The world’s largest beaver dam, in Wood Buffalo National Park. extends for 2,790 feet.

The world’s largest beaver dam, in Wood Buffalo National Park. extends for 2,790 feet. Rob Belanger / Parks Canada

Deep in the Wilderness, the World’s Largest Beaver Dam Endures

The largest beaver dam on Earth was discovered via satellite imagery in 2007, and since then only one person has trekked into the Canadian wild to see it. It’s a half-mile long and has created a 17-acre lake in the northern forest — a testament to the beaver’s resilience.

Wood Buffalo National Park, the largest national park in Canada, covers an area the size of Switzerland and stretches from Northern Alberta into the Northwest Territories. Only one road enters it from Alberta, and one from the NWT. If not for people observing it from airplanes and helicopters, and satellites photographing it, little would be known about big parts of it. The park is a variety of landscapes — boreal swamps, fens, bogs, black spruce forests, salt flats, gypsum karst, permafrost islands, and prairies that extend the continent’s central plains to their northern limit. The wood buffalo in the park’s name are bison related to the Great Plains bison. In this remoteness, the buffalo descend from the original population, and the wolves that prey on them are also the wild originals. Millions of birds summer and breed here. The park holds one of the last remaining breeding grounds of the whooping crane.

Other superlatives and near-superlatives: the delta in the park’s southeast where the Peace River and the Athabasca River come together is one of the largest freshwater deltas in the world; last summer, some of Canada’s largest forest fires burned in the park and around it; and — just inside the park’s southern border — is the largest beaver dam in the world.

Animal technology created the largest beaver dam in the world, but human technology revealed it.

The dam is about a half-mile long and in the shape of an arc made of connected arcs, like a recurve bow. The media has known about it for 16 years, and in that time no bigger beaver dam has come to light, so it’s still known as the biggest, and scientists believe it almost certainly is. Animal technology created it, but human technology revealed it. In 2007, Jean Thie, a Dutch-born landscape ecologist who lives near Ottawa, was looking at the latest satellite imagery of places he had examined via satellite in 1973 and 1974, when he was studying permafrost. It’s hard to remember, but in the early ‘70s some scientists thought the Earth might be cooling. Thie’s research had showed evidence of the opposite; the paper about permafrost melting that he published in 1974 is now considered one of the pioneering studies of climate change.

As he looked over 1970s images taken by NASA’s Landsat satellite and compared them with the latest images from Google Earth and other sources, he noticed that in certain landscapes the evidence of beavers now was everywhere. From being almost wiped out by the fur trade between about 1600 and the 20th century, beavers had bounced back. Just one example was a belt about 1,100 miles long that extended into Wood Buffalo Park. Among the hundreds of beaver dams in this area Thie came across one that looked bigger. He measured it and found it to be 2,790 feet long, or about a half-mile. The 17-acre lake created by the dam reposed undisturbed, shiny and opaque in its swampy northern forest, and in the middle of the lake the small brown dot of a beaver lodge could be seen.

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On October 5, 2007, Thie posted the satellite photo of the dam on the Google Earth Community Forum, with text explaining that it was probably the world’s largest. Seven months later, a reporter for Canadian Broadcasting Company Radio saw the posting and did a story about it. Other outlets picked up the story, and “the world’s largest beaver dam,” a phrase that’s satisfying to say and think about, achieved a modest international fame.

Many of the beavers that have reestablished themselves globally are descended from beavers that were planted by wildlife biologists. The thriving beaver population of Tierra del Fuego (another place Thie has studied) is descended from beavers brought to Argentina from Canada’s Saskatchewan River, who are themselves scions of beavers transplanted from upstate New York. No reintroduction of beavers was done in Wood Buffalo Park. Thie believes that the beavers who built the dam are of original stock. Like the wood buffalo and the wolves, they were too remote to be wiped out.

At the request of the Mikisew Cree First Nation, UNESCO has investigated environmental threats to Wood Buffalo National Park.

The officials who run the park heard about the world’s largest beaver dam because of Thie’s discovery, like everybody else. Until the CBC reporter called them for comment, they had not known that their park contained the world’s largest beaver dam. None of the park’s personnel had ever been to it, or has visited it on the ground (or what passes for ground there), to this day. When I called Tim Gauthier, the park’s external relations manager, he said that he had flown over the dam many times but never stood on or near it. He did not know if the water in the lake was still deep enough to cover the entrance to the lodge or lodges. In these remoter areas of the park, he said, “we tend to let such things regulate themselves.”

