Pioneering river restoration declared a success delivering incredible benefits for nature and people within 12 months of completion

The ‘ctrl alt delete’ of the river was the first large-scale attempt to reset a UK river to fully reconnect its waters with the surrounding floodplain by filling in a 1.2km managed, straightened and deepened section of the River Aller to transform the area and dramatically create seven hectares of waterscapes and wetlands (equivalent to more than ten football pitches[2]).

A priority habitat for nature, wetlands are extremely important to not only slow the flow of water and to hold it during times of drought, but they are also significant for their ability to store carbon and act as homes for wildlife. However, sadly over 90 per cent[3] of wetland habitat in the UK has been lost in the last 100 years, and over 10 per cent of our freshwater and wetland species are threatened with extinction.

Put to the test almost immediately after the project completed, and in the midst of England’s wettest 18-month period on record[4], the project team at Holnicote were both pleased, and relieved in equal measures when the landscape both improved for wildlife and helped slow the flow of water after several heavy rainfall events, helping to protect local communities downstream.

National Trust, Project Manager Ben Eardley said: “Trying any new technique is of course challenging but we need to be bold in order to tackle the climate and nature crisis.

“We had just the worst winter you can imagine post restoration in terms of the number of storms and sheer volume of rain. But despite it being record breaking conditions with high flowing water levels, the site responded really well, increasing the ability to store water within the site and lessening downstream storm flows, demonstrating the value of the restoration in providing resilience to hydrological extremes.”

To help the National Trust understand how the site has developed, researchers from several Universities (Exeter, Loughborough, Nottingham and Umeå in Sweden) have been involved with monitoring water flow, water quality and changes to habitat.

Dr Matthew Johnson from Nottingham University and Dr Richard Mason from Umeå University have been working to understand the changes to the water table and development of the river channels and habitats.

Dr Richard Mason said: “Ground water levels rose dramatically across the site, in some places by over a metre. This allows the site to act as a large sponge or filter in the river catchment, helping to store winter flood water, reducing flood risk downstream and releasing cleaner water more slowly in the drier months to help alleviate drought conditions.

“The twists and turns and numerous flow paths that are now present in the floodplain as the water wends its way over the site through thick vegetation, act as a filter to capture sediment eroded from upstream.

“The development of habitat to include faster flowing riffles and gravel beds has also been key for benefitting fish and other aquatic species. The site has moved from a very simple, static system with little opportunity for wildlife to one with significant dynamic complexity which has seen an explosion in life.

“Put simply, the system has moved from a very tidy area with little for wildlife to a messy complex jumble of waterscapes and diverse habitats that is full to the brim with a huge variety of plants and animals.”

Dr Alan Puttock and Professor Richard Brazier from Exeter University commented: “Using drones and in-river sensors to monitor the project, we found that the dramatic changes following the reconnection of the river with its floodplain, have resulted in an attenuation (lessening) in downstream flow regimes.

“This is demonstrated by increased lag times between rainfall and stormflow, in addition to reduced downstream flood peaks, which were on average 38 per cent, lower. These results mean during storms less water is leaving the site and at a slower rate, demonstrating such river restoration projects can help downstream communities at risk of flooding.

“These results mean that for the first time in the UK, we have seen the potential of allowing rivers to reset themselves to function naturally.

“Enabling water to find its own pathway across a floodplain, undoing the long history of drainage of the landscape, has embodied the very definition of a nature-based solution to river restoration.

They concluded: “As we learn more about how to work with natural processes, the work here will become a pioneering example of how to manage floodplains in an environmentally progressive way.”

Other results from the monitoring of the site reveal a reduction in levels of fine sediment further downstream of the site, improving habitat for migratory fish and reducing the impacts of diffuse pollution from agriculture.

Water quality monitoring showed a reduction of average turbidity, ie water cloudiness, by 41 per cent following restoration, indicating the site is now trapping and storing sediment therefore reducing potentially damaging pulses of downstream sediment and nutrients.

The amount of aquatic habitat has also increased significantly from 0.18 ha3 to 3.42 ha3, a massive 1780 per cent increase. This aquatic habitat has developed in diversity with a huge increase in slower flow areas of wetland, and an overall increase in areas of faster flowing water and ponds, resulting in a new mosaic of wetland habitat types providing opportunities and refuge for wildlife leading to greater abundance and resilience.

