Using Beavers to Naturally Manage Flooding

Martin Hesp
Tuesday, 9th February 2016

An experiment in Devon has found that a pair of introduced beavers have created dams that alter the water flow, preventing flooding and helping to maintain a constant release of water through dryer months.

Beavers are common creatures in some countries, so you’d have thought someone somewhere would have done scientific studies to find out how their habitat-changing lifestyle – complete with river dams and deep channels – affects the water flows in a landscape. But no one has. Which is why the experiment which has been running for five years in West Devon has been both a world-first and a profound surprise for the scientific community.

The results of the experiment are startling. During the five years the two beavers have lived a wild and natural existence in their seven-acre enclosure. They have built 13 dams using the kind of scrub willow and hazel that tend to line Westcountry streams and these have had a profound effect on both water flows and on pollution.

For the first time ever, scientists now have precise figures to prove how beavers affect both a landscape and its water flows. They have been able to attain such precision in their findings because they have mounted high-tech monitoring equipment on the one tiny stream that flows into the beaver enclosure, and at the point where it flows out…

The scientists have been looking at three different areas in the changing hydrology – the amount of flow which goes into the beaver-zone and then comes out; the increase in the area’s water storage capacity; and the effect on pollutants.

In all three cases the scientists have been astounded. On a tour of the area recently, the WMN was told by Professor Richard Brazier, of the University of Exeter’s geography department: “I’d hypothesised what might occur, and seeing the numbers confirms what we thought might happen – but to a far greater extent.

“There has been much more of an impact than we thought a couple of beavers could possibly have in such a small site. 650,000 litres can now be stored here. When this was just a woodland with a small stream, that would have been just a few hundred litres.

“So it’s many orders of magnitude of change in the storage of water, which is of great benefit both during floods and in dry summers or drought.”

Prof Brazier explained: “My job is to monitor the changes, then to see what difference these animals make over a number of years. So what I’m doing here is looking at flood attenuation. Is beaver activity changing the flow regime – the water that’s coming in compared with what’s leaving the site? Can they play a role in mitigating flooding?

“The second is water storage, specifically in dry times. Can more water be stored in this woodland landscape and therefore benefit streams and reservoirs downstream?

“The third is water quality. Have the beavers essentially created a natural filter for quite polluted water coming off agricultural landscapes?”

Asked why such an experiment had never been done before Prof Brazier replied: “It’s not easy in the wider landscape. In an experiment like this there is one stream in, one stream out, so we can control what’s going on. But there is another aspect – studying it in the UK, in this lowland intensively farmed landscape that we have, is important. If you want evidence to discover what beavers will do in our landscape you really have to study the animals here.”

He explained how the 13 beaver dams are slowing the water flow. “The water comes in at the top and fills up behind the first dam, overflows and fills the next. It is like a staircase. There is a constant release of water – each pond draws down and is replenished before the next rainfall.”

The professor showed us a graph on which a blue line showed flows measured at the input to the site, and a red line showed the outflow. As you’d expect, the blue line zig-zagged up and down as heavy rains were followed by dry periods – but the red line measuring output remained more-or-less straight across the middle of the graph.

And there was another reading. “From this landscape here we are seeing an average of 150mg per litre of sediment coming off farmland in storms. But what we see leaving the site here is just 15mg per litre. Behind every one of these dams the water slows until it’s practically not moving – the sediment settles and fills the pond.

“If you are water company and a river has high sediment, it costs a lot of money to treat. Nitrogen and phosphorus both enter this site at reasonably high levels especially in storms – but at the bottom end we see so little in nitrogen and phosphate, the university’s equipment cannot actually detect the minute amount.

“This is a small-scale experiment but there is no reason why these results shouldn’t scale up,” Prof Brazier concluded. “You could actually farm harder upstream even than they are now – because the wetland that has been created soaks up the problem for you. If you saw that on a whole catchment scale, you could see huge benefits.”

Mark Elliott, the beaver project lead for Devon Wildlife Trust, told the WMN: “We put a pair of beavers in here in March 2011 and we were looking to see if they would help keep the grassland here clear of encroaching scrub.

“The site is secure, but within it they have created whole new habitats. Beavers are a ‘keystone species’ because the habitats they create help a lot of other species. We had just 10 frogspawn clumps here in 2011 – and we had 580 clumps last week.

“With this – and the trial we are running on the River Otter – we are really trying to understand how beavers and people interact and where the potential conflicts are. We are finding the beavers stay primarily in the river and feed on young willow.

“We need to manage the population here, because it’s only a small area,” added Mr Elliott, explaining that the pair in the enclosure had given birth to three kits which had eventually been removed to live elsewhere. “If the population gets too high the youngsters try and disperse; that’s when we would put pressure on the fence.

“The funding for this project comes to an end this year,” he said. “So we are seeking additional funding to continue this in some form.”

So what about the local landowner – how did he feel about having beavers on his land? John Morgan is retired, but used to keep suckler cows and sheep in the area where the beavers now live.

“I was offered the opportunity to have the beavers and I decided it would be a good thing to do – and I’ve been thoroughly enjoying it. I come up here every day,” he said.

“It’s certainly brought a lot more wildlife into the site – all very interesting to see.

“This water is all going down to Roadford reservoir; it eventually becomes drinking water,” said Mr Morgan. “So if you can make the water cleaner, that is a good thing.

“If they took the fences away, I’d be perfectly happy for the beavers to stay – they can be managed like everything else.”

Financial incentive for beaver-friendly farmers

Mark Elliott, who is in charge of the beaver product for the Devon Wildlife Trust, has been spending a lot of time considering how farmers and landowners might get on with beavers if they were re-introduced to British landscapes after centuries.

“The key challenge is ensuring there is some mechanism by which landowners who store water on their land in the headwaters get some sort of payment for the wider benefits that they are providing for society,” he told the WMN, adding that he was not just talking about beaver-created wetlands.

“There are already Countryside Stewardship schemes that recognise the wildlife benefits, and pay a landowner for changing the way they farm – but if a landowner allows beavers to flood some low-lying pasture, for example, maybe they should receive some sort of payment for the reduction in flooding downstream.”

Cross-posted from

Further resources

Watch: Prevent flooding with multi-functional streamside buffer zones

Using permaculture design to prepare for floods