Citizen science is a valuable source of data for environmental monitoring, but citizen science hasmany more benefits than just producing scientific data – it benefits society by informing effectivedecision-making, and benefits participants directly. Here we evaluated the benefits toparticipants in taking part in river water monitoring. This study was commissioned by Anglian Water as part of the multi-stakeholder CatchmentSystems Thinking Cooperative (CaSTCo) project. CaSTCo is creating a national standardisedframework for how citizen science can be integrated with professional monitoring to generateimpactful data and decisions for rivers. We explored the range of benefits to citizen scientist participants through three approaches: a review of evidence in the scientific literature; a survey of CaSTCo-trained volunteers; and a case study of the ‘photovoice’ methodology used by volunteers in Norfolk.Based on these, we make recommendations for future citizen science project design.