Bristol Avon Rivers Trust, with the support of a team of local volunteers, have been restoring river habitat along a tributary of the Bristol Frome, the Stoke Brook which flows through the Three Brooks Nature Reserve in north Bristol.
This urban stream has been heavily modified to primarily function as a conduit for water to get from A to B as quickly as possible. Over time, the stream has become disconnected, hidden away, and stripped of valuable habitat for fish and aquatic life.
The over-wide and artificially straight jacketed channel was shallow and devoid of cross-sectional habitat diversity, providing very little refuge for coarse and minor fish species or invertebrates. Conditions were characteristic of slow depositing flows; the bed was uniformly silty which smothered riverbed gravels. In places, the brook was also over-shaded, preventing light reaching the watercourse, impacting the colonisation of macrophytes (aquatic plants) thus limiting habitat diversity and the niches that fish rely upon to thrive.
The primary objectives of the restoration work were to improve conditions for macrophyte (aquatic vegetation) and macroinvertebrate (e.g. aquatic insect) communities in this watercourse, acknowledged to be failing under current Water Framework Directive (WFD) assessments, as well as creating habitat for both coarse and minor fish species, including European eel, and bullhead.
Between October 2021 and November 2022, 30 brushwood berms were added to a 1km reach of the Stoke Brook, a tributary of the Bristol Frome. Brushwood berms are woody structures built in the river to encourage natural processes and increase flow and habitat diversity for river wildlife.
Each structure required large quantities of brushwood (small branches) to be harvested from adjacent woodland, floated down and carefully positioned into the river behind a larger tree limb which acts as a flow deflector, pinching the channel and overtime introducing flow diversity that will begin to reshape the river bed, scouring river bed gravels and creating riffle pool habitat. Once positioned, the brushwood was secured in place using ‘cross braces’ (tree limbs) and wired to chestnut stakes that were hand driven into the riverbed. The process is labour intensive, physically demanding not to mention cold and soggy during early winter! The commitment from the volunteers was truly inspiring!
To build the berms, a team of volunteers assisted BART, many from the local Three Brooks Conservation group and several corporate groups undertaking volunteering days with BART. The team were briefed how to harvest the brushwood for the berms from the banks of the river and surrounding woodland including learning how to undertake the work safely and using correct techniques. Areas of dense canopy were targeted to reduce over-shading and encourage growth of aquatic plants, which themselves provide valuable river habitat. Small branches were collected and tightly packed behind the main deflector arm. The woody deflectors were positioned in the channel in suitable locations to encourage natural processes to begin creating new good quality habitat. The structures were secured to the riverbed and banks to create a series of pinch points to diversify flow conditions and in channel habitat throughout the stretch of river.
When the flow in the brook becomes elevated, the berms are designed to over-top and could be underwater for months at a time. As the water flows quickly over and through them, they will capture fine sediment, eventually filling up and forming a low-level extension of the riverbank. As the brushwood berms capture silt and become self-sustaining, they will create a more diverse river cross-section, creating deeper pools where flow is quickest and scouring silt from the riverbed which providing valuable habitat for fish. Importantly they also create marginal habitat which has been removed from most of our rivers. The brushwood berms will eventually grow up with vegetation and provide valuable new habitat for mammals, birds and insects.
The material scoured from the newly formed pools will deposit downstream in slower flowing water, with material forming riffles and runs. The faster flowing sections generated by the restoration will provide suitable flow conditions for species such as water crowfoot and starwort and the silty structure will encourage species such as reeds to colonise along with the species present in the seed bank transported from upstream. The bankside tree roots exposed by scour will provide fantastic refuge for fry as will the backwater habitat created downstream of the structures.
It will take a few more years before the restoration sites settle into their new equilibrium, and the dry winter resulted in less morphological change than we’d have seen in a wet year. Nevertheless, the changes we’re seeing are encouraging and with more extreme weather patterns impacting our watercourses, the restoration will provide greater resilience to the pressures impacting our local rivers and the life that depend on them.
A big thank you to South Gloucestershire Council for granting permission to BART to carry out the works throughout the council’s land. Thank you to the Environment Agency for funding the project to date.
