Outfall Safaris

The South East Rivers Trust completes regular Outfall Safaris on urban rivers in South London to help improve water quality.

The Outfall Safari is a powerful citizen science scheme that systematically surveys the entire length of a river to identify and locate polluting outfalls or pipes. This allows the pollution to be reported to the local water company and the Environment Agency so that it can be tackled. Without this survey, many polluting outfalls would still be unknown.


  • Trained citizen scientists


  • Km of river surveyed


  • Polluted outfalls reported


  • Started in


What is a misconnection?

Many areas in South East England are serviced by two separate drainage systems. One takes foul water from our homes to the sewage treatment works for cleaning, the other allows rainwater to flow straight to the nearest river.

Sometimes appliances can end up misconnected – meaning they have been incorrectly plumbed into the surface water drainage system. This means that untreated foul water is flowing straight into our rivers.

Demonstrating misconnected plumbing © Connect Right

Are misconnections a problem?

Misconnected outfalls often go unnoticed and can become a chronic source of pollution along our rivers.

It is estimated that across London’s 600km of rivers there could be as many as 1,483 misconnected outfalls caused by thousands of appliances being plumbed incorrectly. With more households than ever choosing to renovate rather than relocate, the chance of more misconnections occurring is high.

Polluted outfall © South East Rivers Trust

What’s an Outfall Safari?

The Outfall Safari method was developed by the Zoological Society of London (ZSL) in 2016 to survey London’s rivers for the presence of misconnected outfalls. Before this, there was no method for surveying and prioritising polluting outfalls and the scale of the issue was completely unknown. It has been so successful that Outfall Safaris have now been carried out on rivers across the country.

Citizen scientists are trained in how to score the pollution levels at all the outfalls they come across whilst walking along each river. Outfalls scoring more than four have moderate pollution with any outfall scoring 10 or more indicating a major source of pollution.

Outfall safari © South East Rivers Trust

The results

One of the first rivers to be surveyed using the Outfall Safari method was the Hogsmill back in 2016. Since we have also conducted surveys on the Beverley Brook and Wandle. The results were shocking:

  • Hogsmill – 23% of outfalls showed signs of pollution
  • Beverley Brook – 29% of outfalls showed signs of pollution
  • Wandle – 12% of outfalls showed signs of pollution

The Hogsmill was resurveyed in 2021 and the number of polluting outfalls reported fell to 49 (from 63 in 2016).

What happens with the data?

The results of each Outfall Safari are sent to the Environment Agency and Thames Water. Each outfall is connected to multiple properties, sometimes numbering in the thousands. It is the job of Thames Water to trace these misconnections back to the misconnected appliances so that the homeowner can rectify the issue.

For example, the original safari on the Hogsmill led to Thames Water opening many new investigations into previously unreported sources of pollution. Since then Thames Water has traced and helped homeowners to rectify more than 400 misconnected appliances.

Without the data from our Outfall Safari volunteers, many of these polluted outfalls would have continued polluting our rivers unnoticed and untreated.

How can you help?

Look out for these signs of pollution around outfalls on your local river:

  • Cloudy or milk discoloured water
  • Smell of sewage
  • Grey slimy growth of sewage fungus on the face of the outfall.

If you spot any of these call the Environment Agency on 0800 80 70 60 with photos of your sighting.

Your property could be misconnected. Visit the Connect Right website to learn how to check.

Check my house for a misconnection

Outfall safari volunteering © South East Rivers Trust

Thanks to our supporters

The Prince of Wales Charitable Fund
Thames Water
The Rivers Trust