Pitkin County aims to learn more about eager beavers of North Star | AspenTimes.com
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Pitkin County aims to learn more about eager beavers of North Star

Motion camera may provide info on winter activity, spring dam dismantling

After the stand-up paddle boarders disappeared and water levels on the Roaring Fork River dropped this fall, some residents of North Star Nature Preserve got busy. Very busy.

While beavers reside on the nature preserve east of Aspen year-round, they have to wait until late fall to build dams. At least two dams spanning the width of the river were constructed in November.

All that effort goes down the drain, so to speak, when spring runoff swells the Roaring Fork and destroys the engineering marvels.



It is a cycle that has been repeated for hundreds, if not thousands of years. Now, Pitkin County Open Space and Trails is launching a research project to learn more about the most industrious residents of North Star.

“We’re here to try to set up a camera and see what happens over the course of the winter,” Liza Mitchell said while hiking through the closed preserve one recent sunny afternoon.



Mitchell is the natural resource planner and ecologist with the open space program, which oversees North Star. She was on a scouting mission to find the dams and beaver lodges along the 3-mile stretch of the river in the preserve and gauge if there were opportunities to place one or more motion-activated cameras nearby without disrupting the residents.

Open space officials are eager to see what the beaver are up to during the winter.

“We’ve seen the aftermath of the spring flood break-up and we’ve seen the creation,” Mitchell said. “We’re just trying to fill up the gap in between.

“We could get a beaver crossing a dam. We could get a leaf blowing. We could get a moose crossing the river, anything that triggers that motion sensor,” she continued.

In addition to learning more about beavers during the winter, open space officials hope to see how exactly the dams get dismantled.

“When does the dam break up?” Mitchell asked. “Is it from high water flows or the ice pressure, maybe that breaks it up before high flows even come down. We’re not out there day-in and day-out to see that, so we’re kind of curious to see how the beavers that are recovering at North Star are operating and influencing the ecosystem.”

The hope is the ice or water triggers the camera, which takes still photographs rather than video. But, it could be that dams fall apart a stick at a time without enough motion to trigger the camera.

Water flows downstream from a dam, as it is frozen above the dam in the Roaring Fork River in Northstar Nature Preserve on Friday, Dec. 4, 2020. The dam is believed to have been built sometime this fall. (Kelsey Brunner/The Aspen Times)

The beavers are undeterred by the annual destruction of their handiwork. Every fall they build them back up, though not necessarily at the same locations.

The North Star Nature Preserve management plan includes a history section that covers the property’s ownership by the James Smith family. Two of Smith’s children, Morgan and Sandy, recalled their job on the ranch one summer in the 1950s was to drain a beaver pond to dry out a field upstream.

“The duo dismantled the dam by day and the beavers rebuilt it by night, (Sandy) said,” according to the passage in the management plan.

Decades later the current population of beavers remain just as persistent through the natural destruction of their dams.

“If they want the dam there, they’re busy beavers, right?” Mitchell said. “They’ll take on that fight.”

Most of North Star Nature Preserve is closed during the winter and the section Mitchell scouted is closed year-round to human uses, except for management and research. So the beaver are left undisturbed.

Beaver activity can be seen throughout Northstar Nature Preserve where the mammals have eaten the bark and chewed down branches.

On the hike along the oxbows and braids of the river, signs of beaver popped up around nearly every bend. Sharp little beaver teeth had lopped off small diameter willow branches just as cleanly as a gardener’s tool. Significantly larger cottonwood trees, with trunks more than a foot in diameter near the ground, were gnawed through and toppled into the water. The larger branches that were underwater were stripped of bark. The smaller branches had been de-limbed for food and dam material. Willows, cottonwoods and aspens are all part of their diet.

Walking along the riverbank, Mitchell pointed out occasional areas where the snow was smoothed out on the opposite bank at a slight pitch into the river, creating perfect slides that the beaver use. Elsewhere were small canals off the main channel where beaver sometime build their lodges.

Mitchell’s exploration stopped a respectable distance away from a beaver lodge built not in the water but on the bank. Sticks and mud were piled several feet in the air.

Pitkin County Open Space and Trails commissioned a 2018 Beaver Occupancy Survey by Jonathan Lowsky, principal biologist of Colorado Wildlife Science. He found two active freestanding surface lodges in the water, two active bank lodges and one freestanding inactive lodge. Each lodge, the study said, has an adult pair, yearlings and kits. The young beavers get booted out of the lodge at 2 years old.

The study said the population at North Star appears to have increased in recent years, paralleling the improved health of the riparian ecosystem.

“The current density of beavers appears to be sustainable and beavers are likely improving ecological conditions at North Star,” the study said. “Beaver cutting stimulates vigorous sprouting in willow and beaver and willow can persist indefinitely in a stable equilibrium.”

A muskrat swims downstream of a beaver dam in Northstar Nature Preserve on Friday, Dec. 4, 2020. (Kelsey Brunner/The Aspen Times)

The dams that were built in November are a masterful mix of mud, sticks and bark. The Roaring Fork was backed up into deep pools right behind the dam. In the pool behind one of the dams, a muskrat swam, pulled itself onto the bank for a brief time, then dove back into water and disappeared. Further upstream and downstream from the dams, the water flow is so low that the river has iced over.

The dams haven’t survived spring since the explosion of stand-up paddle boarding, so there hasn’t been a conflict with river runners encountering an obstacle. The 2020 management plan prohibits removal of beaver dams or lodges. If obstacles survive into summer, the open space program will consider the surrounding terrain and provide information on how to negotiate them.

Meanwhile, the open space program hopes the motion cameras reveal some winter- and spring-time secrets of their flat-tailed friends.

More on the beavers

For more about the North Star beavers, check out the open space program’s Pitkin Outside newsletter.

scondon@aspentimes.com


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