Commenting on the ground water and habitat monitoring, Dr Richard Mason added: “The speed with which the site developed into a diverse patchwork of habitats has been a real surprise. After only a few months the river was already adjusting, creating deep pools, fast riffles, and a mosaic of wetlands which will all support different plants and animals.

“What amazes me is the amount of wetland habitat created; the river has transformed from a pipe into a sponge. When you look at the river upstream or downstream it’s still a small stream, a few meters across. It’s the same amount of water, it’s just that it is stored for much longer and it’s great to see how quickly it’s benefiting people and nature.

“Connecting rivers to their floodplains is really important for many reasons. At this site, fish now have access to a lot more space, which means more food, more shelter and the ability to move and survive if one part of the river is disturbed (e.g. in floods). Furthermore, access to connected wetland habitats is important to young fish and critically lacking in the UK landscape.

He concluded: “It’s an exciting time for river restoration and this novel technique has real potential for transforming our riverscapes, helping animal and plant life and increasing resilience to climate change. What we learn from our detailed monitoring at this site will help river restoration techniques globally. I’m excited to see how the site continues to develop.”

Alongside the various studies, the local team have also been recording the benefits to wildlife. Jack Siviter, National Trust ranger at Holnicote commented: “The site has gone bananas in terms of the amount of wildlife and lush vegetation growth that we’ve seen.

“There is a huge variety of plants and trees starting to shoot up across the wetlands such as Greater Tussock sedge which can grow to over two meters tall. This will provide a rich landscape for wildlife to thrive within.

“Thanks to the increased wetted area we’ve seen loads of water loving creatures like wild waterfowl, water voles, eels, lamprey, grass snake, trout, and birds such as red kite, buzzard, kestrel, sparrow hawk, swallow, swift and sand martin.

“The martins, swifts and swallows are all using the site for feeding and nest building, while the birds of prey are using the site for foraging – which is in part due to the rewetting but also the longer sward.

“The constant supply of water has also resulted in the site being very green and lush with thousands of wildflowers like wild carrot, providing food for pollinators.

“As a consequence, we’ve seen raptors such as kestrels, herons and deer and even barn owls.”

The site has seen such an increase in insect life that at times there have been so many swallows the effect has been a feeding frenzy with hundreds and hundreds of birds swarming to feed on the abundance of food.

Jack concluded: “One of the coolest species we’ve seen is a green sandpiper – which is a great indicator species for healthy, lowland wetland habitat.”

A particular species that the team are monitoring for signs of the site developing to be a healthy wetland habitat is the water vole.

In 2023, there were only limited signs of water voles and only along the edges of the restored section, while the habitat was being established. However, recent surveys carried out by Derek Gow Consultancy have revealed a dense population colonising a central area, with scattered signs of them moving throughout the new wetland, showing that they are happily exploring their new habitat.

Hazel Reading, Ecologist at Derek Gow Consultancy said: “The restoration area started as a linear ditch with patches of suitable water vole habitat along it.

“This patchiness can limit the voles’ ability to migrate and find the best seasonal food, or even each other, without risking predation.

“The new habitat is expansive, connected, and has started growing up wonderfully. With an abundance of their favourite food plants and a link to the surrounding water vole populations, they’ve ended up with a little slice of vole heaven.

“Although 12 months in we can only really say it’s a foothold for now, I wouldn’t be surprised to see a big increase in their population in coming years!”

The Environment Agency were also a partner in the project and funder of part of the scheme. Matt Pang, Catchment Co-ordinator at the Environment Agency said: “One year on and the environmental response to the River Aller restoration exceed expectations. A rich and diverse wildlife haven has exploded from this back-to-basics, nature-based approach and the wider environment is better for it.

“Less flood risk, more wildlife, plus some rare species - the results are mind blowing and we can’t wait to see what the coming years bring.”

The project has been made possible thanks to the Interreg 2 Seas Co-Adapt programme and the Environment Agency. Both organisations have helped fund the project together with the Somerset Rivers Authority (SRA), Green Recovery Challenge Fund and Frugi.

Careful monitoring of this pioneering project will now continue to further understand how the topography develops as well as the ecology of the site.

The project was part of the National Trust’s multi-million-pound Riverlands[5] project announced in August 2018, supporting five river catchment schemes around England and Wales.