The death of "dozens" of fish in the River Lea last week is under investigation by the Environment Agency.
Following heavy rain on Tuesda, June 20, the agency, which has a fisheries team, responded to reports of dying fish and eels in the Lea Bridge area.
The Environment Agency believes the deaths were caused by low levels of dissolved oxygen in the river and deployed aerators next to the bridge to "create a safe haven".
Local resident David Ogle, who visited on June 21, told the Local Democracy Reporting Service he saw "dozens" of dead fish in a 100-yard area south of Lea Bridge.
He added: "This never used to happen, and certainly not with the frequency our waterways are being destroyed right now."
Yesterday's starkest images were dead fish close to the oxygenators in the River Lea.
— London Waterkeeper (@LDNWaterkeeper) June 22, 2023
Dispiriting for frontline Environment Agency staff when, despite your best efforts, countless fish are killed by stormwater pollution.
We need systemic solutions, not 'sticking plasters'. pic.twitter.com/yMHgD9rN7Z
A spokesperson for the Environment Agency said that although its officers are still investigating, they believe the cause of the deaths was a combination of high temperatures and "low atmospheric pressure" caused by the thunderstorms.
However, they added: "Our officers are investigating to rule out any other causes such as pollution.
"Our officers are continuing to monitor the incident and we will deploy additional resources as necessary.
"It is important that members of the public who spot fish in distress report it to our 24/7 incident hotline 0800 80 70 60 so that we can investigate."
But Theo Thomas, who founded the London branch of Waterkeeper campaign group, has offered an alternative theory.
In a series of social media posts last week he suggested that "detritus" built up on London's streets and other hard surfaces may have been carried into the river by the rain.
He said: "In clear water the sun reaches plants and they produce lots of oxygen.
"But on Tuesday detritus that had built up on 80 square miles of roads etc was washed into the river.
"The sun was blocked and bacteria fed on the muck, oxygen fell."
Monitoring data showing the quality of the river water in the Walthamstow Marshes area on the day of the rains shows that oxygen levels plummeted at the same time as turbidity - or cloudiness - increased.
Mr Thomas told the LDRS the River Lea "teeters on the brink" every summer due to the way "industrial amounts" of pollution on roads wash straight into rivers.
He added: "Instead of creating areas to slow down and filter this water, we've increasingly paved so much of London."
Theo said other issues the river faces are "high levels of nutrients" that cause large stretches of green duckweed to cover the surface and block other plants from receiving sunlight.
He believes the river has higher nutrient levels due to the amount of treated effluent released from Deephams Sewage Works, in Edmonton.
The Lee Valley Park Authority and Waltham Forest Council have been contacted for comment by LDRS.
