Who’s Dam Busy?

Written by Jacey Van Wert

beaver cartoon

What exactly does “busy as a beaver” actually imply? It turns out this isn’t easy to answer because beavers work secretly under the cloak of night. Undaunted, a team of scientists led by Jordan Kennedy, Doctoral candidate in the Harvard School of Engineering and Mathematics, and Candice Chen, a Harvard undergraduate student studying Environmental Engineering, were willing to take on this challenge. Kennedy and Chen documented the construction of fast and efficient damming complexes built by beavers to discover how a group of animal engineers have successfully managed water for millions of years.

Beavers meticulously maintain their freshwater habitats. They build robust damming complexes that conserve, clean, and stabilize water sources for the local ecosystem, harboring habitats for local animals (birds, fish, insects, bears, to name a few). Not only are beavers essential engineers that benefit a slew of animals and waterways, they also are sacred to the Blackfeet people. Blackfeet tell a story of a raft full of animals and man. Man instructed animals to dive down and bring up mud to the surface to create land. And the first animal tasked with this job was, of course, the beaver.  The Blackfeet people regard the beaver as sacred and do not hunt them.  As such, it was important for the researchers to enlist members of the Blackfeet community into this research. Historically, the Blackfeet Nation lands are one of the few sites where beaver populations prevailed throughout the North American fur trade and still persist today. The waterways on these tribal lands remain naturally maintained by these incredible beaver engineers.
beaver on riverbank
The North American beaver (L. Castor canadensis) on the field site in the foothills of the Rocky Mountains in northwestern Montana
Kennedy and Chen’s study took place on the Blackfeet Nation Reserve in the foothills of Montana. Every summer, snow melt washes out the beaver builds from the previous year. Within three to five months, beaver complexes are fully reconstructed under the cover of night. Previously, the onset of dam construction and the creation of sprawling networks of trails and canals were relatively undocumented. Kennedy identified four sites in the Blackfeet Nation reserve with reliable beaver activity. Every week for three months, Kennedy and her team of Blackfeet community college students surveyed these sites with high resolution (1.3 cm/px) drone photography and flow measurements. She then constructed high resolution orthomosaics (many images stitched together) into 3D maps that could then be annotated and tracked across hundreds of square meters. Kennedy and Chen first identified hints of beaver activity. What started off as minor grass impressions scattered along riverbanks and gnawed-on trees became cleared pathways in just weeks. Soon, the team mapped out the entire formation and construction of beaver damming complexes.
scientist in river
Jordan Kennedy taking flow data in a river occupied by beavers
We asked Kennedy our pressing question: just how busy are beavers exactly? The answer was astounding. “A single beaver colony can construct over 2500 cubic meters of dams over a span of 3 months spanning a distance of 800 meters. They can clear over 3200 meters worth of trails in 3 months. They can also maintain over 1700 meters worth of canals.”

Every year beavers must engineer a new and complex system of trails, canals, dams, and lodges on the scars of old dam sites. To create their central lodge for living, beavers engineer the surrounding water system, quickly and efficiently. They build dams across many nearby waterways to create a moat around their lodge. When water from the snowmelt is sufficiently low, the sound of the river changes. A longstanding observation is that beavers respond to this change in sound, and indeed, Kennedy showed that beavers wait for just the right flow to begin construction. This means that beavers have some sense of when to build a dam that won’t get washed out by rushing water. They also build canals and excavate networks of trails to provide building materials. Chen mapped one beaver colony that cleared 16000 meters of trail and canal networks around the lodge complex. That is the equivalent of 175 football fields long! Despite their tireless nocturnal construction, beavers in action were hard to spot –  in fact, Kennedy spotted more grizzly bears and moose than she did beavers.

  aerial photo of winding riverriver map

Drone imagery of a river colonized by beavers paired with an annotated map of trail and canal networks

Beavers are not alone in their engineering prowess. Kennedy is now working with Dr. Radhika Nagpal and her research group at Harvard University. Kennedy hopes to find insight into beaver behavior from other self-organizing systems such as ants and termites. These insects work as a large collective group to make optimally designed nests. Kennedy believes that beaver colonies may work like these other, smaller self-organizing systems. But unlike small insect nests, beaver networks are two to three orders of magnitude larger. Kennedy plans to use the annotated maps to identify how these networks propagate across an expansive landscape over a small timescale. She plans to use a network analysis to understand the collective behavior of beavers, something that has not yet been discovered in nature on this scale.  Practical, human-based applications for this work include, but are not limited to, informing hydrologists about dam construction and canal networks for improved water management and using beaver transport strategies to inspire engineers when building effective strategies for moving supplies across large landscapes.

Beavers are critical ecosystem engineers. Their environmental impacts have provided a thriving aquatic ecosystem for millions of years and are treasured by the Blackfeet people. Although we still cannot say for certainty just how busy a beaver truly is, for now, when you hear the term, you know that it is a lot of “dam” work.

Kennedy and Chen presented these findings at the Society for Integrative and Comparative Biology’s 2021 virtual annual meeting.

the Society for
Integrative &