Support Provided By
Learn More
Planet EarthPlanet Earth

The Undamming of America

We’ve impounded rivers for hundreds of years, but as our society has evolved, so has our relationship with dams.

ByAnna LiebNOVA NextNOVA Next

Gordon Grant didn’t really get excited about the dam he blew up until the night a few weeks later when the rain came. It was October of 2007, and the concrete carnage of the former Marmot Dam had been cleared. A haphazard mound of earth was the only thing holding back the rising waters of the Sandy River. But not for long. Soon the river punched through, devouring the earthen blockade within hours. Later, salmon would swim upstream for the first time in 100 years.

Grant, a hydrologist with the U.S. Forest Service, was part of the team of scientists and engineers who orchestrated the removal of Marmot Dam. Armed with experimental predictions, Grant was nonetheless astonished by the reality of the dam’s dramatic ending . For two days after the breach, the river moved enough gravel and sand to fill up a dump truck every ten seconds. “I was literally quivering,” Grant says. “I got to watch what happens when a river gets its teeth into a dam, and in the course of about an hour, I saw what would otherwise be about 10,000 years of river evolution.”

Over 3 million miles of rivers and streams have been etched into the geology of the United States, and many of those rivers flow into and over somewhere between 80,000 and two million dams. “We as a nation have been building, on average, one dam per day since the signing of the Declaration of Independence,” explains Frank Magilligan, a professor of geography at Dartmouth College. Just writing out the names of inventoried dams gives you more words than Steinbeck’s novel East of Eden.

Explore this interactive map of dams in the United States or search to find a dam close to you.

Some of the names are charming: Lake O’ the Woods Dam, Boys & Girls Camp # 3 Dam, Little Nirvana Dam, Fawn Lake Dam. Others are vaguely sinister: Dead Woman Dam, Mad River Dam, Dark Dam. There’s the unappetizing Kosciusko Sewage Lagoon Dam, the fiercely specific Mrs. Roland Stacey Lake Dam and the disconcertingly generic Tailings Pond #3 Dam. There’s a touch of deluded grandeur in the Kingdom Bog Dam and an oddly suggestive air to the River Queen Slurry Dam.

The names arose over the course of a long and tumultuous relationship. We’ve built a lot of dams, in a lot of places, for a lot of reasons—but lately, we’ve gone to considerable lengths to destroy some of them. Marmot Dam is just one of a thousand that have been removed from U.S. rivers over the last 70 years. Over half the demolitions occurred in the last decade. To understand this flurry of dynamiting and digging—and whether it will continue—you have to understand why dams went up first place and how the world has transformed around them.

The Dams We Love

A sedate pool of murky water occupies the space between a pizzeria, a baseball field, and the oldest dam in the United States, built in 1640 in what is now Scituate, Massachusetts.

When a group of settlers arrived in the New World, the first major structure they built was usually a church. Next, they built a dam. The dams plugged streams and set them to work, turning gears to grind corn, saw lumber, and carve shingles. During King Phillip’s War in 1676, the Wampanoag tribe attacked colonist’s dams and millhouses, recognizing that without them, settlers could not eat or put roofs over their heads.

Robert Chessia of the Scituate Historical Society shows me a map of the area, circa 1795. On every windy line indicating a stream, there is a triangle and curly script label: “gristmill.”

Map of area surrounding Scituate, Massachusetts, circa 1795.

In the 19th century, dams controlled the rivers that powered the mills that produced goods like flour and textiles. Some dams are historical structures, beautiful relics of centuries past. Not far from Scituate stands a dam owned by Mordecai Lincoln, great-great grandfather of Abraham Lincoln. Some dams have been incorporated into local identity—as in the town of LaValle, Wisconsin, which dubbed itself as the “best dam town in Wisconsin.”

Before refrigerators, frozen, dammed streams offered up chunks of ice to be sawed out and saved for the summer. Before skating rinks, we skated over impounded waters.

In the 20th century, the pace accelerated. We completed 10,000 new dams between 1920 and 1950 and 40,000 between 1950 and 1980. Some were marvels. Grand Coulee Dam contains enough concrete to cover the entirety of Manhattan with four inches of pavement . Hoover Dam is tall enough to dwarf nearly every building in San Francisco. Glen Canyon Dam scribbled a 186-mile-long lake in the arid heart of a desert.