Since 1983, Wood Buffalo National Park has been listed as a World Heritage Site by UNESCO, the environmental and cultural agency of the United Nations. In more recent years, this designation has become shaky; at the request of the Mikisew Cree First Nation, whose members gather traditional resources in the park and depend on it for cultural survival, UNESCO has twice investigated environmental threats to the park and has come close to declaring it officially endangered. Wood Buffalo Park is now on UNESCO probation, and the governments of Canada and Alberta are supposed to fix its problems.

A North American beaver (Castor canadensis).

A North American beaver (Castor canadensis). Rainer Erl / McPhoto / ullstein bild via Getty Images

The park is suffering the worst drought in its history. Flows are down by half in many places, owing to climate change, water diversion, poor seasonal snowpack, and dams on the Peace River, upstream in British Columbia. A danger that seems inescapable comes from the oil sands that are being mined for crude-oil-containing bitumen, and from tailing ponds that hold trillions of liters of mine-contaminated water. The ponds are near the banks of the Athabasca River, just upstream from the park boundary. They are fatal to birds that land on them. Given the direction that water flows, conservationists and native people fear the tailings will pollute the park eventually. Toxic chemicals have already been found in McClelland Lake, just southeast of the park. Locals stopped taking their drinking water from the lake years ago.

Gillian Chow-Fraser, the boreal program manager for the Northern Alberta chapter of the Canadian Parks and Wilderness Society, in Edmonton, travels in the park often by helicopter, canoe, and foot. She has described the park’s environment as “super degraded.” When I spoke with her by phone not long ago, she talked about a recent tailing basin leak that was not reported to the First Nations downstream of it for nine months. In places that used to flood regularly but now don’t, the land is drying out and vegetation disappearing. Though she crisscrosses the park, she has never seen the world’s largest beaver dam, but she’s grateful that it’s there and bringing the park attention.

The idea of going to the world’s largest beaver dam came to Rob Mark after reading about Thie’s discovery.

Another expert, Phillip Meintzer, conservation specialist with the Alberta Wilderness Association, told me that he hadn’t seen the dam, either, but that the park’s difficulty of access is a good thing, in a way, because it keeps people from visiting in large numbers and putting stress on the area. The downside is that environmental degradation, like the recent tailings seepage, can happen without many watchers finding out. Meintzer’s main worry is that when the economy shifts to renewables, the oil sands will be abandoned and taxpayers stuck with the cleanup. What will be done about the multi-trillion liters of toxic tailings is unknown. “Last summer I was on a trip to test water quality in and around McClelland Lake,” he said. “We camped by the shore, and all night in this remote and uninhabited place we could hear the propane cannons at the nearby tailings ponds firing to scare off the birds.”

As far as is known, only one person has ever been to the world’s largest beaver dam. In July 2014, Rob Mark, of Maplewood, New Jersey, 44 years old at the time, reached the dam after a challenging journey. Holding the flag of the Explorers Club, the international organization with headquarters in New York City, he took a photo of himself standing on the dam. The top of the structure was the only solid ground he had encountered for miles. After he got back, a newspaper in Edmonton did a story about him, and he appeared in other newspapers and a travel magazine. His achievement is like the dam in that so far no one has said it isn’t unique.

A beaver lodge sits in the middle of a 17-acre lake created by the dam.

A beaver lodge sits in the middle of a 17-acre lake created by the dam. Parks Canada

Mark is now a blueberry farmer in Virginia. When I reached him by phone, he told me he did solo extreme treks unsponsored and for his own pleasure. In 2007, he crossed South America from the Pacific Ocean to the Amazon River by hiking over the Andes. The idea of going to the world’s largest beaver dam occurred to him after he read about Jean Thie’s discovery. He planned the trek for several years, and in 2011, he flew to Fort McMurray — the Alberta town, more than 100 miles from the dam, that is the hub of the oil sands industry — to see how he could get from A to B.

