Overview of Outfall Safari
One of the major threats to water quality in urban rivers is misconnected pipes. These send pollution into rivers via the surface water drainage system and compromise the biodiversity and amenity value of our waterways. In partnership with Thames Water, the Environment Agency (EA), fellow NGO’s and communities in Greater London, the Zoological Society of London (ZSL) has developed the Outfall Safari citizen science method for locating, assessing the impact of, and reporting on these polluted surface water outfalls (PSWOs).
In dry weather, trained volunteers walk the riverbanks with a mobile app that allows them to geolocate, photograph and assess outfalls for evidence of pollution. The assessment gives outfalls a score between zero and 20. The higher the score, the greater the visible impact of the outfall – more points indicate more pollution. Data are sent directly to a database for analysis and reporting to the regulator and water company.
Suitability for monitoring purpose
| Monitoring purpose category | Likely tier(s) What’s this? | Suitability |
| Engagement: Education and raising awareness | 0-1 | ✓ |
| Surveillance: Ecosystem health screening | 1 (2) | ✓ |
| Investigation: Helping to target further action | 2 (3) | ✓ |
| Evaluation: Assessing the impact of actions | 2 (3) | ✓ |
Data system: A national data system is in development. A beta version of the Cartographer app and database has been trialled in London, with full release expected for the 2026 Outfall Safari season. In the meantime, templates are provided in the download below.
Cost: Significant. Guide and data collection templates are available free of charge. The main costs are time for local coordinating groups to plan and run the scheme, train and coordinate volunteers, interpret and share data and engage with regulators and water companies.
Ease of use: There is a manual, volunteer guide, and training resources on the website. A short introductory film is also being developed to explain the why, what, and where of Outfall Safaris.
Quality control: Local coordinators are responsible for quality control.
Coverage / scale: The scheme is available nationally but has only been implemented in selected catchments and regions, including selected catchments in the Thames, Trent and Mersey basins and the south coast of England, as well as selected catchments in Scotland.
Health & safety: The guidance manual covers the health and safety aspects of the survey. Different groups take different approaches to guidance regarding entering the water versus only surveying from the bankside.
Method sponsor/ owner / primary user: Zoological Society of London
Weblink: https://catchmentbasedapproach.org/learn/outfall-safari-guide/
CaSTCo Investment: CaSTCo is reviewing and investing in this method to optimise data quality, ease of use and cost, including rolling out training, producing an introductory film, and investing in national data systems (Cartographer) for 2026.
Download: Outfall Safari Guidance
As of 2025, the updated Outfall Safari Guide is available, and CaSTCo partners continue to refine training and data systems in preparation for a national roll-out.