Climate-driven droughts that create water-quality issues are proving to have a formidable foe: beavers.

The furry semi-aquatic rodents' industrious dam building actually improves water quality, scientists at Stanford University and their colleagues have found.

The study, published on Nov. 8 in the journal Nature Communications, found that rising water levels caused by the dams push water in secondary channels into the surrounding upstream plant communities, or riparian zones. These zones filter out nutrients and contaminants, and cleaner water then flows downstream and back into the main waterway.

American beavers are often considered pests due to their prolific dam building, which raises water levels. Their populations are booming in the western United States as conditions grow hotter and drier, the scientists said. But the new research shows the dam building benefits river water quality so much that it outweighs damage caused by droughts.

Less water flows through rivers and streams during droughts, causing concentrations of contaminants and excess nutrients, such as nitrogen, to rise. Major rainfall and snowmelt flush out contaminants and restore water quality. But as climates dry out, these contaminants aren't being flushed through natural rainfall cycles.

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

The researchers studied dam-building effects on the East River, a main tributary of the Colorado River, and a mountainous watershed near Crested Butte in central Colorado, after a family of beavers coincidentally appeared in their research-study area. Then- Stanford Ph.D. student Christian Dewey, now an Oregon State University postdoctoral scholar, was initially tracking seasonal changes in hydrology, and the riparian zone impacts on nutrients and contaminants in a mountainous watershed.

His study compared water quality along a part of the East River during an extreme drought year, 2018, to unusually high water levels in 2019. Dewey and his colleagues reviewed hourly water-level data gathered by sensors installed in the river and throughout the riparian area. The team collected water samples to monitor nutrient and contaminant levels, including from below the ground's surface.

Then, the beavers arrived. The scientists compared their yearlong 2018-2019 data to water quality during nearly three months starting in late July 2018 when the beaver dam blocked the river.

The dam dramatically increased removal of nitrate, a form of nitrogen, by creating a steep drop in water levels above and below the dam.

Warm, dry summers that follow spring snowmelt in the West create a pressure gradient — an increase or decrease in the magnitude of pressure or concentration — that pushes water into surrounding soils. Water and nitrate are pushed into soils where microbes transform the nitrate into an innocuous gas. The greater the gradient, the more water and nitrate are moved into the soil.

The beaver dam increased the gradient at least 10 times more compared to an average day during the summer peak without the dam, for both the 2019 high-water year and the 2018 drought year.

"Beavers are countering water quality degradation and improving water quality by producing simulated hydrological extremes that dwarf what the climate is doing," Fendorf, a professor in the Stanford Doerr School of Sustainability, said.

The beaver dam also boosted nitrogen removal from the studied river section by 44% above the seasonal extremes. Nitrogen harms water quality by promoting algae overgrowth. Decomposed algae starves water of oxygen needed to support animal life and a healthy ecosystem, the researchers said.

"We would expect climate change to induce hydrological extremes and degradation of water quality during drought periods, and in this study, we're seeing that would have indeed been true if it weren't for this other ecological change taking place, which is the beavers, their proliferating dams, and their growing populations," Fendorf said.

The study's co-authors are affiliated with Lawrence Berkeley National Laboratory.