What is the Stage Zero approach to river restoration?

‘Stage Zero’ or ‘Stage 0’ is a recently developed concept in the scientific literature that refers to the fact that following to human disturbance  to land use, sedimentation and channel shape for land drainage and flood risk management, many streams and rivers have become incised or entrenched within their floodplains. Others have become perched above the natural valley floor.

As illustrated in the diagram there is strong evidence that prior to human disturbance, (ie at ‘Stage 0’) many watercourses in depositional reaches naturally flowed through multiple, low banked anabranching or anastomosing, channels that were well connected hydrologically to wetlands within the floodplain.

River restoration projects have typically focused on ‘re-meandering’ straightened streams, working on the assumption that these streams had single-thread meandering channel planforms initially.

The philosophy of Stage Zero restoration is to work with natural processes to rehabilitate a modified and incised, or aggrading, channel network and restore the water connection to its floodplain. Even if it can only be returned to stage 8 (see diagram), this approach delivers a more resilient mosaic of habitats than in-channel restoration.

At River Restoration Centre conferences, Colin Thorne presented his work, with Brian Cluer, on their understanding of river evolution. He provided examples of some 2 dozen restoration projects in the US State of Oregon that successfully used a ‘Stage Zero’ restoration approach. This process-based approach, aims at restoring the river to its wider floodplains by raising the channel bed and regrading valley floors, allowing anastomosing channels, and therefore greater complexity.

In the USA, restoring a stream to ‘Stage Zero’ initially requires identifying the topography and slope of the river-floodplain system before it was impacted by human activities - this is termed the 'Geomorphic Grade Line'. Earth works are then undertaken that may involve localised lowering of the floodplain and in-filling of the existing single-thread channel. For larger rivers, an undersized proto-channel is cut through the valley bottom, increasing connectivity with the new floodplain surface. Large woody material and pioneer planting of trees takes place across the floodplain to act as flow deflectors, spreading flows and dissipating the erosive forces of the increased floodplain flow. Natural geomorphic processes then take over developing a fully-connected, stream-wetland-floodplain system, often resulting in the creation of multi-thread channels. For smaller streams and ditches, the same process is undertaken but instead of cutting a proto-channel, the land is allowed to become saturated, creating wetland and marsh habitat.

This return of the natural stream processes and ecosystem has produced remarkable benefits to river health, heritage, wildlife, sustainability and resilience. Evidence of improved salmonid populations and the return of beaver and other wildlife (birds, mammals, insects) have been found. 

Inset: pre-restoration, incised channel. Main image: post-restoration ‘wet meadow’ at Dick Creek, Oregon. Note abundant use of large wood and lush vegetation only a year after restoration.

Of particular interest in the UK are the improvements in water storage and water quality shown, with restored valleys floors showing moderated extremes of floods and low summer flows, and reductions in land-derived sediment and eutrophication issues in the channels downstream. Restoration of wetland habitats could contribute to the Government targets for nature recovery at a landscape scale so the interest in the UK in ‘Stage Zero’ is growing.

The River Nairn at Aberarder, one of 2 projects in Scotland to adopt the stage zero approach, demonstrates these benefits. Both it ,and the Allt Lorgy (tributary of the River Dulnain on the Spey system), have been monitored in detail since 2012 and 2016 respectively. Their evolution towards their respective reference states (i.e. ‘wandering’ channel morphology  and connected wet woodlands and ‘online’ perennial wetlands) is clearly evident, with associated ecological improvements. In 2020 two further sites on tributaries of the Aberdeenshire Dee will have Stage 0 or assisted recovery restoration designs implemented. Both are important 'sub-sets' of Stage 0, the re-naturalisation of an alluvial fan (the Garbh Allt) and the reinstatement of an anastomosing wetland environment (Betlie Burn). It is intended that detailed physical and ecological monitoring will be conducted at both sites post-implementation.

In England, National Trust identified Riverlands Porlock Vale Streams Project as a ‘Stage Zero’ trial. A site known as Mud Pool Meadow, in West Somerset, was originally 4.5ha of improved pasture land which had been historically drained by digging a ditch through and along the perimeter of the fields. In 2019, the National Trust re-shaped the floodplain and infilled the ditch and stream on the site, to revert the site back to a 'Stage Zero' wetland.

In 2019, the Environment Agency, with the Dorset Wildlife Trust, let a project to develop 'Stage 0 Opportunity Maps' and a series of worked example case studies within the Dorset Frome, Corfe, Piddle and Dorset Stour catchments. The case studies will explore in detail the process, risks and uncertainties of undertaking 'Stage 0' projects in a range of river types. It follows their trial of the site analysis required to restore a stretch of the lower Corfe River, Dorset. This work is being funded by Wessex Flood Defence Committee Local Levy and will deliver information which contributes to the development of mapping and analysis tools, supporting a national 'step-change' in how we work with natural processes to deliver multiple benefits. It will support the development of a programme of 'Stage 0' restoration sites within the Wessex region.

Technical risks identified so far in this approach include:

• Ensuring that a 'geomorphic control' is present that prevents renewed channel incision at the downstream end of the restoration reach, or upstream for perched channels. This is because taking a reach of river back to 'Stage Zero' creates a sudden change in gradient and therefore the long-term geomorphic response MUST be understood and planned for in the project design.
• The extent to which the evolution and biodiversity of the site requires a continued supply of coarse sediment from upstream (eg gravel) and its availability.
• Potential impacts on fish passage through multi-thread channels, during low flows.
• Short term ecological impacts as the site develops and naturalises.
• Hydrological changes due to reduced conveyance and increased floodplain storage could have flood risk implications and therefore a flood risk assessment may be required.

Whilst the USA experience has shown the significant benefits that this re-naturalised river can provide, there are a number of barriers to achieving such significant land use and cultural change. In the UK, the river floodplains have historically been reclaimed for farming and infrastructure, such as roads, railways or housing. The current river channels are part of our cultural and landscape heritage, and are cherished by anglers. Gaining consensus to agree to larger scale restoration of floodplains with multiple land and riparian ownership is a significant constraint. Changes in funding for land owners for public goods may offset the commercial concerns longer term. In the meantime, the existing and future trials can test the effectiveness of the stage zero approach to restoring natural river processes in the UK.

For further information.

Scottish projects contact: Hamish Moir h.moir@cbecoeng.co.uk

Wessex Area Stage 0 opportunity mapping project contact: john.Phillips@environment-agency.gov.uk

Presentations on US projects at RRC conference RRC 2018

Presentation on Porlock project and Stage zero https://vimeo.com/417996381