose in the event of failure, can make them economic liabilities that their owners are eager to offload—often as significant a motivator for dam removal as fish passage.

Meanwhile, the Trump administration is hampering dam removal efforts in multiple ways. The National Oceanic and Atmospheric Administration, which has spearheaded dozens of dam removals around New England, faces dramatic cuts to its staff and budget. The Culvert Aquatic Organism Passage Program, a bipartisan infrastructure initiative that earmarked $1 billion to remove road culverts and other fish barriers, has at times seen delays in the review and funding of applications. And widespread cancellations of solar and wind energy projects have deprived grids of power sources that could replace hydroelectric dams.

“The lack of belief that government can be an important partner, the reductions in funding, the assault on the staffing of partner agencies—it’s not making this work any easier at all,” says Andrew Fisk, northeast regional director for American Rivers.

Plymouth, Massachusetts, has long been an enthusiastic purger of dams, and one weekend I visited during its Herring Run Festival, a celebration of the resurgent alewife population that annually runs up Town Brook, the stream at Plymouth’s heart. The day was cool and damp, and I followed a posse of dads in Red Sox sweatshirts and kids in rainboots up the path that paralleled the creek. Someone had chalked “LOOK FISH” on the asphalt, and I glanced streamward to find alewives holding in small pods, weaving around each other in their effortless way.

The higher I walked, the thicker the fish got, until I reached the base of the Plimoth Grist Mill, the last dam left in Town Brook, where they queued up to enter a ladder in an aggregation as thick and black as tar. A guy holding a Dunkin’ cup eyed a belly-up alewife and quipped, “He ain’t doing so hot.”

Above the dam, which was preserved for cultural education and fitted with fish and eel ladders, the herring festival was in full swing: an acoustic guitarist jamming, kids grinding corn at the antique mill, enviros tabling for fish-adjacent causes. “My family looks forward to the herring run every year,” Malissa Costa, a Mashpee Wampanoag cultural leader, told me when I stopped at her tribe’s kiosk. “We consider these fish our ancestors, connected beings to us.” Though Massachusetts has prohibited alewife harvest since 2006, the Wampanoag are exempt, and Costa takes around 20 each spring with nets and sometimes her bare hands.

Funny enough, Costa doesn’t much like the taste of the fish: “You ever smelled herring?” she said, wrinkling her nose. “It’s like the fishiest fish.” Instead, she gifts her catch to elders and tribal members in need. “Every year I feel an obligation to go out there and get the herring.”

As they emerged from the gristmill’s ladder, alewives slid through a camera box to be counted by volunteers monitoring the feed from home—a literal live stream. For centuries, these fish would have faced five additional dams along a span of just 1.5 miles, which powered the production of grain, textiles, leather shoes, anchors and more, en route to their headwater spawning pond. Now their journey is free and clear, and I couldn’t blame the gawkers applauding each fish that emerged from the ladder. “It’s hard not to root them on after all we’ve done to them,” agreed David Gould, director of the town’s environment department, as we fish-watched.

Gould is the person most responsible for Town Brook’s transformation, and I suggested we walk upstream to see what he’d accomplished. “This used to be underwater,” Gould said every few minutes as we followed the creek up a trail, gesturing at some field or park or woodlot once submerged by a reservoir. Occasionally we saw vestiges of former impoundments: a dock’s rotting skeleton, a high-water stain on a glacial erratic. The masonry of a couple of old dams, which had doubled as town streets, had been artfully converted into bridges; the ruins of others faded into the greenery.

In Plymouth, Massachusetts, a former dam has been converted to a bridge that lets fish pass unimpeded beneath it. Ben Goldfarb

Though Town Brook’s dams had been outfitted with fish ladders, the passages were shoddy, overlong and hard for fish to find amid turbulent pools. “You’d see thousands of fish washing back down, just bruised and battered and cut up,” Gould said. Among the most egregious offenders was the Billington Street Dam, and in the early 2000s the town of Plymouth sought federal assistance to repair its faulty ladder. The request reached Eric Hutchins, a now-retired restoration biologist at NOAA, who proposed removing the dam altogether instead of fixing it. In 2002 the destruction, overseen by Gould, was carried out by an Army Reserve engineering battalion.

Energized, Gould began to pick off the brook’s other dams—Water Street, Plymco, Holmes—with various pots of funding. In a town with a history as layered as Plymouth’s, he discovered, so much of dam removal turned out not to be dam removal. Asbestos shingles had to be peeled off mill buildings; rights of way had to be extinguished; contaminated sediment had to be landfilled. The stream itself had to be rebuilt, boulders and bends and riparian plants restored to destitute reservoir bottoms. And the permits—so many permits. “You’re using the same regulatory framework that permits, like, a Walmart,” Gould said.

By the time I visited Plymouth, Gould had attained almost mythic status among river restorationists. “Dave has probably removed more dams single-handedly than anybody east of the Mississippi, and maybe west,” Hutchins tells me. Gould didn’t seem entirely comfortable with being a living legend. He’d recently spoken at a fish-passage conference alongside delegates from the Klamath and other massive western watersheds. “All these people are talking about opening up 50 miles of habitat for these giant salmon, and I’m talking about these projects where we opened up, like, a third of a mile at a time for these little herring,” he recalled sheepishly. Yet few dam removals could rival Town Brook for sheer production. Before Gould began ripping out dams, around 25,000 herring returned to the brook annually. Now, many runs exceed 200,000.

Replicating such success on a nationwide scale will require a raft of reforms: vastly more funding, smoother permitting processes, more solar energy to replace hydropower. To achieve American Rivers’ goal of removing 30,000 dams by mid-century, the pace of dam removal would have to accelerate by roughly an order of magnitude. Yet all those policy changes must be preceded by social transformation—by humans, millions of them, witnessing free-flowing streams again running chock full of fish. As American Rivers’ Fisk puts it: “Proximity to abundance is transformational.”

Forty minutes of walking brought me and Gould to the freshwater pond where the alewives swarming in Town Brook would soon spawn. Often, Gould pushes out a kayak to watch breeding fish swirl among the lakeshore willows, his own private celebration of a herring run that had fed pilgrims and sustained the Wampanoag since the end of the last ice age. “To me, the single-most historic thing in Plymouth is this brook,” he said. Rather than being a force acting against history, fish were history—and unlike mill dams, they had a future.

A version of this article was originally published by bioGraphic, an independent, nonprofit magazine that publishes journalism about biodiversity loss and conservation.

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