I received a phone call this evening informing me that the Irwell had turned white downstream of Bury. So I went up to see how bad it actually was.
The foaming below Warth Fold Weir was as bad as I've ever seen, but thankfully I couldn't see any fish in distress.
Upstream of the weir, the river looked normal, and there was no foaming at the mini weir underneath the Metrolink bridge making me think that the pollution may have been coming from the adjacent industrial park. The EA turned up to investigate just as I was leaving so hopefully they will get to the bottom of it.