An animation of the number of dam completions in the U.S. between 1800-2000

Behind those big new dams were big new dreams. A 1926 dam on the Susquehanna River produced so much hydroelectric power that the owners needed to set up a network of wires to sell the electricity far and wide. This became the PNJ Interchange, “the seed of the electricity grid as we know it,” explains Martin Doyle, a professor of river science and policy at Duke University. Grand Coulee Dam, which stopped the Columbia River in 1942, supplied vast quantities of electrical power that turned aluminum into airplanes and uranium into plutonium. President Harry Truman said that power from Grand Coulee turned the tide of World War II.

Rivers in the U.S. flow into and over more than 80,000 dams.

Yet only 3% of dams in the US are hydropower facilities—together supplying about just under 7% of U.S. power demand. Most dams were built for other reasons. They restrained rivers to control floods and facilitate shipping. They stored enormous volumes of water for irrigating the desert and in doing so reshaped the landscape of half the country. “The West developed through the construction of dams because it allowed the control of water for development,” says Emily Stanley, a limnologist at the University of Wisconsin, Madison.

But for most dams, none of these are their primary purpose. Nearly one-third of dams in the national inventory list “recreation” as their raison d’être, a rather vague description. I inquired about this with the Army Corps of Engineers, which maintains the inventory, and their reply merely offered a cursory explanation of “purpose” codes in the database. Mark Ogden, the project manager for the Association of State Dam Safety Officials, says many small private dams were indeed built for recreational activities like fishing.

Support Provided By
Learn More

Grant, Magilligan, and Doyle have a different theory, however. Dams may get the recreational label, Doyle says, “when we have no idea what they are for now, and we can’t stitch together what they were for when they were built.” But while many of the original uses have disappeared, the dams have not.

The Dams We Love to Hate

In the very center of conservationist hell, mused John McPhee, surrounded by chainsaws and bulldozers and stinking pools of DDT, stands a dam. He’s not the only one to feel that way. “They take away the essence of what a river is,” Stanley says.

A dam fragments a watershed, Magilligan explains. A flowing river carries sediment and nutrients downstream and allows flora and fauna to move freely along its length. When a dam slices through this moving ecosystem, it slows and warms the water. In the reservoir behind the dam, lake creatures and plants start to replace the former riverine occupants. Sediment eddies and drops to the bottom, rather than continuing downstream.

Migratory fish can be visceral reminders of how a dam changes a river. Salmon hatch in freshwater rivers, swim out to sea, and then return to their birthplace to reproduce, a circle-of-life story that has captured people’s imaginations for generations. At the Elwha Dam in Washington state, Martin Doyle recalls looking down to see salmon paddling against the base of the dam, trying in vain to reach their spawning grounds upriver. Roughly 98% of the salmon population on the Elwha River disappeared after the dam went up, says Amy East, a research geologist at the U.S. Geological Survey (USGS). Doyle points out that salmon are just one of many species affected by dams. Migratory shad, mussels, humpback chub, herring—the list goes on. He notes that the charismatic salmon are a more popular example than the “really butt-ugly fish we’ve got on the East Coast.”

Dams not only upend ecosystems, they also erase portions of our culture and history. Gordon Grant points out that on the Columbia River, people fished at Celilo Falls for thousands of years, making it one of the oldest continually inhabited places in the country. The falls are now covered in 100 feet of water at the bottom of the reservoir behind the Dalles Dam.

Salmon live a circle-of-life story that has captured people’s imaginations for generations.

Hundreds of archaeological sites, going back 10,000 years, dot the riverbanks and the walls of the Grand Canyon. For millennia, East explains, many of these potsherds, dwellings, and other artifacts had been protected by a covering of sand. But that sand is disappearing because the upstream Glen Canyon Dam traps most of the would-be replacement sand coming down the Colorado River. Furthermore, snowmelt used to swell the river with monstrous spring floods, redistributing sediment throughout the canyon. Now, demand for power in Las Vegas and Phoenix regulates the flow. “They turn the river on when people are awake and turn the river off when people go to sleep,” explains Jack Schmidt, a river geomorphologist at Utah State University. Without “gangbuster” spring floods, he says, the sandbars are disappearing and the archaeological sites are increasingly more exposed. “There’s a lot of human history in the river corridor, and unfortunately a lot of it is being eroded away in the modern era,” East says.

