Blaming beavers for flood damage is bad policy and bad science, Concordia research shows

(Press-News.org) Beaver dams are critical to river health and a source of biodiversity. They create wetlands, slow water and improve water quality. They also reduce flood peaks and delay runoff.

But beaver dams are often blamed when extreme rainstorms cause flooding — especially when they fail.

This blame had serious consequences following the extraordinary rainstorms that hit Quebec’s Charlevoix region in 2005 and 2011 in the wake of Hurricanes Katrina and Irene. Flooding along the Port-au-Persil watershed caused considerable damage to a riverside inn downstream, leading its owners to successfully sue the Charlevoix-Est Regional County Municipality (RCM) both times.

The owners argued that the RCM was liable under Article 105 of Quebec’s Municipal Powers Act, which states that municipalities are responsible for keeping rivers free of obstacles — including beaver dams. The courts agreed, despite an independent, in-depth hydrology and hydraulics report presented by the defense in the second court case. The report, written by an engineer, argued that the failed beaver dams could not have reasonably been responsible for the damage.

Pascale Biron, a professor in the Department of Geography, Planning and Environment, found the decisions baffling. An expert in river management and river dynamics, she says it was impossible for a failing beaver dam on a tributary many kilometres upstream to have caused the large-scale flooding that damaged the inn.

 A new study published in Earth Surface Processes and Landforms co-written by Biron and the author of the independent report explains why. It uses an updated version of the original model with the latest developments in hydraulic modelling.

“The original was really impressive work,” Biron says. “We were able to improve on it with state-of-the-art modelling tools and new data such as a LiDAR digital elevation model, which recreates the river levels during the Irene 2011 flood event.”

Dam failures had minor consequences The results were clear. In both floods, the failed beaver dams had only a small and short-lived effect on water levels downstream.

During the 2011 flood, the dam failure raised river levels near the inn by only about 20 centimetres — and for just a few minutes. Even if the dam had remained intact, the river would still have overflowed because of the extreme rainfall, according to the researchers.

When they modelled beaver ponds containing four times the observed water volume, the flooding was still minimal. Only unrealistically high dams could have made a meaningful difference, the researchers said.

Log jams a likelier culprit The study also found that the river’s steep slope, combined with intense rainfall, naturally created fast-moving water, capable of eroding banks and moving large logs. Fallen trees and other kinds of wood jams near bridges were likely a much bigger factor in the flooding and damage than beaver dams far upstream.

These conditions alone were enough to explain the dramatic “walls of water” cascading down the river towards the inn, as witnesses reported.

“We don’t want rivers to be canals, with the same shape and depth — we want trees and beaver dams. It would be completely counterproductive to remove them, and impossible anyway,“ Biron says.

“When we see something incorrect, scientists feel it is our duty speak out, especially when there are legal implications. The wording in Article 105 is key, so lawyers need to be involved, not just scientists.”

The study was co-written by Jean Gauthier and Mathieu Dubé of Tetra Tech QI, inc., Thomas Buffin Bélanger at Université du Québec à Rimouski and Maxime Boivin at Université du Québec à Chicoutimi.

Read the cited paper: “Beaver dam failures: Reconciling science, perception and policy for sustainable river management in Quebec (Canada)

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