Beavers and flood risk management
Beaver man, Derek Gow, and Mark Elliott of the Devon Wildlife Trust, tell us about the role beavers could play in managing flood risk in Britain
Beavers are well known for building dams. The wetland landscapes they create include an abundance of standing, fallen and submerged deadwood providing living space for a wealth of associated wildlife. These habitats are typically established where narrow, shallow stream systems trigger dam building activity, as water depths are too low for beavers to feel safe.
'A series of six beaver dams on the River Chevral resulted in a significant lowering of flood peaks on the downstream reaches of the river'
The number of dams created by beavers in any given territory will vary according to the number and density of beavers in a colony, the availability of building materials and the topography of the surrounding landscape. Beavers generally use timber for dam construction but are innovative creatures; in rocky landscapes they will combine stones bound together with mud, or use the roots of aquatic plants or rushes to create very broad dams.
The impact of beaver dams on hydrology can be dramatic. In one North American study, water took 3-4 hours to travel 2.6 km where there were no beaver dams. When a single, leaky beaver dam, 1.5m high, was established, it took 11 days to travel the same distance (Müller-Schwarze and Sun 2003).
In the Belgian Ardennes where beavers were reintroduced in 2003 a series of six beaver dams on the River Chevral resulted in a significant lowering of flood peaks on the downstream reaches of the river, and an increase in the interval between flood events (Nyssen et al. 2011).
The Devon Beaver Project In March 2011, a pair of Eurasian beavers was released into a three-hectare enclosure as part of the Devon Beaver Project. This project, led by the Devon Wildlife Trust in collaboration with a private landowner and the University of Exeter, is designed to investigate the impact of beavers on the ecology and hydrology of a mosaic of semi-natural habitats including remnant grassland encroached by willow scrub.
For more information on the official Devon beaver trial Visit the Devon Beaver project
Prior to the introduction of the beavers, a small release pond was constructed in the enclosure. Apart from this, and a few temporary pools that had formed under the root plates of fallen trees, the only open water was a trickle, which flowed over the ground surface of the wet woodland from a small spring on its boundary.
The pair of beavers (and the three kits they produced in 2013) have dramatically altered this environment. The trickle of water has been extensively dammed, and a series of approximately 13 ponds, associated marshes and interconnecting beaver ‘canals’ have been constructed. Professor Brazier’s research group at the University of Exeter has demonstrated that the 13 ponds alone have a total volume of ca. 650 m3. If this pond creation was extrapolated throughout a larger catchment, the impacts on flood flows and droughts would be profound.
The change from a single rivulet passing through the woodland to a highly complex mosaic of ponds and braided channels emanating from the numerous dams has been particularly interesting (Puttock et al., 2015). Many of the dams are up to two metres in height and although their impact was anticipated, the complexity of the wetland habitats that have rapidly developed has been surprising.
The dams are permeable and seep at a number of points along their length. This process creates a dynamic pattern of new, complex channels slowing the flow of the water. This aspect of their character is of note. A series of ponds that are permanently full afford a limited capacity to capture floodwater. The constant flow of water from the beaver dams results in their levels falling between rainfall events, which renews their capacity for further storage.
'The research has also shown that beaver activity improves water quality.'
The early stages of the Devon Beaver Project clearly demonstrate the potential of beaver-generated wetlands to retain water. Professor Brazier’s work shows that in dry periods, flow leaving the site is greater than flow entering the site, enhancing baseflows in the river downstream. In contrast, during periods of rainfall, the series of beaver dams and ponds gently fill up with water to mitigate downstream flooding, as less water is released from the site than enters. When the rainfall stops, the system reverts back to supporting enhanced baseflows, as the storm water slowly leaves the site.
The research has also shown that beaver activity improves water quality, with levels of suspended sediment, nitrate and phosphate entering the site from intensively managed farmland upslope, being significantly reduced by the time that the water leaves the site. These diffuse pollutants settle out in the series of ponds, creating nutrient-rich sediment, which has become a fertile location for a wide range of aquatic plants to colonize.
In the lower lying reaches of river systems in Western Europe there are occasional conflicts between beaver activity and the requirements of human infrastructure. However, while beavers sometimes burrow into flood defence structures or riverbanks, block drainage ditches or road culverts and flood areas of productive farmland, there are established management techniques that mitigate their impact (Gurnell et al. 2008). It is clear that beavers can adapt to and exist quite successfully in highly engineered, riparian landscapes.
For information on the official beaver trial in Scotland Visit Scottish Beavers
Beavers have a low reproductive rate and are slow to cross catchment boundaries (Halley and Rosell 2002). These aspects of their ecology provide the opportunity to gradually assess their effectiveness as agents of water management without committing to a national process of reintroduction in the first instance. Over time, as the results of release trials are clarified, any process or restoration could be expanded or terminated with ease.
Beaver-generated landscapes have the clear potential to provide natural, sustainable, expanding systems of effective water management. In their most basic form their pertinence would apply equally to flood dissipation and water storage. Although there would be costs associated with the species’ restoration, three independent feasibility studies commissioned respectively by Natural England, Scottish Natural Heritage and the Countryside Council for Wales (Macdonald et al. 2000, Gurnell et al. 2008, Jones et al. 2012) have all recommended that trial reintroductions of beavers should be explored in Britain.
As flood risks increase and the costs of conventional flood defence rise, the case for the restoration of the Eurasian beaver to Britain becomes stronger.
About the authors
Derek Gow has worked with Beavers since 1994 when he organised the first import of Eurasian beavers to Britain from the Polish Academy of Sciences breeding farm at Popielno. Since then he has advised the statutory restoration process for this species in Scotland, England and Wales. He has travelled widely to see beaver generated landscapes from Western Oregon to Russia. He has written many articles regarding the species and was a co-author of the recent Mammal Society field guide to beavers.
Mark Elliott is the Devon Beaver Project Lead for the Devon Wildlife Trust. He is a wetland ecologists with 20 years experience working in wetland creation and management, working for the Environment Agency, the Wildlife Trusts and West Sussex County Council. As well as being fascinated with beavers, his particular interest is the use of wetlands to buffer human communities against the impacts of extreme weather.
Puttock, A.K., Cunliffe, A., Anderson, K.A. and Brazier, R.E. (2015) Aerial photography collected with a multirotor drone reveals impact of Eurasian beaver reintroduction on ecosystem structure. Journal of Unmanned Vehicle Systems. doi.org/10.1139/juvs-2015-0005
The River Otter Beaver Trial is led by Devon Wildlife Trust working in partnership with The University of Exeter, the Derek Gow Consultancy, and Clinton Devon Estates. Expert independent advice is also provided by the Royal Zoological Society of Scotland, Professor John Gurnell, and Gerhard Schwab, an international beaver expert based in Bavaria. In addition to the generous support of DWT members and others who have donated to our appeal, the trial is also funded by The Royal Society of Wildlife Trusts (RSWT). The ongoing research work at the enclosed beaver trial near Okehampton is funded by Westland Countryside Stewards.