Receive emails about upcoming NOVA programs and related content, as well as featured reporting about current events through a science lens.

As the ecological and cultural toll dams take became clearer, our relationship with them started to show its cracks. Fights over dams grew increasingly loud. At the turn of the century, John Muir and a small band of hirsute outdoorsmen opposed construction of the O’Shaughnessy Dam in the Hetch Hetchy Valley of Yosemite. They failed. By the 1960s, pricy full-page ads in the New York Times opposed the Echo Park Dam on a tributary of the Colorado. They succeeded. Echo Park Dam was never built—but downstream, Glen Canyon Dam went up instead, inspiring new levels of resentment and vitriol among dam opponents. In a 1975 novel by cantankerous conservationist Edward Abbey, environmental activists blow up Glen Canyon Dam. The novel’s title entered the popular lexicon as a term for destructive activism: “monkeywrenching.”

Abbey once described his enemies as “desk-bound men and women with their hearts in a safe deposit box, and their eyes hypnotized by desk calculators.” Now, 40 years later, Abbey might be surprised to learn that it’s men and women crunching numbers at desks who actually incite the dynamiting of dams.

hetch hetchy dam
O'Shaughnessy Dam in Hetch Hetchy Valley, California

Why They’re Coming Down

The decision to remove a dam is surprisingly simple. Ultimately, it comes down to dollars. “The bottom line is usually the bottom line,” says Jim O’Connor, a research geologist at USGS. As dams age, they often require expensive maintenance to comply with safety regulations or just to continue functioning. Sometimes, environmental issues drive up the cost; for example, the Endangered Species Act may require the owner to provide a way for fish to get past the dam. Consideration for Native American tribal rights may also influence decisions over whether to keep or kill a dam. “In my experience, economics lurks behind virtually all decisions to take dams off or to keep ’em. But the nature of what’s driving the economics is changing,” says Grant, the Forest Service hydrologist. Dam owners—who are overwhelmingly private, but also include state, local, and federal governments—have to weigh repair costs against the benefits the dam provides.

In some cases, those benefits don’t exist. The age of waterwheel-powered looms and saws is long gone, but thousands of forlorn mill ponds still linger. “You’re left with a structure isn’t doing anything for anybody and is quietly and happily rotting in place,” says Gordon Grant. Others like Kendrick Dam in Vermont supplied blocks of ice. “We’ve got refrigerators now,” Magilligan says. “This one should probably come out.”

This old mill in Tennessee is now a restaurant.

Other dams don’t live long enough to become obsolete. The designers of California’s Matilaja Dam, which was completed in 1948, said it would last for 900 years, says Toby Minear, a USGS geologist. But the reservoir behind Matilaja silted up so quickly that within 50 years the reservoir was 95% full of sediment. Though the surrounding community still wanted its water, the dam could no longer provide storage. Congress approved a removal plan in 2007, but the estimated $140 million dollar project has stalled after proving more expensive and technically challenging than anticipated.

For most dams, the story is more complicated. Two dams on the Elwha River generated hydropower, but when the owner was legally required to add fish ladders—a series of small waterfalls that salmon can use to easily scale the dam—future sales of hydroelectricity paled in comparison to the repair cost. Furthermore, the neighboring Elwha Tribe had fought for decades to restore the salmon catch—half of which legally belonged to them. The owner opted to sell the dams to the federal government in 1992, and after nearly two decades of study and negotiation, the Department of the Interior, the Elwha Tribe, and the surrounding community had agreed on a removal plan. In September, 2011, construction crews began breaking up the two largest dams ever removed from U.S. rivers.

Beginning of the End

“Removing these big, concrete riverine sarcophagi, and salmon swimming past that gaping hole—that is the mental image that people will always have of dam removal,” Doyle says. But in reality, not all rivers host salmon and not all dams are removed with explosives. Each river, each dam, and each removal are totally different, says Laura Wildman, an engineer at a firm specializing in dam removal.

Doyle remembers one particularly dramatic example of a “blow and go” removal, where the US Marines exploded a small dam slated for removal as part of a training exercise. When a dam disappears suddenly, the river responds violently. O’Connor was at the “blow and go” removal of the Condit Dam on the White Salmon River in Washington. “At first it was like a flash flood of water—just mostly water, definitely dirty water. It came up fast, it was turbulent, it was noisy,” he says. “Then it was brown, stinky, and chock full of organic material mud flow.”

