It said swimming is “not advised”, as storm overflows and misconnected sewers are discharging effluent into the water and run off from livestock farms upstream are also having an impact.
The Environment Agency took weekly samples from that stretch of water last summer to test for bacteria, specifically E coli and intestinal enterococci, after it was awarded bathing status in 2020.
The Ilkley Clean River Group said the data shows “it is not safe for people to paddle, play or swim” in the water “after rainfall when sewage pollution exceeds safe limits”.
Yorkshire Water said it is investing £13m in improving the water quality, implementing extra disinfection measures and a new scheme to reroute the sewage network in areas upstream of the bathing water site.
But Ben Roche, director of wastewater at Yorkshire Water, said the investment “will not guarantee an improvement in bathing water status”.
“Our modelling indicates pollution is entering the watercourses from a variety of sources, including misconnections and agricultural land which the river and its tributaries run through," he said.
“It is important other landowners and stakeholders take action to ensure water quality is improved in the future, with the ultimate aim of improving the bathing water classification.”
Martin Christmas, Environment Agency area environment manager, said the organisation will conduct “enhanced monitoring” of the River Wharfe in Ilkley to understand why the water quality is so poor.
He added: ”We are at the beginning of the journey at the site in Ilkley and we’re committed to working with the community, Yorkshire Water, local farmers, local authorities and other organisations to improve bathing water quality in the years to come.”
It comes a week after a committee of MPs found England’s rivers are filled with a “chemical cocktail” of sewage, agricultural waste and plastic, which is putting public health at risk, and just 14 per cent of rivers can currently claim to have good ecological status.
The other 18 bathing sites in Yorkshire, which are all on the coast between Withernsea in the south and Runswick Bay in the north, passed water quality standards. The Environment Agency said 16 have been rated as excellent or good.
The seven Yorkshire bathing waters classified as excellent are Flamborough South, Danes Dyke Flamborough, Scarborough North Bay, Cayton Bay, Reighton, Runswick Bay, and Whitby.
Emma Howard Boyd, chair of the Environment Agency, said: “With billions spent on seaside visits every year, we know good water quality helps coastal towns prosper.
“Twenty years of improvements in bathing water took targeted regulation and significant investment. While this is reflected in today’s results we must continue to work together to maintain this trend.
“We cannot afford to be complacent. Public confidence in water quality has faltered in recent years with new evidence of pollution incidents getting much needed attention as a result of some excellent campaigning.
“The polluter must pay. To restore trust, water companies, industry and farmers need to get the basics right or face legal action.”