Beavers can help reduce soil erosion and pollution in rivers, research on a family of the mammals in south-west England suggests.
The study looked at beavers which have lived in a fenced site at a secret location in west Devon since 2011 as part of a trial by Devon Wildlife Trust to assess their impact on wildlife and water management and quality.
The aquatic mammals have built 13 dams on the site, creating a series of ponds along what was once a small stream.
University of Exeter scientists assessed the impact their activities have had on reducing the flow of tonnes of soil and nutrients from nearby agricultural land into the local river system.
They found the pond and dam network had trapped more than 100 tonnes of sediment, around 70% of which is thought to be soil washing away from the "intensively managed grassland" fields upstream.
The ponds are also storing tonnes of carbon in the sediment, along with nitrogen and phosphorus which are nutrients that can cause problems for wildlife and water quality in streams and rivers.
The team has also found that the water leaving the site had lower levels of suspended sediment and nitrogen than the water flowing in.
Beavers were hunted to extinction in the UK by the 16th century, but populations have been reintroduced, released or escaped into the wild in Scotland and Devon.
Conservationists back the return of the aquatic mammals, which manage the landscape by cutting down trees and damming rivers, for the benefit they can provide in preventing flooding, and boosting water quality and other wildlife.
The findings that beavers could help reduce soil erosion and pollution come in the wake of research in 2009, which estimated soil loss from UK agricultural land cost £45 million a year, largely due to sediment and nutrient pollution downstream.
Busy beavers. One beaver family has removed 100tonnes of polluting sediment from one Devon stream. New research by @UniofExeter shows positive impact #beavers can have on pollutants in our water supplies. https://t.co/yyqEm5aU38#cleanwater#naturebasedsolutionspic.twitter.com/NeCInmSdKI
-- Devon Wildlife Trust (@DevonWildlife) May 9, 2018
The new research, published in the journal Earth Processes And Landforms, was led by Professor Richard Brazier, who 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."
In addition to its enclosed beaver trial, Devon Wildlife Trust is running another project involving England's only wild beavers on the River Otter in east Devon, where a pair were released in 2016 to boost the genetic diversity of the population which has been living on the river for a decade.
Devon Wildlife Trust's Peter Burgess said: "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."