It's one of the picture-postcard scenes of East Anglia - but new figures suggest that almost half of the water flowing through Cambridge's Sheep's Green may in fact be raw sewage.
Sewage effluent is estimated to comprise 40% of the river flow through the picturesque city-centre spot near King's College, according to research done by the Liberal Democrats.
The findings have sparked fresh criticism of Anglian Water, which was ranked worst for failures in monitoring how much sewage it discharges - with nearly half of all its discharges (49%) not measured.
The figures - which the company says are out of date - show that it discharged untreated sewage for at least 1,689 hours in 2021, equivalent to more than four-and-a-half hours every day.
Environmental journalist Terry Macalister has compared the damage being done to the River Cam to vandalism of Cambridge's iconic King's College Chapel.
He told ITV News Anglia that on one occasion while swimming in the river he thought he could smell sewage, and said that clean-up action needs to be immediate.
"I don't know what I'm swimming in and I don't know to what extent I'm endangering my health," he said.
"People have been sick from swimming in the river. These are chalk streams and they are very, very valuable in biodiversity terms.
"King’s College Chapel - which we all revere as a cultural icon for humans - we’d never let that be damaged in any way. You could argue that the river in its biodiversity terms is the equivalent of King’s College Chapel and yet we’re abusing it and that needs to stop.”
Campaigners such as Feargal Sharkey, the former lead singer of The Undertones, who lives near the River Lee in Hertfordshire, say not enough is being done to tackle pollution.
"The whole thing is quite simply an organised rip-off, it's a crime being perpetrated upon all bill holders, that should fill all of us with an almighty righteous anger and fury," he said.
"The River Cam does not have the legal protection but suffers the same impact, over-abstraction for water supply and what remains of it is now being pumped full of sewage.
"It's a disgrace and the water companies now need to be held accountable for their actions and the impact their activities have had on the environment."
Anglian Water said the figures being discussed by the Lib Dems were historic and that the company was now monitoring nearly 75% of all discharges.
Company spokesperson Regan Harris told ITV News Anglia the number of storm spills was declining and that the company was investing money to deal with the issue.
“Monitoring is obviously very important, but it’s actually equally, if not more important, that we’re tackling the problem. So we’re investing £200m over a five-year period to basically stop storm spills or at least significantly reduce them.”
She said sewage was not the only pollution in the river, pointing to other causes, including from animals swimming in the water.
"We know that there is work to do there, we recognise we don't want our assets to be the reason for unhealthy rivers and seas, but there are lots of things in the natural environment that can also cause poor water quality for rivers - from ducks and other animals swimming in rivers, to agricultural run off, urban run off. There's lots of things people need to be mindful of when swimming."
The government has previously said it wanted to reduce discharges into bathing waters by 70% by 2035, and eliminate 160,000 incidents of sewage overflows by 2040.
Water minister Steve Double said earlier this week: "We are the first government to take action to tackle sewage overflows. Work on tackling sewage overflows continues at pace."
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