Paint spilt in a motorway crash last week has made its way into watercourses in Kirklees.
The Environment Agency said it was investigating pollution in rivers and streams in the area, after the major spillage on the M606 on Friday.
Some 12,000 litres of paint was spilt between junction 2 at the Euroway Trading Centre and junction 3 at Odsal following a collision between a lorry and a car, the Highways Agency said.
Containers from the lorry were also strewn across a large section of the road, which was closed until Sunday.
A specialist clear-up operation was carried out to remove the paint and the southbound carriageway also had to be resurfaced.
Paint has been spotted in nearby waterways since the incident.
Local resident Kath Larkin, who regularly walks along the River Spen and around Spen Bottoms, said she was concerned to see the water coloured white.
She said: “The River Spen is still running the colour of milk and polluting the environment and ecology along its wake.
“I’m very concerned. I don’t think the River Spen is not the cleanest of rivers under normal circumstances but over the past few years there has been progress made to try and clean it up..
“The Environment Agency do a great job. And the water has been clean enough to attract Kingfishers, Swans, Cranes and Herons, not to mention the flora and other wildlife that lives in the area.
“It has been said it was emulsion paint but it begs the question, of the ease it got into the waterways. Other substances that could be washed off the Euroway could equally get into the tributaries and then the River Spen and beyond.”
The River Spen, known locally as Spen Beck, runs close to the M606, past Oakenshaw, Cleckheaton, Liversedge and Heckmondwike and into the River Calder at Ravensthorpe.
The Environment Agency said it was continuing to investigate following the paint spillage.
A spokesman said: “Our officers have been out to carry out assessments of watercourses downstream, and we have taken water samples from some locations for laboratory analysis.
“The becks around the motorway are now running clear. Some of the pollutant is being washed through the system and can be seen in the Calder, although no significant impacts on wildlife have been reported.
“Our officers are out again this week to carry out further assessments.
“Anyone who spots any problems as a result of pollution, such as dead fish, are urged to report the matter to our incident hotline on 0800 807060 so we can investigate.”