More than 6,000 fish have died in Essex, Norfolk and Suffolk during the recent heatwave, it has been confirmed.
The Environment Agency said the Rivers Can, Colne, Chelmer, Blackwater, Stour, Deben, Gipping, Brett, Thurne, Hickling Broad, Yare and Tud had been affected.
Fast-growing algae has been blamed for cutting oxygen levels and thunderstorms have also caused river pollution in some areas.
Teams have responded by using equipment to boost oxygen in the rivers.
A spokesman for the Environment Agency said: "At the weekend we received an unprecedented number of reports relating to fish in distress across East Anglia.
"Our teams responded by deploying aeration equipment and using hydrogen peroxide in affected stretches of river to boost oxygen levels."
A burst water main in Thetford also caused sewage to flow into the River Thet during the weekend, adding to the death toll.