The River Morda lies on the outskirts of Oswestry. Agency staff discovered the pollution during the night.
A warning remains in place to keep people and animals out of a Shropshire river after an "acidic pollutant" killed thousands of fish.
The Environment Agency said the water in the River Morda, south of Oswestry, was improving since the chemicals were discovered on Thursday.
The agency said it had not seen any more fish killed or in distress.
United Utilities said it was investigating an issue at its water treatment works in Oswestry.
The guidance to stay out of the water is a precaution, the agency tweeted., external
The Environment Agency set up an incident room at the scene
"Since we were notified of the incident, our officers have been on site to reduce the impacts on the environment... we're starting to see the situation improve," a spokesman said.
Chris Bainger, a fisheries officer with the agency, said it could take "a few years" for the river to recover.
Farmers have been warned to keep their livestock out of the river
- Published20 January 2017
