THE death of dozens of fish found floating in the river Ching may have been caused by a sudden, heavy rainstorm, it has emerged.

A dog walker spotted the roaches and sticklebacks in the water on Tuesday night.

Yesterday an inspector from the Environment Agency took samples from the river, checking oxygen, ammonia and pH levels.

The agency conculded the most likely cause of death was a short thunderstorm or a burst of heavy rain that pushed debris and sewage into the river, which already had low levels of oxygen because of the dry weather.

The inspector also checked the lake at Highams Park which runs into the Ching brook.

A spokesman said about 20 to 30 dead fish were found gathered in the lower part of the river.

He added the likely cause was pollution within the last 48 hours, which travelled down the Ching brook, along the stream and trapped the fish.

Alex Chown, team leader for the Lower Lee Valley at the Environment Agency, said: "Something has come down the Ching. The fish died because of a lack of oxygen.

"A thunderstorm event was likely which caused a flushing of the drainage system in the Ching area very quickly possibly from Whitehead Road. It resulted in organic matter going into the water, possibly highway run off and sewage.

"It has been quite a dry summer so the water levels have been low. So when it rained and all the run off went into the river it was like sludge and the fish couldn't cope. As a result the fish got stressed and trapped in poor quality water."

But he added that the water is not harmful to people or animals now.

"Any river has the potential for bacteria, especially in an urban catchment."

"The Environment Agency will be removing the dead fish today so they do not pollute the water further."

He added investigators are still trying to work out if the problem is in the river or the lake.

The water quality in the lake is lower than they expected and the oxygen is low, so it will be monitored over the coming days.

They are asking members of the public to contact them if they see any changes in the river or lake on their 24-hour helpline 0800 80 70 60.

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