The River Spen could turn bright green tomorrow, as pollution tests with a harmless dye are carried out.
Environment experts will be using the dye to locate the source of a leak in the sewage system, which they say could be allowing pollution to flow into Sykes Beck and the river.
The pollution has caused fungus to grow on the river bed in recent weeks, leading to complaints from residents about the appearance and smell.
A tributary of the River Calder, the stretch of water runs through Cleckheaton, Liversedge, Heckmondwike and Ravensthorpe, Dewsbury.
Mark West, from the Environment Agency, which is carrying out the work, said: “We’ve had a lot of complaints in recent weeks about this issue, which has seen the beck and river visibly polluted, with quite an unpleasant odour.
“We’re keen to get to the bottom of it, and once we’ve identified the source we’ll be working with whoever is responsible to resolve it as quickly as possible.
“The dye we are using will help us to trace where the pollution is coming from, and how it flows downstream. It won’t harm wildlife, and will be diluted naturally by the river.”
Anyone with concerns about pollution in the River Spen or elsewhere, is encouraged to report it to the Environment Agency’s 24-hour incident hotline on 0800 807060.