How will Biodiversity Net Gain restore nature in England’s rivers?
The UK contains over 200,000 km [1] of rivers and streams, forming intricate sprawling networks which permeate and connect habitats throughout the landscape, from turbulent mountain streams to sluggish, meandering lowland rivers. These waterways are home to highly diverse ecosystems, including internationally important habitats such as chalk streams—of which England supports 85% of the global total [2]. Despite covering less than 1% of the Earth’s surface, freshwater habitats including rivers are biodiversity hotspots, supporting over 10% of all plants and animals globally [3]. However, these critical habitats are under severe pressure from human activities, since the 1970’s there has been an 85% decline in freshwater species [4], far outstripping species declines in terrestrial and marine systems. Focusing on England’s rivers, its not hard to see how this has unfolded. These rivers suffer from heavy pollution, water abstraction, and physical modification, with only 15% of them meeting “good ecological status” according to The Rivers Trust’s 2024 State of Our Rivers report [5].
The introduction of Biodiversity Net Gain (BNG), underpinned by the Environment Act 2021, represents a significant opportunity for restoring rivers and reversing these alarming biodiversity losses. But what part will BNG play in addressing the plethora of pressures that our rivers face, and how can it drive meaningful restoration to river ecosystems?
The Legacy of the Water Framework Directive
For over two decades, the Water Framework Directive (WFD) has been central to European river protection and restoration. It sets ambitious goals for all water bodies to achieve “good ecological status.” However, ecological monitoring evidences how most rivers continue to fall short of this target. This presents across biological communities, such as invertebrate communities with low species richness and deficient in pollution sensitive species; macrophyte (aquatic plant) communities dominated by filamentous algae and species which thrive in nutrient rich waters; and fish populations with poor diversity and absence of flag ship migratory species such as Atlantic salmon and European eel. Key reasons include agricultural pollution (affecting 40% of rivers), water company discharges (35%), and physical modifications to rivers (30%). Furthermore, over-abstraction, which harms natural flows vital to ecosystems, impacts 25% of rivers. The WFD provides a fairly comprehensive evaluation of the facets of rivers which determine their capacity to support diverse, functioning ecosystems, including assessments of water quality, hydromorphology and biological communities. However, it doesn’t provide the full picture and recent research has identified nearly 500 chemicals [6], many unmonitored, in England’s rivers, adding further complexity to the challenges. Pharmaceuticals, microplastics, and industrial chemicals are increasingly degrading water quality, with many of these pollutants not fully addressed by current regulations.
Enter Biodiversity Net Gain (BNG)
Undertaking a MoRPh survey on the River Calder in West Yorkshire
BNG, as mandated by the Environment Act 2021, aims to ensure that development projects leave nature in a better state than before by delivering at least a 10% net gain in biodiversity. This approach uses habitats as a proxy for biodiversity with net gains in biodiversity delivered through the creation and enhancement of habitats. In terrestrial environments this is underpinned by plant community assessments to classify habitats, determine condition and enable calculation of biodiversity value using the statutory biodiversity metric. Rivers are however considered separately to terrestrial area (e.g woodlands) and linear habitats (e.g hedgerows). Rivers are accounted for within the rivers and streams module of the statutory biodiversity metric, and whilst the principle of using habitat as a proxy for biodiversity remains, the approach differs with assessment including determining the extent of artificial encroachment into the riparian zone (habitat within 10m of the crest of the river bank) and the watercourse channel itself. As well as river condition which utilises the Modular River Physical Survey (MoRPh) survey method. These assessments alongside other parameters including river type and strategic significance inform the biodiversity unit value of a watercourse.
Focus on physical restoration
As you would expect from the title, the MoRPh method focuses on the physical features (natural and artificial) and vegetation features of a watercourse, assessing the channel, bank face and banktop (land within 10m of the crest of the bank). Represented across 32 ‘condition indicators’ this provides a detailed appraisal of the physical characteristics of the watercourse. However, only one of the 32 condition indicators ‘channel bed filamentous algae extent’ could be considered a measure of water quality and only one ‘channel bed hydraulic feature richness’ a measure of flow within the river. The remaining 30 condition criteria are therefore focused on the physical characteristics of the watercourse.
While a river’s physical characteristics can serve as a useful proxy for biodiversity, this approach has notable limitations. If natural processes within a river are uninhibited by modifications such as channel reinforcements, weirs and over-deepening then this can enable diverse habitats to be created and maintained. Diverse habitats can provide a wide range of ecological niches for organisms such as aquatic invertebrates to occupy whilst providing all the various habitat needed for different life-stages of more complex organisms such as salmonid fish. But, this doesn’t account for the significant and extensive impact of poor water quality and to slightly lesser extent low flows; pressures which the WFD already evidences are major, if not the most significant factors in riverine biodiversity.
