What is the Stage Zero approach to river restoration?

Guest blog - Fiona Bowles, Hamish Moir, Ben Eardley, John Phillips, Will Bond & Matt Parr

‘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.

River morphology is influenced by biology, hydrology and geology as shown in the diagram (Castro & Thorne 2019). So restoration of the valley requires thinking beyond the river channel.

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.