Beavers reduce river pollution by building dams, study shows

Beavers should be reintroduced into the wild to help clean up polluted rivers and stem the loss of valuable soils from farms, new research suggests.

The study,  by scientists at the University of Exeter, found that a single family of beavers removed high levels of sediment, nitrogen and phosphorus from water that flowed through a 2.5 hectare enclosure in Devon.

The group of beavers, which have lived in fenced-off site at a secret location in West Devon since 2011, have built 13 dams, slowing the flow of water and creating a series of deep ponds along the course of what was once a small stream.

Researchers measured the amount of sediment, phosphorus and nitrogen in water running into the site and then compared this to water as it ran out of the site after it had passed through the beavers’ ponds and dams system.

They also measured how much of the chemicals, and sediment had been trapped by the dams in each of the ponds.

The results showed the dams had trapped more than 100 tonnes of sediment, 70 per cent of which was soil, which had eroded from ‘intensively managed grassland’ fields upstream.

Beavers were thought to be extinct in Britain but were spotted for the first time around 10 years ago
Beavers were thought to be extinct in Britain but were spotted for the first time around 10 years ago

And that sediment contained high concentrations of nitrogen and phosphorus, which are nutrients known to create problems for the wildlife in rivers and streams and which also need to be removed from human water supplies to meet drinking-quality standards.

Professor of Earth Surface Processes, Richard Brazier of Exeter said: “It is of serious concern that we observe such high rates of soil loss from agricultural land, which are well in excess of soil formation rates.

“However, we are heartened to discover that beaver dams can go a long way to mitigate this soil loss and also trap pollutants which lead to the degradation of our water bodies.

“Were beaver dams to be commonplace in the landscape we would no doubt see these effects delivering multiple benefits across whole ecosystems, as they do elsewhere around the world.”

Beavers were thought to have been extinct in Britain since the 16th century after being hunted for their pelts, meat and scent glands.

But 10 years ago a colony was discovered on the River Tay in Scotland, followed by a group in the Knapdale Forest, Argyll in 2009 which led to a reintroduction programme by Scottish Heritage.

In 2010 a wild family were also spotted in the River Otter, in Devon, and are currently monitored by the Devon Wildlife Trust. They have since bred to six family groups.

The new study lends further weight to reintroduction schemes.

Devon Wildlife Trust’s Director of Conservation and Development, Peter Burgess which monitors the River Otter beavers: “Our partnership with Exeter University working on both our fenced and unfenced beaver trials is revealing information which shows the critical role beavers can play, not only for wildlife, but the future sustainability of our land and water.

“It is truly inspiring to have our observations confirmed by detailed scientific investigations.”

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