Even the slower removals, which take place over months or years, can have dramatic moments. Doyle describes how a backhoe slowly taking out the Rockdale Dam in Wisconsin “looked kind of prehistoric, like a long-necked dinosaur reaching out and eating away at the dam.”

Jennifer Bountry, a Bureau of Reclamation hydrologist who helped plan the Elwha Dam removal, explains that initially the engineers would gingerly shave off a foot of concrete off the dam and wait to see what happened. But as the removal progressed, the river was changing so fast that she had to keep a close eye on the currents as she was recording her observations. “You had to be careful where you parked your boat,” Bountry says. The freed Elwha River rapidly carved out a new channel, carrying with it roughly the same volume of sediment as Mt St. Helens belched out during the infamous 1980 eruption.

Aftermath of the End

A small stream trickles through the YMCA’s Camp Gordon Clark in Hanover, Massachusetts. Freshly tie-dyed t-shirts hanging from the chain link fence sway in the breeze. Summer camp is in full swing, and Samantha Woods, director of the North and South Rivers Watershed Association, a nonprofit, walks me down a shallow slope to an ox-bow stream curling through a wide plain covered in cattails. The heavy, humid air is thick with buzzing cicadas and singing birds. Less than a year ago, this plain was a blank, wet canvas. Where the cattails stand now was submerged beneath several feet of water impounded by a 10-foot-tall earthen dam that had stood for at least 300 years. In 2001, the state determined that the dam could catastrophically collapse in a flood and required the owner—the YMCA—to fix or remove it.

The dam hung in limbo for nearly a decade until storm damage reignited fears of collapse. By then, the public had started to embrace the idea that removing the dam could be a good thing for the river. Plus, repairing the dam would have cost an estimated $1 million. Taking it out would cost half that amount. So in October of 2014, crews tore down the earthen blockade, drained the pond, and planted native plant seeds in the newly exposed earth. Less than a year later, the transformation to wetlands is well underway. Woods is optimistic that if one more downstream dam comes out, herring would swim up this creek for the first time in centuries.

Many rivers are starting to resemble their pre-dam selves.

But no one knows for sure if the herring will come back. In general, scientists are just beginning to unravel what happens when a dam is removed after tens or hundreds of years. “Dam removals help us understand how rivers behave,” Magilligan says. Magilligan, along with Bountry, East, Grant, O’Connor, and Schmidt, is part of a group called the Powell Center , which is studying how rivers respond when they’re set free.

In the hundred or so dam removals for which data is available, fish, lamprey, and eel populations are rebounding, and more sediment and nutrients are heading downstream, both expected outcomes. But the Powell Center scientists are surprised at just how fast recovery takes place. Formerly trapped sediment clears out within weeks or months. For example, a recent study showed that this freed sediment is quickly rebuilding the Elwha River delta. Some fish populations revive within a few years, not a few decades as many had expected. Many rivers are starting to resemble their pre-dam selves.

But the Powell Center members also point out that dam removals may sometimes have undesirable consequences, like allowing non-native species formerly trapped upstream to colonize the rest of the river, or releasing contaminated sediment downstream. They agree there’s much more to figure out.

Dam New World

Few people may be more emblematic of the subtle shift in attitudes about dam removal in recent years than Gordon Grant. A much younger Grant spent a dozen years as a rafting guide. Back then, he’d sat around campfires singing “Damn the man who dams the river!” with people who chained themselves to boulders at the bottom of a valley slated to become a reservoir. One day Grant got curious enough about the forces shaping the rapids he ran that he went to graduate school. For nearly 30 years now, he’s been conducting research in fluvial geomorphology—the study of how rivers reshape the surface of the earth. I asked Gordon Grant if a dam is still, for him, at the inner circle of hell. “It used to be more than it is now,” he says. “It may be slippage, it may be gray hair, it may be something else, but I see dams in a somewhat different light now.”

