A massive yellow Cat excavator pounded away at a 200-year-old dam in Norwood on a recent morning.
"The sound of the hydraulic hammer, to me, is one of the best sounds in the world," said Beth Lambert, head of the Massachusetts Division of Ecological Restoration. "When you hear that hammer, you know that another dam is coming down, and the river will recover its health."
The old stone masonry crumbled away easily, falling into the riverbed of Traphole Brook below.
There are over 3,000 dams in Massachusetts. The state says only few of them still serve their original purpose, and many are in disrepair. A growing movement of environmentalists, fishermen and community groups are pushing to remove them in order to restore the health of the rivers they block.
Advocates pushed for years to destroy this old mill dam on Traphole Brook.
The stream is home to the largest number of native brook trout in this part of the state. But the dam has severed their population in two.
"We have trout upstream of the dam, trout downstream of the dam, that are sort of like families that have been isolated in different countries that can’t cross the border for years," said Ian Cooke, executive director of the Neponset River Watershed Association.
Cooke and his organization were key in rallying community support to remove the dam, which cost about $2 million.
"So many things had to go right, and so many people had to be excited about this to get us to this day," he said. "It’s really wonderful to just see it finally come to fruition."
These dams often look unassuming, even bucolic. But experts say they have significant environmental impacts.
"One of the things that dams like this, especially small dams, cause is artificial warming of the water," said Chris Hirsch, the state’s project manager for the Norwood dam removal. "And trout and other cold water animals require very cold, highly oxygenated water."
"Cold water streams used to be very common in the greater Boston area," he added. "But unfortunately, because of development, damming and climate change, they're becoming increasingly rare. This is one of the last remaining really good examples with really robust populations of trout."
Neglected dams can also pose flooding risks. In 2005, downtown Taunton was ordered to evacuate after an earthen dam upstream buckled during a heavy rainstorm. Fortunately the dam held, and was safely removed.
A database compiled by the state lists 745 significant-hazard and 330 high-hazard dams in Massachusetts. "Significant" means they could cause serious damage to property or the environment if they broke; "high" means a break could be fatal. And experts warn that number could grow as climate change leads to more intense rain and flooding.
A recent climate resiliency study of the Mystic River found the Amelia Earhart Dam between Somerville and Everett would be vulnerable to significant damage in a historic Nor'easter, threatening to flood parts of both communities, the MBTA Orange Line, and areas of Malden and Medford.
The Norwood dam was considered low-hazard, but still caused concern — "the upstream road, downstream houses, sensitive eastern brook trout," Lambert said. The dam backed up water above a dense residential neighborhood.
"Now just multiply that by 3,000 across the commonwealth, and you can see what a cumulative impact these dams have on public safety and the environment," she said.
Most of those 3,000 dams were built in the 1700s and 1800s to power small mills. Today, some are owned by the state, some by cities and towns, and some by private owners.
In the western part of the country, where most dams are used for hydropower or irrigation, there is organized resistance from farmers, ranchers, and utilities to removing them.
But in Massachusetts, some advocates think the state is not moving fast enough in its dam removal efforts.
A few miles up the road from Norwood, the 200-foot-long Watertown dam backs up water on the Charles River. A vocal group of environmental activists wants it gone.
"It has been been a somewhat frustratingly slow process," said Emily Norton, the executive director of the Charles River Watershed Association.
"It's an anachronism. It's serving no purpose," she said. "It's really harming fish populations and native ecology, and a very basic part of river restoration is removing blockages on them."
The Watertown dam is the first obstacle fish returning from the ocean encounter as they make their way up the main stem of the Charles River, looking to spawn.
A recent study by the Department of Marine Resources tracking American shad found very few were able to cross the Watertown dam, despite a nearby fish ladder.
"We know the fish populations overall are just a fraction of what they were historically before colonization of this area," Norton said.
Records before dams show alewives, shad, blueback herring, and even salmon used to migrate much further up the Charles River watershed.
A state feasibility study found removing the dam would help fish populations rebound. But the Department of Conservation and Recreation, which owns the dam, said a decision on removing the dam "has not been determined at this time."
The pace of dam removal in Massachusetts could soon speed up. The recently approved bipartisan infrastructure package includes about $2.4 billion to support the removal, rehabilitation and retrofit of dams.
Beth Lambert, of the Division of Ecological Restoration, said they have a "backlog" of projects that could use the funding.
Lambert said she hopes that money helps them move closer to her vision of streams all over the state freed of their dams.
On demolition day at Traphole Brook in February, restoration seemed a long way off. Excavators scraped away the crushed stone, leaving a gaping hole of mud and rock in the riverbed.
But Lambert said the stream would look transformed by next year.
"You won't know that there was a dam," she said. "You'll just basically see a stream flowing through a grassy meadow type setting with trees and shrubs that have been planted there. It'll look amazing."
Lambert said in addition, all that sediment that’s been building up behind the dam for years acts as a natural seed bank.
So as the pond drains down into the original riverbed, those seeds will be exposed to earth, air, and sun. They’ll spring to life, after waiting for years to grow.