Introducing River of the Month – The River Mole
Thank you to Nigel Bond and the River Mole River Watch for much of the content provided in this blog
Introducing River of the Month!
Every month, we’ll be spotlighting a different river, sharing its wonders, its challenges, and the incredible people fighting to protect it. First up is…
The River Mole.
The River Mole is a tributary of the River Thames in southern England. It is a waterway steeped in history, once powering dozens of mills and referenced in literature for its unusual underground flows. Today, it remains ecologically important, supporting diverse fish species such as brown trout, chub and stone loach. However, its biodiversity is under threat from persistent pollution caused by sewage, urban runoff, and agricultural practices, with most sections rated only Moderate or Poor in water quality.
Did you know?
The name ‘Mole’ likely derived from the Latin mola, meaning “mill,” reflecting the numerous mills that once operated along its course.
The river has captured the imagination of several authors and poets and features in “The Faerie Queene” by Edmund Spencer, the “Poly-Olbion” by Michael Drayton and “Wildflowers” by Robert Bloomfield.
Key Facts:
- Length: 80 km
- Catchment Area: 512 km²
- Counties: 2 (West Sussex and Surrey)
- Fish Biodiversity: High – 13 species
- Water Quality Tests (poor/bad): 51%
- Sewage discharges in 2024: 254
- Sewage hours in 2024: Over 13,000 hours
- Ecological Status: Poor
The River Guardians
The River Mole River Watch is a community group founded in 2022 to tackle pollution in the River Mole. Frustrated by worsening water quality, locals came together to protect and restore their river, working with councils, charities, and residents.
River Mole River Watch organises protest walks like March the Mole, runs citizen science monitoring, and empowers local people to become river guardians.
Key Challenges
The Mole faces many challenges. It is a “flashy” river, with river levels rising and falling rapidly in response to rain. In winter in places it is prone to flood, in summer many tributaries become mere trickles and in extreme drought (e.g. 2022) the main river can run dry for several miles. 11 Thames Water sewage treatment works discharge into the Mole and tributaries, contributing a substantial proportion of the total flow particularly in summer. Storm overflows discharged untreated sewage for a total of 13,000 hours in 2024.
Summer conditions are particularly stressful for wildlife with high concentrations of pollutants and high water temperatures. There is a large and growing population with housing developments adding load on water supply and wastewater infrastructure. The river flows under and around Gatwick Airport. The M25 runs across the catchment with the runoff from this and other major roads adding pollutant load on the river system. Domestic misconnections are a particular problem for smaller tributaries.
Much of the catchment is rural with agriculture contributing pollution from legacy phosphate, chemicals, animal sewage and soil loss. Invasive species including floating pennywort, himalayan balsam, american mink, and red-signal crayfish are widespread. Climate change increases the stressors and also highlights the importance of the river as a natural corridor for wildlife.
Key Solutions
- Fix & Upgrade Wastewater Treatment – Properly maintain existing sewage facilities, increase capacity, install better filtration (tertiary treatment), and set strict discharge limits, especially for summer.
- Better Management of Road Runoff – Stop pollution from roads and infrastructure before it hits the river.
- Nature-Based Solutions – Use reedbeds to treat sewage, build sustainable drainage systems in new housing, and promote farming that protects rivers (catchment-sensitive farming).
- Natural Flood Management – Restore wetlands, create space for water, and slow the flow to prevent flash floods.
- Restore River Health – Re-meander rivers, re-seed gravel beds, remove barriers to fish, and tackle invasive species.
- Tackle Hidden Pollutants – From pet insecticides to sewer misconnections, more action is needed to stop hidden pollution sources.
Find out more about the River Mole River Watch
If you are in the Sussex or Surrey area and would like to help out or join the group, you can find out more here.
If you found this inspiring and would like to find out how you can protect your local river by joining or creating your own river community group, visit our River Rescue Kit.