An inherent focus on the physical environment means that physical interventions will be most effective in moving the dial on river condition and yielding the watercourse biodiversity units needed to achieve statutory BNG requirements. It should be acknowledged however that pressures which impact water quality and quantity may influence hydromorphological processes which are then exhibited in the physical characteristics of a river. Nevertheless, this indicates how the approach to assessing biodiversity in watercourses could on the face of it lead to a bias towards enhancing the physical characteristics of a river in favour of measures which directly tackle issues with water quality for example.
Fowlea Brook in Stoke-on-Trent. Example of a highly modified stream with completely artificial channel bed and banks.
The emphasis on rivers’ physical characteristics in BNG aligns with the kinds of impacts most developments cause, such as habitat encroachment and channel modifications. Consequently, BNG encourages developers to adopt mitigation strategies that focus on the physical environment, such as avoiding development within 10 meters of a watercourse, or opting for softer engineering approaches.
This focus on physical restoration can manifest in several practical actions. For instance, developers might enhance rivers by stabilizing eroding banks, restoring natural river morphology, or adding features such as gravel bars and marginal vegetation. These changes can improve the ecological condition of rivers by restoring habitats for aquatic species and promoting natural processes. BNG also offers an excellent opportunity to reverse historic modification to rivers from over 36,000 river obstacles [7] including weirs and culverts, impacting longitudinal connectivity in rivers, fragmenting habitats and preventing fish movement and migration.
In some cases, these actions are more feasible for developers to implement than tackling broader issues like diffuse agricultural pollution or over-abstraction. Restoring the physical environment is often within the scope and capability of development projects, whereas addressing issues like water quality requires systemic changes and coordination across sectors, such as agriculture and water management.
Nevertheless, physical restoration is not a panacea for the broader challenges facing rivers. While it can restore some of the natural structure and function of river ecosystems, it often overlooks root causes of poor river health, such as nutrient loading from agricultural runoff or untreated wastewater. Furthermore, hydrological pressures—particularly over-abstraction—remain largely unaddressed by the physical measures that BNG incentivizes. Without tackling these fundamental issues, many BNG efforts risk being limited to surface-level improvements that fail to resolve the deeper problems preventing rivers from achieving good ecological status and providing real improvement to biodiversity.
The Potential of Local Nature Recovery Strategies (LNRS)
This is where Local Nature Recovery Strategies (LNRS) could play a pivotal role. Developed at the local authority level, LNRS provide a strategic framework for nature recovery across landscapes, identifying priority areas for restoration and coordinating actions to maximise biodiversity gains. By integrating BNG with LNRS, river restoration efforts could become more holistic—enabling catchment-based approaches that address not only physical modifications but also the underlying issues of water quality and flow regulation.
Crucially, LNRS also offer a mechanism to tackle diffuse pollution—one of the most pervasive and complex pressures on river ecosystems. By highlighting where land-use changes such as riparian buffer zones, wetland creation or regenerative farming practices could help reduce nutrient and sediment runoff, LNRS can guide nature-based solutions that benefit both biodiversity and water quality. When aligned with other initiatives like Environmental Land Management schemes, catchment partnerships and water company investment plans, LNRS could help unlock more systemic improvements across entire catchments.
The effectiveness of LNRS in this role, however, will depend on how robustly they are developed. Strong evidence bases, adequate resourcing, and close coordination with stakeholders—particularly landowners and farmers—will be critical. If delivered well, LNRS could ensure that BNG investments are targeted in the right places, tackling the root causes of ecological decline and not just the symptoms. Rather than piecemeal improvements, this would support cohesive, long-term recovery of England’s rivers and the ecosystems they sustain.
The recently deculverted River Medlock in Manchester
Conclusion
So, how will BNG restore nature in England’s rivers? This remains to be seen. Physical restoration alone cannot fully address the complex, interrelated challenges facing our rivers. To fully restore our rivers, we must look beyond the immediate improvements and work towards addressing the deeper systemic challenges facing these vital ecosystems. BNG’s success in contributing to this will likely be determined by how effective LNRS are at directing efforts and investment towards the pressures which are most significant in determining river health and biodiversity.
Despite this, I can’t help but feel extremely hopeful about the opportunities BNG presents for restoring our rivers. As an ecologist, I often find myself quoting that “perfection shouldn’t be the enemy of the good”, and physical restoration is a critical piece of the puzzle for unlocking nature recovery in river systems. The potential for BNG to enhance riverine habitats and restore some of their natural functions is significant—while also building greater resilience within these ecosystems to withstand pressures such as pollution and over-abstraction.
References
[1] UK River and Flow Regimes – Centre for Hydrology and Ecology
[2] Chalk Rivers – The Wildlife Trust
[3] Freshwater Biodiversity – WWF
[4] Living Planet Report 2024 – WWF
[5] State of our Rivers Report 2024 – The Rivers Trust
[6] Almost 500 chemicals found in England’s rivers and groundwater – The Guardian
[7] The River Obstacles Initiative
About the Author
Simon Fleming MCIEEM is a Principal Aquatic Ecologist within the Nature Services team at Arup. He has particular interest in river and wetland restoration to support nature’s recovery in freshwater systems.