This paper explores the mainstreaming and upscaling of Nature-Based Solutions (NbS) in Northwest Europe, focusing on experiences from the INTERREG ResiRiver project. It highlights key concepts such as NbS, mainstreaming, and upscaling, using case studies from nine pilot projects across Belgium, France, Germany, Ireland, and the Netherlands. The study applies the IUCN Global Standard for NbS to establish a baseline, emphasizing the importance of iterative assessments and co-benefit valuation. Challenges in governance, economic feasibility, and adaptive management are discussed. The paper concludes by advocating for robust communication strategies and international collaboration to accelerate NbS adoption.

River managers today are faced with the challenge of adapting to climate change while also having to sustainably secure all important functions in a healthy river system for society. Nature-based Solutions (NbS) have proven themselves effective across a multitude of contexts; providing integrative approaches for river restoration, conservation and sustainable management, ensuring both climate change adaptation and contribute to climate change mitigation and biodiversity recovery for generations to come. NbS are multi-faceted and more importantly, they are effective when it comes to addressing complex societal challenges (e.g. reducing flood risk, increasing natural values and biodiversity, ecosystem services and human well-being), as they provide a novel, integrative and coherent approach. Despite the significant and rapidly growing base of scientific evidence regarding the effectiveness of NbS in riverine systems management, the actual uptake and application of NbS on a larger (EU) scale is still in its early phase. From where we stand today, a major barrier to the wider uptake and application of NbS in riverine systems remains (a) our limited experience in scaling solutions beyond their local contexts (so called ‘Upscaling’), and (b) make Nbs as a standard work practice within water management organisations throughout North-West Europe (so called ‘Mainstreaming’). Also, our lack of standardised methods for quantitative assessment and monitoring of ecosystem services and benefits related to NbS hinders replication and application at a wider scale.