On his first long trip down Hat Hill Creek Canyon in the Blue Mountains, David Noble could barely cope with the putrid stink from partly treated sewage pouring from a plant serving the nearby town of Blackheath.

At one point, the polluted torrents had churned up a wall of foam reaching some three metres high and 20 metres long, prompting Mr Noble and his friends to use an inflated airbed to push their way through.

"We knew the water was polluted, but we didn't realise it was that bad," the retired science teacher and canyon explorer said. "We were amazed to find so much foam."

Between his first visits in the mid-70s to one this week, Mr Noble has witnessed the remarkable recovery of the waterway.