Thames Water, which operates the local drainage network, said it was “urgently investigating the pollution incident” but its cause is yet to be determined.
The company has deployed aeration at key locations along the River Pool to increase dissolved oxygen levels and help mitigate further impacts.
A Thames Water spokesperson said tests were currently being carried out and it “understands the concerns residents will have”.
Beckenham Town and Copers Cope councillor, Steven Jefferies, praised the EA for reacting swiftly to the “tragic incident”.
He said: “Beckenham’s rivers play a crucial role in supporting the biodiversity of our area, and bring great benefits to the health and wellbeing of residents living in our community.”
