Beavers bring benefits to British waters

Posted: Tuesday 6th June 2017

A summary of the initial findings from the Devon Beaver Projects reveals that these 'ecosystem engineers' could have a role to play in flood and drought management, as well as help to improve water quality.

The Eurasian beaver (Castor fiber) is a native species to Britain. After an absence of around 400 years, it is now returning to the British Landscape. A new report "Beavers - Nature's Water Engineers", published by the Devon Wildlife Trust - summarises the results of the research being undertaken in Devon to investigate the effects of beavers on the water environment. The ongoing research being carried out in the Enclosed Beaver Project in the western side of Devon is now being extended and applied to the free-living beavers that form part of the River Otter Beaver Trial in East Devon.

In 2011 a male and female beaver were introduced into a three hectare fenced enclosure in the Tamar headwaters, where their impacts are being studied in detail. Most of the results reporte are from this research site.

The beavers live in a large lodge situated on the banks of a pond. The lodge has increased in size every winter as more sticks and silt are built on top by the beavers, and willow sticks and branches are placed in the water to create a nearby winter food cache.

Since 2011, 13 ponds of varying sizes have been constructed by the beavers. The dramatic engineering of the watercourse in this site has provided a perfect opportunity to study the impacts of beaver dams on a wide range of different subjects.

Beavers are widely referred to as ecosystem engineers. They modify river systems and surrounding riparian areas to create habitats which they can exploit and which benefit many other species. Beavers are also termed keystone species, having a disproportionately large impact upon aquatic ecosystems, relative to their abundance. The most significant hydrological impact of beavers, results from their dam building activity and the consequent impoundment of large volumes of water in ponds. Dam and pond features alter hydrology, both locally and downstream. Dams hold water, push water sideways and release water slowly, re-wetting surrounding areas and creating complex wetland environments.

Impacts on water storage

Between 2011 and 2016, 13 ponds were created. The surface area of ponded water rose from 90 m2 to a maximum of 1800 m2, with the largest pond holding 220,000 litres of water.

Water storage is constantly changing with the inflow and outflow of water, but also as dams are maintained and enlarged by the beavers. Even though the number of ponds has stabilised since 2014, the volume of water stored has continued to rise.

Beaver dams slow the flow of water. In storms more water is stored; in droughts more water is released. The potential for beavers to reduce flooding and maintain baseflows downstream is significant and will be explored further.

Experimental design

The amount of water entering, being stored within, and leaving the site has been monitored via a network of instrumented weirs and dipwells.

The potential for beavers to reduce flooding and maintain baseflows downstream is significant and will be explored further. By continuously measuring flow upstream and downstream of the beaver-created wetland, it is possible to quantify the impacts of beavers on the hydrology. Recording rainfall, water table levels and pond depths allows a detailed water 'budget' to be constructed. Understanding the mechanisms of water storage to explain the hydrological responses is a key part of the research.

Impacts on flooding

This research provides strong evidence for the role that beavers might play in reducing flooding downstream, even during prolonged wet periods. Flow into the site (blue line) increases more rapidly in response to rainfall, peaking at a higher flow rate, and falling more rapidly than flow out of the site, as water is quickly moved through the intensively-farmed landscape. The flow leaving the site is attenuated (squashed), rising less rapidly, peaking at a lower rate and falling very slowly after rainfall ceases. This flood attenuation effect is controlled by the storage of water in each beaver pond and the enhanced 'hydraulic roughness' of the landscape, together which slows the flow of the water.

This profound impact persists even during the wettest times of the year as the leaky dams constantly drain, freeing-up storage capacity in each pond. For example, during storm Frank in December 2015, 29 mm of rain fell on the site; peak flow leaving the site was 3 times lower than into the site and the water took 30 min to travel through the site (a distance of 183 m). Holding water back and desynchronising peak flows from minor watercourses into major rivers is a key principle in flood alleviation and something that beaver dams and increased channel complexity are likely to contribute to.

Impacts on river baseflow during drought

As beaver dams are leaky, stored water drains from the site for a long time after rainfall has stopped. This 'baseflow maintenance' means that flow from the site is greater than flow into the site after the storm event. This reversal of hydrological behaviour also relates to the leakiness of the dams, which provide a constant supply of water downstream, even as in summer 2016, when a month long drought left the incoming channel and the top two ponds completely dry, whilst baseflow leaving the site continued. Low flows cause serious environmental problems as oxygen levels are depleted and any pollutants are concentrated.