His plan was to go down the Athabasca River by boat, then hike through the muskeg peatland. That proving impracticable, he returned home and decided he would come at the dam from another direction, by way of Lake Claire, whose southwestern edge is about 10 miles from it. Crossing the lake by boat, a distance of about 25 miles, and then hiking to the dam, seemed straightforward enough. But the lake is more like a wetter spot in a swamp than a lake. Sometimes it does not have enough water for boats. Mark waited three years for that problem to improve. In 2014 it did, and Mark went to the town of Fort Chipewyan, east of Lake Claire, and hired a man to ferry him.

The lake has no real shore, it just gets shallower at the edges. At a chosen point Mark got out and arranged for the boatman to return and pick him up there in six days. Mark noted the coordinates in his handheld GPS and told them to the boatman. The boatman replied that he had no GPS. That was a detail Mark had not thought of. The boatman told him to cut one of the nearby willows and stick it in a more conspicuous place in the swamp-lake, and they arranged to meet by it. Then the boatman left, and Mark began his trek.

Mostly the route, which required two days of slogging, was just swamp. The last mile to the dam took Mark five hours.

The mosquitos swarmed like nothing he’d seen in the Amazon. He was ready for that and for trying not to go crazy from their noise. The sphagnum moss islands submerged slowly under his weight, step by step, as he grasped at willows to sort of brachiate on. By looking at the tree species shown on satellite photos he had plotted a route along comparatively higher ground, and he tried to keep to that. Mostly the route, which required two days of slogging, was just swamp. The last mile to the beaver dam took him five hours.

Late in the long subarctic afternoon he emerged into the clear patch of sky created by the dam’s lake, waded to the dam, and stepped onto it. The dam is no more than three feet high at any point. He realized that a person seeing it up close would never guess it extended for half a mile. To grasp its full size and the ingenuity of its construction you needed a photograph from space. A lone beaver appeared, looked at him, and slapped its tail. Mark got a sense that his presence enraged the beaver.

Bringing out his Explorers Club flag from his pack, he took the selfie. To be allowed to carry that flag he had had to apply to the club, which reviewed his plan of exploration and deemed it worthwhile. Mark became the 851st explorer in the club’s 110 years to carry the flag, joining a list that includes Thor Heyerdahl and James Cameron. After a supper of granola and peanut butter, he hiked to some larger spruce nearby, lashed his hammock between two of them, draped the mosquito netting, and prepared to spend the night.

Rob Mark at the dam, with the beaver lodge behind him.

Rob Mark at the dam, with the beaver lodge behind him. Rob Mark

Hiking out occupied three more days. When he reached the lake, he could not wait next to the willow marker for his ride, because that would mean standing thigh-deep in water. He sat on a drier patch of ground back in the trees, too far from the lake to see it, and listened for the engine. At mid-morning of the day appointed, he heard a sound that got louder. The boatman went right to the unlikely willow and Mark walked through swamp to the lake and waded out to the boat, so exhausted he could barely climb in.

The world’s largest beaver dam is not like human dams. It does not stopper a river, or even a stream or rivulet. Its low half-mile barrier collects small trickles that come off a plateau called the Birch Mountains. Along the margin of this comparatively higher ground, it accommodates itself to a slope of less than two percent. The gathered-up trickles have amounted to a lake, and after the beavers eat the plants that grow in it, they may relocate to another dam and another pond, graze that area, then move on again, in a sort of crop rotation. Other dams in this beaver belt are up to three-quarters the length of the longest dam. These long, low dams may help the beavers adapt to drought.

Places almost impossible to get to undergird all of existence. In my car there are regions under the front seats where, when my cell phone falls into them, I must almost take the car apart to get it out. Beavers create hard-to-access places that are good for them, less so for us. Jean Thie had beavers on land he owned near Ottawa, and they built dams and made swampy ponds and cut down trees. He got a trapper to remove them but they or other beavers came back. Finally, he gave up and just put chicken wire around the trunks of trees on the property and lived with the beaver landscape.

In the big picture, Thie is pro-beaver nonetheless. “Of course, I’m not very positively minded about our own future on the planet,” he told me. “But I am an optimist about beavers. Their presence improves water management, reduces water flows, reduces the loss of runoff, and creates and improves wetlands. In drier landscapes of the future all this could be of benefit. I think the worldwide flourishing of beavers is a small step in a good direction.”