Officers from Natural Resources Wales (NRW) are dealing with a silt pollution incident in Llangattock.
A stretch of water from the canal aqueduct, through to the Nant Onnau and the confluence of the River Usk near Crickhowell is affected and is heavily discoloured.
After taking swift action, NRW officers located the source of the pollution and stopped further silt from entering the water.
Water specialists are on site taking water samples to assess any potential damage to fish and wildlife, but presently no dead fish have been spotted.
Simon Worrall, Senior Environment Officer at NRW said:
“The pollution is now clearing but people may still notice a dirty brown colour in the river for a little while.
“People will also notice that the canal has been partially drained. This has been done intentionally and is due to ongoing maintenance works by a third party.
“Our investigation into cause of the pollution is ongoing.
“If anyone is concerned about further pollution in the area, they should contact our incident hotline on 0300 065 3000.”