The River Morda lies on the outskirts of Oswestry. Agency staff discovered the pollution during the night.
Thousands of fish are thought to have died in a Shropshire river.
The Environment Agency (EA) says it is dealing with "significant fish mortality" after an "acidic pollutant" was found in the River Morda, south of Oswestry.
The pollution was discovered during the night. Agency staff have flooded the river in an attempt to flush out the chemicals.
Pet owners and farmers are warned to keep their animals out of the river.
'Completely devoid of life'
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The EA has set up an incident room at the scene.
Chris Bainger, a fisheries officer with the EA, said it could take "a few years" for the river to recover, but said the area had been affected by pollution previously and "bounced back" well.
"Our role is to make sure we minimise these impacts and stop these pollutions, and hopefully we will do," he said.
Farmers have been warned to keep their livestock out of the river
A section of the River Morda, south of Oswestry, is "completely devoid of life", a local fishery says.
Giles Cochrane, who manages the Weston Pools, said the fishery itself was unaffected, but pollution had killed not just fish in the river, but other wildlife as well.
"It's absolutely devastated the brook. There's no living organism whatsoever.
"The trout population has been wiped out - the fry, next year's fry, the food on which they live - the shrimp, the caddisfly."
United Utilities said there has been an issue at their water treatment works in Oswestry and were investigating.
A spokesperson added: "We are working closely with the Environment Agency to resolve this."
