Beavers preserve wetlands in water-stressed areas

by Sarah Derouin
Wednesday, January 10, 2018

Once considered detrimental to ecosystems and nuisances where, for example, dams flooded farmland, beavers have been rhetorically touted in recent years as a potential boon for wetland health and water conservation. Anecdotal accounts and qualitative findings have suggested beavers improve water quality and availability in drought-stressed ecosystems, but just how much influence they have was not known. In new research, scientists have examined two creeks in Nevada to directly measure how effective beaver dams are at slowing water flows and storing water through the dry summer months.

In 2013, news reports in Nevada and nationwide highlighted the benefits of beavers when dams on creeks flowing into the Humboldt River created year-round wetland habitats that lasted through the driest part of the year.

“I’ve always been interested in beaver ponds in general, but I never realized until I saw these newscasts that beavers lived in deserts,” says Emily Fairfax, a hydrology graduate student at the University of Colorado Boulder. Fairfax decided to dive into the available research to see how the animals might affect drought-stricken rivers in Nevada, finding that while there were qualitative observations linking increased vegetation and sustained wetlands to the presence of beaver dams, quantitative data were lacking.

“In this part of Nevada, the primary stressor on the vegetation is water,” Fairfax says, adding that water availability plays the strongest role in summertime growth patterns. Fairfax analyzed Landsat 8 satellite data using two different measures of water availability — evapotranspiration (ET) and normalized difference vegetation index (NDVI) — to see which plants had access to water around a pair of beaver-dammed creeks. Evapotranspiration describes water leaving a landscape as vapor, after being used by plants, so higher ET measurements mean plants are thriving. The NDVI, meanwhile, distinguishes living green vegetation from surrounding brown dormant plants. At the Geological Society of America’s annual meeting in Seattle last October, Fairfax reported that ET was 50 to 100 percent higher in the beaver-dammed areas compared to surrounding non-dammed areas, and the NDVI was 6 to 88 percent higher.

Beaver dams, as the name suggests, impound flows on waterways, forming upstream ponds that recharge the water table and drain slowly. Fairfax and her colleague, Eric Small, also at the University of Colorado Boulder, found that beaver ponds in the dammed areas stored water from the spring surge of snowmelt, and this water persisted long into the dry season. “Basically, the beavers were irrigating the riparian areas” around the stream, she says.

Two hundred years ago, beavers were plentiful in the western U.S., according to diaries from fur trappers, says Mark McKinstry, a biologist at the U.S. Bureau of Reclamation who was not involved in the study. Remnants of beaver dams can be seen in drainages all over the West, he says, but it has been hard to quantify the animals' effects on stream ecosystems. This study is a great step forward, he says, and “helpful because [such studies] point out the benefits [of beavers].”

There is an intrinsic value of wetlands that is sometimes overlooked, McKinstry says, noting that restoring wetlands benefits both animals and any “landowner who’s trying to make a living off that land.” McKinstry says he has seen the benefits of reintroducing beavers to streams in Wyoming. He stresses that in dry areas, riparian landscapes are oases for animals, and beavers are the reason that many of these wetlands exist.

Fairfax says the next step in her team’s research is to create a physically based stream model — incorporating information about channel hydraulics and groundwater infiltration rates — that would be useful for water managers and those interested in reintroducing beavers to particular streams.

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