Can beavers clean our streams? A study from three agricultural catchments in south-west England

In common with many European streams, most streams in south-west England have failed to reach Good Ecological Status under the Water Framework Directive. The landscape comprises mainly pastoral and arable fields, from which rainwater can run off, carrying huge quantities of sediment and surplus fertiliser. The receiving streams, often highly modified through channelisation, are degraded from the physical, chemical and biological effects of these pollutant loads, most notably high nutrient and fine sediment inputs leading to eutrophication. 

After an absence of 400 years, Eurasian beavers Castor fiber are now being re-introduced into some of these landscapes, or are colonising naturally from nearby introductions. Through the building of their dams and creation of diverse, ponded wetland environments, beavers have been shown to deliver multiple ecosystem services, including flow moderation, habitat provision and water quality improvements.

 
Encompassing highly productive, vegetation-rich shallow areas and deeper, oxygen-limited areas with different nutrient-cycling pathways, beaver wetlands have the potential to improve water quality through the settling out of solids and uptake and cycling of nutrients. By contrast there are periodic releases of solids and nutrients due to burrow and canal excavations, dam breaches and nutrient inputs from the beavers themselves and the diverse fauna and flora supported in their wetlands.

 
To examine the potentially dynamic effects of beavers on the transfer of sediments and contaminants (nutrients) in catchments, this study used fortnightly water sampling at the inflow, outflow and upstream and downstream of three beaver re-introduction enclosures over two years. In addition, automated samplers were deployed to investigate finer temporal resolution responses to rainfall events. 

For each site, suspended solids, nitrogen, carbon and phosphorus concentrations and loads were determined. Sediment storage dynamics were revealed through the novel use of sonar monitoring in ponds and continuous in-situ turbidity sensor measurements at the inflow and outflow.

 
Results demonstrate the dynamic nature of sediment and nutrient reduction in beaver-engineered wetlands, with switches between source and sink states depending on inflow conditions and pond-specific factors. Beaver wetlands were shown to remove nutrient pollution where inflow loads were high and the mixed temporal and spatial dimensions of this study help resolve differences in results between previously published studies.