Impacts on water quality?

As well as affecting the storage and flow of water, impoundment behind dams can affect the quality of water leaving beaver-engineered sites and the amount of diffuse pollutants being transported downstream. By slowing and filtering the water, beaver dams cause sediment and nutrients to be deposited in ponds. In this case, the source of the material is the intensively managed farmland upstream, which provides elevated levels of not only sediment (from soil erosion), but also nitrogen and phosphorus, from manures, slurries and fertilisers that are added to the land. By the time the water has flowed through the sequence of beaver dams, a high proportion of these diffuse pollutants have been removed from the water, settling out in the ponds.

Impacts on sediment

During storm events, each litre of surface water leaving the beaver-modified site has 3x less sediment than the water entering the site. On average 112 mg l-1 of sediment enters the site, but under 40 mg l-1 of sediment leaves the site.

Nitrogen

During storm events, each litre of surface water leaving the beaver-modified site has 0.7x less nitrogen. On average each litre of water draining from agricultural land and entering the channel contains 3.35 mg of total oxidised nitrogen, but only 2.19 mg when it leaves the site.

Phosphorus

During storm events, each litre of surface water leaving the beaver-modified site has 5x less phosphate than the water entering the site from agricultural land upstream. On average each litre of water draining from agricultural land and entering the channel contains 0.10 mg of phosphate, but only 0.02 mg when it leaves the site.

Carbon

Dissolved organic carbon concentrations were significantly lower entering the site, compared to those leaving the site (5.11 mg l-1 versus 11.87mg l-1). This difference reflects the fact that intensively managed grasslands are depleted of soil organic carbon. In contrast, fully functioning wetlands are carbon rich environments, which both store and release more carbon to water than their agricultural counterparts.

Mitigating diffuse pollution from agriculture

Loss of sediment and nutrients from our farmed landscapes can cause problems with land becoming depleted of nutrients and requiring greater fertiliser use and can also cause downstream water quality problems such as eutrophication. For sediment, nitrogen and phosphate, variables which contribute directly to poor ecological status and water quality downstream, the presence of beavers at the Enclosed Beaver Project has led to significant improvement in water quality.

"I have always said that Eurasian beavers have the potential to be one of the most cost-effective tools in the Water Framework Directive toolbox, but it is really important that we know just how effective they are and what the real pros and cons of beavers are. Trials involving monitoring of hydrology and water quality in enclosures like the North Devon Beaver project are therefore really important because they are significantly improving our evidence base, so that when actual releases to the wild are proposed in future, as they surely will be, all consultees will be much better informed about the ecosystem impacts - both positive and negative."

Alastair Driver, National Biodiversity Manager for the Environment Agency.

In related news, the New Scientist recently reported on a US beaver research project, which suggests that beaver dams could lower maximum water temperatures in streams. This is good news for (high) temperature-sensitive fish, and goes against the findings of previous studies, which concluded that beaver dams warm up water.

In late 2016, the Scottish Government decided to allow beavers to remain in Scotland.

Environment Secretary Roseanna Cunningham said that the species would have to be actively managed, in line with practices in other European countries.

Work has now begun to ensure beavers can be added to Scotland's list of protected species as soon as possible. It will be the first time a mammal has been officially reintroduced to the UK.

Scottish Ministers have agreed that:

Research has shown beavers, which were native to Scotland before being hunted to extinction in the 16th century, provide important biodiversity benefits.

However, the animals can also cause significant difficulties for farmers and land managers in vital agricultural areas.

The impacts of beavers in Scotland have been closely monitored by Scottish Natural Heritage (SNH) at both the official Scottish Beaver Trial site in Knapdale in Argyll and also on Tayside, where the species has become established after being released illegally.

Environment Secretary Roseanna Cunningham said: "I have been determined to find a pragmatic approach, which balances the biodiversity benefits of reintroducing beavers with the obvious need to limit difficulties for our farmers.

"Beavers promote biodiversity by creating new ponds and wetlands, which in turn provide valuable habitats for a wide range of other species.

"We want to realise these biodiversity benefits while limiting adverse impacts on farmers and other land users. This will require careful management."




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December 2018

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