The Environment Agency Officers have been carrying out an assessment of the Beverley Brook this week following a number of reports of over 100 dead fish.
Their staff are working with Thames Water Utilities to try and find the source of pollution.
It is believed the cause is due to the low dissolved oxygen levels in the watercourse, following on from the heatwave.
Continued monitoring is showing the water quality is now recovering but if you see fish in distress – please call the EA incident hotline on 0800 80 70 60.