After 400 years, beavers are back in England; 9 dams are estimated to store 3 million litres of water and slow flood flows

Beavers are reshaping parts of England’s waterways, and their dams can change how water moves and is stored across the landscape.

A government evidence review published on GOV.UK says Eurasian beavers can build dams and canals that push water sideways, creating ponds and wetland features. These structures can slow and store water by increasing the roughness of the landscape.

But the evidence does not mean beavers can prevent flooding everywhere. The government report gives medium confidence to the evidence on the effect of beavers on flood risk, with the effect depending on local conditions.

How Beaver Dams Can Slow Water

Beaver dams can change the way water moves through a river system.

The UK government report says beaver dams and the complex habitats they create can increase surface-water storage in ponds and canals. They can also raise the water table and contribute to groundwater recharge.

The review says beaver dams can attenuate flows during both low- and high-flow periods. They can also slow flood flows by encouraging rivers to reconnect with their floodplains and allowing floodwater to be stored there.

However, damaged dams may release floodwater.

River Otter Research Found a Delay in Peak Flow

The River Otter provides one example of what happened after beavers returned.

The GOV.UK government report says the effects of beaver colonisation were observed after three years. A delay in peak flow was recorded and was believed to be associated with increased storage capacity and a general slowing of the flow.

In this example, the increase in storage was linked to temporary floodplain storage and increased surface roughness. The study also highlighted the role of local topography in determining how much beaver dams can slow water flow.

Four Sites Recorded Reductions in Peak Flows

Another analysis examined storm events at four sites where beavers had been reintroduced.

The report says the analysis found significant reductions in peak flows after beavers were reintroduced. The impact was greater at sites with well-established beaver activity.

The four sites were considered representative of landscapes where beavers are returning in England.

Research in the headwaters of the Tamar Catchment in Devon also found that total flows below the beaver-affected area were smaller than those above it. Peak flows also showed a reduction, with significant differences recorded during the largest storm events.

Essex Beaver Project Stored an Estimated 3 Million Litres of Water

Another example comes from Spains Hall Estate in Essex.

In 2019, the estate and the Environment Agency released the first pair of beavers in East Anglia for more than 400 years. Five years later, the beaver family had grown to 11 animals and had created nine dams across a 10-acre woodland stream.

The Environment Agency estimated that the dams stored 3 million litres of water in ponds covering almost a quarter of the woodland. Data collected by King’s College London showed that the way water flowed across the farm had changed.

Data collected by King’s College London showed that the beavers were increasingly building bigger dams to trap more water when it rained. The changes meant stormwater flowed downstream more slowly, with some water held for several months before being slowly released back into the river.

Beaver Activity May Increase Groundwater Storage

The effects of beaver activity can extend below the surface.

According to the report, beaver activity may increase groundwater storage and contribute to groundwater recharge. It also identifies drought resilience among the potential wider benefits associated with beaver activity.

However, more research is needed to understand these effects and how they vary between landscapes.

Beavers Can Also Cause Problems

The changes created by beavers can have unwanted effects in some locations.

Government guidance for England says beaver activity may cause flooding to adjacent land and damage to crops, infrastructure, property or machinery. It also says beavers may need to be managed to address problems caused by their activity.

More Research Is Still Needed

The government evidence review gives medium confidence to the effect of beavers on flood risk. It also identifies several areas where further research is needed.

These include understanding the hydrological effects of beaver dams across different scales and land uses, the conditions and consequences of dam failure, and the structure, number and location of dams needed to achieve significant downstream flood reduction.

The wider GOV.UK evidence update reviewed research published from 2017 to 2023. Beavers were added as a new natural flood management measure because of an increase in scientific studies in a UK context since 2017.

Beavers Are One Part of Natural Flood Management

The evidence points to a potential role for beavers in managing water, rather than showing that they can prevent flooding everywhere.

Their dams can increase water storage and slow flows, while studies in several English locations have recorded changes in peak flows. But the government evidence review gives the evidence medium confidence and highlights gaps in understanding how these effects translate into downstream flood-risk reduction across different landscapes.

That makes local conditions important. The impact of beaver dams can vary depending on the landscape and how the dams and wetlands develop over time.

Beavers can therefore form part of natural flood management in some places, but the available evidence does not support describing them as a universal solution to flooding.