A large amount of fish found dead in the lower Lea river and surrounding waterways were starved of oxygen.
The fish were spotted at various locations along the river, including Springfield Marina and Lea Bridge Weir pool in Walthamstow.
Recent hot weather reduced oxygen levels in the river, the Environment Agency said.
And storms on Monday night, which swept toxins from nearby roads into the river, meant oxygen levels were further depeleted.
Fisheries officers from the agency worked through the night carrying out work to return levels to normal by running water through locks and pumping oxygen into the water at Three Mills Boatyard.
The agency reported that oxygen levels have risen around 25 per cent in the area, creating a refuge for fish including bream, perch, pike and roach
A spokesman said: "We received reports of fish that have died in the Lower Lea river this morning.
"Our monitoring equipment showed oxygen levels had dropped.
"Environment officers are at the scene to monitor peroxide and oxygen levels."
Anyone who sees fish in distress is asked to contact the Environment Agency incident hotline on 0800 807060.