Courier photographer Charles Round has taken this video showing the River Calder, which has turned orange.
Hebden Bridge resident Barry Greenwood said: “It looked as if the river had been polluted with some kind of coloured substance. At Todmorden it was swirling and as it poured down the river system it was mixing with the water itself and changing into a completely different light brown colour which looked like straw.
“I have never seen anything like this in my 60 years of living in the Calder Valley,”
Environment Agency officials are now investigating what has happened.
Readers have also been sending us pictures of the river. If you have any, email them to email@example.com.