Sunday, December 4, 2022

Beavers Improve Water Quality

As climate change worsens water quality and threatens ecosystems, the wooden dams of beavers may help lessen the damage.  This was the finding of a recent study by Stanford University researchers. Published November 8th 2022 in Nature Communications the study found that the wooden dams built by beavers raise water levels upstream, diverting water into surrounding soils and secondary waterways, the riparian zone. These zones act like filters, straining out excess nutrients and contaminants before water re-enters the main channel downstream.

The hotter, arid conditions that are expected to be brought by climate change will lessen water quality. However, these same conditions are favorable to the American beaver, and may have also contributed to a resurgence of the population and an explosion of dam building in the western United States.

The discovery of the profound impact of beaver dams came about serendipitously. When he was a  PhD student in 2017, lead study author Christian Dewey had started doing field work along the East River, a main tributary of the Colorado River near Crested Butte in central Colorado. Initially, Dewey had set out to track seasonal changes in hydrology, and riparian zone impacts on nutrients and contaminants in a mountainous watershed.

“Completely by luck, a beaver decided to build a dam at our study site,” said Dewey, who is now a postdoctoral scholar at Oregon State University. “The construction of this beaver dam afforded us the opportunity to run a great natural experiment.”

Beavers are semiaquatic mammals partial to freshwater environments.   They have the ability to create their own ecological niche by building dams.  Dam construction has the potential to alter the hydrology, biogeochemistry, and ecosystems of river corridors. Beavers build dams to help engineer their habitat for food supply (riparian and wetland vegetation), to create water bodies sufficiently deep that do not completely freeze during winter in  colder locations, and as a protection from potential predators. The dams and their ponds create riparian discontinuities baht allow a river to cleanse its waters and allow the created wetlands to absorb excess precipitation preventing catastrophic flooding.

To understand how beaver dams may affect water quality in a future where global warming produces more frequent droughts and extreme swings in rainfall, the Stanford researchers compared water quality along a stretch of the East River during a historically dry year, 2018, to water quality the following year, when water levels were unusually high. They also compared these yearlong datasets to water quality during the nearly three-month period, starting in late July 2018, when the beaver dam blocked the river.

Water quality is a measure of the suitability of water for a particular purpose – ecosystem health or human consumption, for instance. During periods of drought, as less water flows through rivers and streams, the concentrations of contaminants and excess nutrients, such as nitrogen, rise. Major downpours and seasonal snowmelt are then needed to flush out contaminants and restore water quality.

The researchers found that the beaver dam dramatically increased removal of nitrate, a form of nitrogen, by creating a surprisingly steep drop between the water levels above and below the dam.The larger the gradient, the greater the flow of water and nitrate into soils, where microbes transform nitrate into an innocuous gas.




For Further reading see:

Dewey, C., Fox, P.M., Bouskill, N.J. et al. Beaver dams overshadow climate extremes in controlling riparian hydrology and water quality. Nat Commun 13, 6509 (2022). https://doi.org/10.1038/s41467-022-34022-0

Dam builders and their works: Beaver influences on the structure and function of river corridor hydrology, geomorphology, biogeochemistry and ecosystems - ScienceDirect


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