Discovery of a New Beaver Benefit is a Happy Accident

In recent years, the beaver has enjoyed a reversal in its reputation. Once, the bucktoothed rodent was viewed as a fashion accessory or a pest. They were wiped out in much of North America as their pelts were turned into tophats worn by 19th century gentlemen. Ranchers and farmers cursed the surviving beavers for their tireless penchant for damming creeks and flooding low-lying areas.

Today, however, beavers are increasingly hailed as ecological saviors, engineering ecosystems in ways that create more bird habitat, counter wildfire damage and build green oases in a drying world, among other things. Now, scientists are adding another beaver benefit: Their presence can help counter stream pollution worsened by climate change.

“As we’re getting drier and warmer in the mountain watersheds in the American West, that should lead to water quality degradation,” said Scott Fendorf, a professor of Earth system science at Stanford University. “Yet unbeknownst to us prior to this study, the outsized influence of beaver activity on water quality is a positive counter to climate change.”

The discovery of this beaver benefit is a happy accident. In 2018, Christian Dewey, then a PhD student in Fendorf’s lab, was tracking stream dynamics and chemistry in the East River, a tributary of the Colorado River, as it passes near the Colorado mountain town of Crested Butte. Dewey used sensors to track water levels in the river and in the surrounding moist ground and side-channels known as the riparian zone. He also gathered water samples to monitor levels of chemicals such as nitrogen, a potent fertilizer which can fuel algae blooms that lower oxygen levels in the water.

Part way through the year, an industrious beaver built a dam across the river within the stretch monitored by Dewey. While the dam was destroyed several months later, it lasted long enough that the it offered the possibility of illuminating how stream conditions varied with and without beavers.

“The construction of this beaver dam afforded us the opportunity to run a great natural experiment,” said Dewey, who is now a postdoctoral researcher at Oregon State University (mascot: the beaver).

On top of that, stream conditions varied dramatically over the course of two years of data collecting. While 2018 was unusually dry, the following year was unusually wet. That enabled the researchers to compare the effects of beaver on stream conditions with the effects of droughts and dramatic swings in river flows expected to happen more often in the western U.S. (and elsewhere) as the planet warms.

Over the course of the study, the beavers’ presence stood out for its effect on how much river water soaked into the surrounding ground. As the beaver dam blocked the river it raised the water levels upstream. The stark difference in water levels on either side of the dam generated water pressures that  caused more moisture to seep into the riparian zone.

Large fluctuations in river levels driven by a surge in water as mountain snows melt in the spring can have the same effect. But on the East River, Dewey found the beaver dam’s impact on the water pressure was at least 10 times greater during the summer than the seasonal changes in flows during both dry and wet years, the scientists reported Tuesday in Nature Communications.

As the beaver dam pushed more water into the surrounding land, it affected what happened to nutrients in the water. The soil filtered out more nitrogen, and soil-dwelling microbes feasted on the chemical. Overall, nitrogen removal increased by 44% when the beavers were present, compared to seasonal extremes of high and low water flows.

Beaver numbers in the West are rebounding thanks to a combination of less trapping and intentional re-introduction. Ironically, an expansion of beaver-friendly habitat due to rising temperatures could increase beaver numbers in some places.

So, the findings hold out promise that as climate change continues to buffet the region’s water supplies—witness the megadrought currently gripping the southwest—one answer to the problem could come in the form of a paddle-tailed, waddling creature. In other words, more beavers, please.

Dewey, et. al. “Beaver dams overshadow climate extremes in controlling riparian hydrology and water quality.” Nature Communications. Nov. 8, 2022.