It’s a beautiful day on the river Cam in Cambridge. As the sun sparkles on the water at Jesus Green lock, tourists line up for ice-cream and prepare to take a punt around the university’s most celebrated colleges. Few notice how pathetic the flow of water is over the lock.

It is a clear sign that this ancient waterway is faring badly, says Stephen Tomkins, emeritus fellow and former head of the science faculty at Homerton College. “That little bit running through here is the total flow from the whole of south Cambridgeshire,” he says, pointing at an unimpressive trickle. There just is not enough water, he adds: “The river Cam is drying up.”

Tomkins chairs the Cam Valley Forum environmental group, which earlier this month sounded the alarm about the Cam with a manifesto that warns that the river can no longer function properly.

“It is just not just ‘the weather’,” the manifesto states. “Over-abstraction of water from the ground is the main reason. We pump too much water out of the chalk for our domestic supplies and, to a smaller extent, for agriculture.”

A report by the Environment Agency last month also revealed the river’s flow is now exceptionally low, just 33% of its long-term average.

The chalk streams that feed the Cam with crystal-clear groundwater, filtered naturally by the local chalk hills, are dying because of this over-abstraction, says Tomkins. In total, 65% of the groundwater is pumped out of the chalk for drinking water supplies – and a further 20% is used for augmentation by the water companies to “disguise the problem”.

Stephen Tomkins; “The river Cam is drying up.”

“They have to put water back into the headwaters of the streams in the summer to keep those streams running, because they have taken so much water out of the chalk,” he says.

This manoeuvre hides how low the water table has sunk, says Ruth Hawksley, a Wildlife Trust officer in Cambridgeshire. “If they didn’t pump water in, those streams would be dry already. They are very special, fragile habitats which are quite unique to England, and they’re currently at the mercy of a pump and possibly a power cut.”

She worries that this precious water supply for the river Cam will eventually be lost. “That’s what Cambridge is drinking. At some point, the river is going to pay the price.”

At risk are aquatic plants and invertebrates such as mayflies, caddisflies, damselflies and stoneflies, which provide food for fish such as trout, which in turn are eaten by otters.

“We’re losing a lot of the plants, invertebrates and fish we should have on the Cam. It is not the healthy river it should be,” she says.

But in the historic centre of Cambridge, the river is canalised, with tightly controlled locks, dams and weirs – so there, despite the depletion of the chalk streams that feed the river out in the countryside, the water levels can appear to be normal. The only clue to the problem is that the water is not clear and its quality is poorer than it should be. “It’s a big pond, basically,” Tomkins says.

A short walk from Jesus lock, opposite Magdalene College, Cambridge Punt Company guide Theo Land is preparing to take his next group of tourists punting along the river. “There’s nowhere I’d rather be now it’s summertime in Cambridge,” he says. “There’s nowhere else like this in the world. It’s special.”

The heart of Cambridge is the river, he says, and the revelation that it is drying up upsets him. “I wouldn’t feel the same attachment to Cambridge without the river. It would be a different place.”

Facebook Twitter Pinterest Former Undertones singer Feargal Sharkey is campaigning to protect southern English chalk streams – a vital habit for a wide range of interdependent flora and fauna. Photograph: Richard Saker/The Observer

Feargal Sharkey, former frontman of the Undertones, has campaigned to raise the profile of threatened chalk streams across the south of England. “The water companies have been allowed by the Environment Agency to over-abstract and deprive rivers of the very basic resource they need to survive: water,” he says

Chalk streams are the northern hemisphere’s version of the Amazonian rainforest. “What hypocrisy that, as this country is chastising Brazil over fires in the Amazonian rainforest and criticising Indonesia about deforestation, we are destroying a globally rare resource in our own backyard,” Sharkey says.

An Environment Agency spokesperson said: “The River Cam catchment received only 70% of the long-term average rainfall between May 2018 and May 2019, which has largely contributed to the current low flow levels.

“We are working with water companies to protect water levels by limiting abstraction quantities.

“We will continue to monitor conditions in the Cam catchment alongside carrying out a number of river restoration projects, and we are ready to respond to incidents caused by low water levels, including by rescuing fish in distress or oxygenating the water to help them.”