Beaver dams reduce drought damage

As climate change worsens water quality and threatens ecosystems, beavers and the dams they build can help reduce the damage, according to a new study.

When it comes to water quality in upland watersheds, beaver dams may have a much greater influence than climate-driven seasonal extremes of rainfall, the researchers report.

The wooden barriers raise the water level upstream, diverting water into surrounding soils and secondary streams, collectively called the riparian zone. These zones act as filters, filtering excess nutrients and contaminants before the water re-enters the main downstream channel.

This beneficial influence of large, toothy amphibious rodents looks set to grow in the years to come. While hotter, drier conditions caused by climate change will reduce water quality, these same conditions have also contributed to the resurgence of the American beaver in the western United States and consequently the explosion of dam building.

“As we are getting drier and hotter in the upland watersheds of the American West, that should lead to a degradation of water quality,” says Scott Fendorf, professor of earth system sciences at Stanford University and senior author of the paper. in Nature communications.

“However, unknown to us prior to this study, the enormous influence of beaver activity on water quality is a welcome contrast to climate change.”

Beavers and a natural experiment

The discovery of the deep impact of beaver dams happened serendipitously. As a doctoral student in Fendorf’s lab in 2017, lead author Christian Dewey had begun fieldwork along the East River, a major tributary of the Colorado River near Crested Butte in central Colorado.

Initially, Dewey set out to monitor 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,” says Dewey, who is now a postdoctoral scholar at Oregon State University. “Building this beaver dam has given us the opportunity to conduct a great nature experiment.”

For the study, Dewey and colleagues looked at hourly water level data collected by sensors installed in the river and throughout the riparian area. The team also collected water samples, including from below the soil surface, to monitor levels of nutrients and contaminants.

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

Beaver dams rid rivers of nitrates

Water quality is a measure of the suitability of water for a particular purpose, such as ecosystem health or human consumption. During periods of drought, when less water flows through rivers and streams, concentrations of contaminants and excess nutrients, such as nitrogen, increase. Heavy downpours and seasonal snowmelt are therefore needed to wash away contaminants and restore water quality.

Through their measurements and computer modeling of the interrelated biological, chemical and physical processes that affect how contaminants concentrate or flow downstream, the researchers found that the beaver dam dramatically increased nitrate removal, a form of nitrogen, creating a surprisingly steep drop between the water levels above and below the dam.

The hot, dry summers that follow the spring snowmelt also produce large changes in water level, which generate a pressure gradient that pushes water into the surrounding soils. The higher the gradient, the greater the flow of water and nitrate into the soil, where microbes turn the nitrate into a harmless gas.

In the East River, the researchers found that the increase in gradient over an average day was at least 10 times greater with the dam than it was during peak summer without the dam, both for the high-water year (2019) than for the drought year (2018). In other words, the dam’s effects exceeded hydrological climatic extremes—in either direction of drought or heavy snowmelt—by an order of magnitude.

“Beavers are counteracting water quality degradation and improving water quality by producing simulated hydrological extremes that dwarf what the climate is doing,” says Fendorf, a professor at the Stanford Doerr School of Sustainability and a senior fellow at Stanford. Woods Institute for the Environment.

While in place, the beaver dam increased the removal of unwanted nitrogen from the studied section of the East River by 44 percent over seasonal extremes. Nitrogen is a particularly dangerous water quality issue as it promotes excessive algae growth, which once decomposed robs the water of the oxygen it needs to support diverse animal life and a healthy ecosystem.

The study reminds that as future impacts of climate change are assessed holistically, feedback from changes in ecosystems also needs to be included.

“We would expect climate change to induce hydrological extremes and degradation of water quality during dry spells,” says Fendorf, “and in this study, we are seeing that that would indeed have been true were it not for this other ecological change. that is taking place, which are the beavers, their proliferating dams and their growing populations.

The study co-authors are from Lawrence Berkeley National Laboratory.

The US Department of Energy and the Rocky Mountain Biological Laboratory in Gothic, Colorado supported the work.

Source: Stanford University

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