River Beavers: No Dams Required

To dam or not to dam? The ‘river beavers’ in the incredible camera trap photographs below, from William MacFarland, live on the Anacostia River which flows from Maryland into Washington DC. They build lodges to live in but have no need for dams. Beavers build dams to create water that is deep enough (ideally 3 feet or more) to provide a safe place to escape land predators and to ensure that the water doesn’t freeze through during the winter. This spot along the Anacostia is deep enough, and water levels are stable enough, that no dam building is required!

According to Rosell and Campbell-Palmer, “Dam building is a high-cost activity and would only justify the investment in building and maintaining where resources exist to sustain a beaver territory.”¹ Factors that affect dam building behavior also include watercourse width and gradient, the speed at which the water flows, the size of the watershed, the width of the valley floor, the distribution of vegetation, substrate type, riverbank characteristics, and more.

While it’s true that beavers in rivers don’t create wetlands to the extent that many of the beavers in ponds do, they are still important and worthy of our coexistence efforts. These distinctions are based solely on the habitat these beavers inhabit and all still represent the same species, Castor canadensis. Individual beavers may move back-and-forth between ponds and rivers as conditions change.

Here are some benefits of river-dwelling beavers:

• They fell trees which create gaps in the canopy, encourage plant growth, and prevent forest ecosystems from stagnating in older growth stages.
• They dig channels and canals making the area surrounding the riverbanks wetter and creating microhabitats for other aquatic critters.
• Their bank dens, burrows, lodges, and food caches still provide shelter for other animals, including sunning, hunting, hiding, and nesting spots for birds, reptiles, fish, and amphibians.
• They provide food for predators.
• They contribute to local biodiversity.
• They provide educational opportunities for human recreation, enjoyment, and wellbeing.
• Finally, dispersing beavers from river habitats may settle along connecting streams and provide all the additional benefits of beavers who dam!

Thanks to Mohammad Abbasi, Dr. Emily Fairfax, Cherie Westbrook, Chris Jones, and Abby Lee Tee for engaging in this discussion on Twitter and sharing your experiences and expertise 🙂

Thanks to Mike Digout for the cover image.

1- Quote is from pg. 123 of Beavers: Ecology, Behaviour, Conservation, and Management by Frank Rosell and Roisin Campbell-Palmer published in 2022 by Oxford University Press