“I’ve seen dams that provide nothing for anybody and I’ve seen dams that provide a lot of power that otherwise would have been generated by coal,” Grant says of his research career. Both building and demolishing dams have tradeoffs, Grant argues, and as a scientist he’s interested in how economics, ecology, and hydrogeology each play a role. Emily Stanley says, “I’ve learned that it’s not enough to say ‘Yeah, we should blow ’em all up!’ We can’t just wave the wand and take them away. There will be huge consequences. But yeah, there’s too many dams.”

Even Daniel Beard, Commissioner of Reclamation under the first Clinton Administration agrees there’s too many dams. He has been calling loudly and unequivocally for taking out one of the largest in the country, the Glen Canyon Dam. “Do I think that’s controversial? Absolutely. Do I think it’s politically realistic? Eh…not really. But somebody has speak up,” he says.

Glen Canyon Dam on the Colorado River

Most scientists and engineers are skeptical that any dam as large as Glen Canyon will go anywhere, anytime soon. Drought has brought the reservoir down to as low as one-third of its designed capacity in recent years, but even still it currently stores 12 million acre-feet—roughly the average volume of water that goes through the Grand Canyon in a year—and generates enough power for 300,000 homes. And even if the economics change dramatically, the dam itself is a formidable structure and one not easily removed. “I can’t imagine getting dropped into Glen Canyon and having the audacity to start wanting to plug that thing with concrete,” Doyle says. “If we really want to start removing Western dams, then we need an audacity to match that with which they went after building them.”

Even removing the dam in Scituate, which is 370 years old and a mere 10 feet tall, is a tough sell. “This dam isn’t coming down,” David Ball, president of the Scituate Historical Society, told me on two occasions. The pond still provides about half the town’s drinking water.

For some dams that do still serve purposes like Scituate and Glen Canyon, dam owners, conservation groups, and government agencies have worked to manage them more holistically. In Scituate, fish ladders and timed water releases are beginning to restore herring to the upstream watershed.

At the Glen Canyon Dam, operators now create a simulacrum of spring floods by releasing extra water to help restore sediment in the Grand Canyon. The first artificial flood stormed through the Grand Canyon in the spring of 1996, and by 2012, a supportive Bureau of Reclamation had helped clear the way for nearly annual restoration floods. Though these floods surge with less than half of the flow of pre-dam torrents, they were still highly controversial at first, says Jack Schmidt, the Utah State professor. Releasing extra water in the spring means lost revenue, he explains, because it generates electricity that no one is interested in buying.

And at Shasta Dam in California, water releases are now carefully controlled in order to keep the water temperature low enough for downstream Chinook salmon to survive, according to Deputy Interior Secretary Michael Connor. He expects that drought, exacerbated by climate change, will alter our relationship with dams. “There is nothing necessarily permanent. We should be relooking and rethinking the costs and the benefits of our infrastructure,” he says.

Many dams will remain—and as climate change alters precipitation patterns, some new ones will be built. Dams shaped the country of the rivers they divide, and they don’t go down quietly. But time and economics will sweep more dams away.

It’s hard to forget the moment when a once-restrained river breaks free. Connor, for one, vividly recalls the removal of the Elwha dams. “You know, you count on your one hand those days that really stand out, and those events that you really participate in. That is easily, for me, one of those days that I’ll always remember.”

Some had hoped for that moment for a very long time. East, the USGS geologist, recalls meeting an 80-year-old Elwha woman who had never before seen the river untrammeled. The woman had said, joyfully, “I’ve been waiting for these dams to come out my whole life!”

  1. Marmot dame sediment estimate from a U.S. Forest Service report . Estimated 100,000 dump trucks total, 20% in first 48 hours.
  2. Thanks to Robert and Christine Chessia of the Scituate Historical Society for providing scanned images of their historic maps
  3. Surface area of Manhattan = 87 million m 2 , volume of Grand Coulee Dam = 9 million m 3 , so contains enough for a 0.1 m thick surface covering of Manhattan

Photo Credits: Robert and Christine Chessia, Inklein/Wikipedia User: (CC BY-SA 3.0) , Martin LaBar/Flickr (CC BY-NC 2.0)

, Christian Mehlführer/Wikimedia Commons (CC-BY-SA 2.5)

Funding for NOVA Next is provided by the Eleanor and Howard Morgan Family Foundation.

Major funding for NOVA is provided by the David H. Koch Fund for Science, the Corporation for Public Broadcasting, and PBS viewers. Additional funding is provided by the NOVA Science Trust.