A leak of china clay effluent into a river in Cornwall is being investigated by the Environment Agency.
The pollution changed the colour of the water
Officers from the agency were called at about 0800 GMT on Friday to a six-kilometre (3.75-mile) stretch of the River Fal near St Stephen.
The water turned a cloudy milky white colour as a result of the discharge, which was later stopped.
The clay company Imerys is believed to be responsible for the leak. The company said it regretted what happened and its staff were surveying a ruptured pipe.
Samples are being taken from the water to monitor the impact the leak may have on local wildlife.
Phil Christie from the Environment Agency said it could have long-term effects in the area, including the stopping of fish eggs hatching.
He said: "The clay can clog up gravels where fish do spawn. As this can happen in April, it's not a good time to have this sort of pollution occurring."
A spokesman for Imerys, Ivor Bowditch, said it regretted what had happened.
He said: "We never take any of these incidents lightly and we take the advice of our colleagues in the enforcement agencies, such as the Environment Agency.
"People can be reassured that Imerys' management at the most senior level is looking at the incident.
"It may be another one where a belt-and-braces approach has to be put in place on the system."
The Environment Agency said that the recent wet weather and fast river flow should help flush the china clay out to sea.