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
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