Busy beavers dam Bow River side channel near Calgary Zoo

The Bow River is known for being pretty wide, but that hasn't stopped a family of beavers from constructing a dam right across the river in the heart of Calgary. 

The beaver dam is just near the Calgary Zoo, about 50 metres east of the Baines Bridge, which connects the zoo to Memorial Drive, and runs shore to shore in an area where the Bow River splits to go around zoo island.

Naturalist Brian Keating spotted the unlikely dam while out walking along the river. Keating told The Homestrech this is only possible because of the low water.

"At this time of the year, there's only a small flow of water that goes down that side of the island, but the dam has been successful in creating an area of flat water, which is now frozen as a pond on the upside of the stream,"  Keating said, adding that the construction is solid.

The dam is constructed of large and small logs and branches, held together with stones and mud.

"It appears to even have some good-sized, cantaloupe sized stones sitting on top of some of it," Keating said.

"You can just imagine the beaver walking by repeatedly holding that rock, and bringing up the rock and mud and all kinds of debris, lots of branches, even some big logs."

Live under the ice

Keating said keeping some deep water open is essential for beavers to survive under the ice all winter.

"If the beaver can keep the winter pond viable without it freezing all the way to the bottom, even though the dam is going to disappear next spring, it'll at least serve the purpose for the winter," he said.

"It'll provide the beaver with a safe place to feed under the ice all winter long and give the beaver access under the ice to places where it can emerge and go up and to run on fresh food."

Beavers eat bark, about a kilo per day, and they store that food in the form of logs and branches, which they cut down in the fall.

"They then jam those cuttings under the water and then the ice forms over the top, and beavers have lips that they can fold behind their teeth so that they can eat under water," Keating said.

"And until it creates, you know, a situation where it should keep the beavers safe as long as it doesn't freeze all the way to the bottom."

Keating said he did not spot the beavers this weekend, but that he did see one in the fall. 

"We looked down and we saw a beaver pulling a fairly large sized cutting," Keating said. "We watched this beaver wrestle this branch down to the river and then get into the river and swim downstream. So I think this beaver has been working on this dam for quite some time."

Beavers will feed and stay active all winter under the safety of the ice, feasting on their bark supply.

"When they get out on land and they're lumbering and slow and of course, outside the city, in places like Banff and Kananaskis, when they do have to leave the safety of their underwater environment, they're easy prey for wolves and cougars," Keating said.

"So their defense really is a well-built lodge with an entry that is below the ice so they can go in and out in safety. And one would think that it's quiet under that ice."

Ecological engineers

Keating said beavers are one of the only animals that will build themselves a home and alter the landscape.

"They can change water courses, they can flood forests, they create new wetlands. And it makes for a wonderfully diverse bunch of habitats," Keating said.

"A beaver will go into a forest environment, for instance, that's essentially a monoculture. And they'll create edge habitat, which is biologically very rich. Of course, for us, they sometimes cause issues. It's that age-old problem of us versus beasts."

Keating said this dam shouldn't cause a problem during the winter, and will not hold up in the rising waters of the spring runoff.

"It's going to fail next spring when the snow runoff begins," he said. "There's no way that a beaver dam will work, that dam will become history virtually overnight when the water starts to rise. But ... amazingly, it appears to be working now to a degree."

With files from The Homestretch.