Several hundred fish have died after storm water was accidently released into the River Yare near Norwich.
Anglian Water confirmed there was a "discharge" at its nearby pumping station after an electrical storm caused a power outage on Thursday night.
Martin Kentish, who lives in Trowse, said he was woken by a foul smell on Friday morning after falling asleep on his boat.
He said he saw lots of dead fish in the water, along with eels trying to escape onto the river bank.
Mr Kentish said: "There were all these fish on their backs and gasping for air, and hundreds of eels climbing out of the river."
The Environment Agency said officers found "several hundred dead fish, as well as fish clearly showing signs of distress" in the river.
It said there was low oxygen in the water and ammonia levels were higher than normal background levels, which suggested pollution.
Mr Kentish said he had seem similar incidents in the past - usually in the summer months after a heavy downpour - where the river turned "white" and fish died.
The section of affected river runs next to Whitlingham Lane in Trowse before widening as it gets to Thorpe St Andrew.
The Environment Agency said it used aeration equipment to raise oxygen levels in the water - work that continued until the morning on Saturday, July 27.
"Water samples were taken and have been sent to our laboratory for analysis. We are waiting on the results," the agency spokesperson said.
"This incident follows electrical storms and a heavy downpour of rain overnight on Thursday and we believe this may have caused polluting matter to enter the River Yare."
An Anglian Water spokesperson said: "During periods of hot dry weather, it is not uncommon to see reduced oxygen levels in our rivers which means they become a less welcoming environment for wildlife and fish in particular.
"In addition to these reduced oxygen levels, an electrical storm last Thursday night caused a power outage at our pumping station near in Trowse. This resulted in a discharge into the River Yare."
The spokesperson said most of the water would have been run-off from the roads or storm water, containing "an amount of diluted sewage".
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