Beer hasn’t been sold in steel cans for decades. The cans Keith Dopson found in Slough’s Salt Hill stream would be collectors’ items were they in good condition, but they had disintegrated into clumps of rust.

“We filled seven bin bags with rubbish,” he says. “Just from the river, not the banks. Plastic bottles and cans, lots of cans. Those steel ones must have been there for ages.”

Dopson, a retired carpenter, is one of around 20 volunteers who have for the past two years been reclaiming the stream. They have cleared undergrowth, dredged and narrowed the river and built a new footbridge and path. The aim is to reclaim the whole of Salt Hill stream and create a riverside walkway connecting Slough’s parks.

“We’ve seen kingfishers, grey wagtails, an egret,” says Shelley Rowley of the Wildfowl and Wetlands Trust, who works with volunteers, Thames Water, the council and local schools. “It shows that we’ve provided the right habitat for them to thrive, and that there are fish in the river they can eat.”

Litter picking, both in the river and on its banks, is a major part of the project. “We’re going to try ‘plogging’ next – it’s a Scandi thing, a mix of jogging and picking up litter,” Rowley grins, standing in waders, water past her knees. “It sounds exhausting.”

Salt Hill stream is one of hundreds of British rivers that are significantly cleaner than 30 years ago, when most waterways were biologically dead – often killed by raw sewage flowing from misconnected waste pipes down gullys meant for rainwater.

Last year there were 317 serious pollution incidents on British rivers, a drop of two-thirds since 2001, but still enough for the Environment Agency to call for bigger fines for companies and farmers who pollute rivers. Yet a growing body of research indicates that many rivers are polluted by substances that are not systematically measured.

Last week, researchers at the University of Manchester found “extraordinarily” high levels of microplastics in the river Tame in Denton, with 517,000 particles per square metre of river bed – levels not recorded anywhere else in the world. Microplastics are tiny particles that range from microbeads – used as exfoliants in skincare products, and banned in January by the government – to microscopic fragments small enough to cross the blood-brain barrier. A large proportion come from materials such as polyester and new synthetic fabrics, produced when clothes are washed, and from wet wipes and sanitary products.

“We all have a responsibility to fix the problem, starting with what we flush away,” said Catherine Moncrieff, WWF’s freshwater policy and programme manager. “The Environment Agency and water companies must fast-track efforts to better understand and address plastic pollution pathways.

“Microplastics in rivers are not routinely monitored, and the issue has received less attention than marine plastic. The true scale of the problem is only just emerging.”

But microplastics are just one issue, according to Matt Shardlow, the chief executive of Buglife, which campaigns to protect invertebrates. His organisation used Environment Agency data to highlight dangerous levels of neonicotinoids – a type of pesticide linked to bee population collapses – in several British rivers, particularly the Tame and the Great Ouse.

Shardlow and other campaigners are also worried about insecticides used in flea treatments for pets, and pharmaceuticals. Antidepressants change the behaviour of freshwater snails, and antibiotic resistance is growing because microbes in water treatment plants are exposed to drugs present in human waste, and can then evolve into drug-resistant strains.

“People think they put flea treatment on and it just goes away, but it doesn’t. It goes through their pet and comes out when they go to the toilet or jump in a pond,” Shardlow said.

“We want the Environment Agency to really pick up on monitoring, to start looking for problems, rather than simply being told what to monitor by the EU.

“We also want a review group to look at insecticides in water, neonicotinoids and other persistent chemicals. We know they are in orcas in the Antarctic, but not if they are in common rudd or perch in Surrey.”

Richard Benwell, head of government affairs at WWT, called for the government to take practical action on pollution. “That means enhanced monitoring for new risks, regulation as the science emerges, and financial responsibility for producers for the whole lifecycle of polluting products,” he said.

In the next few weeks proposals are expected to be published for a new environment watchdog, to replace the EU commission and European court of justice in holding the government to account on environmental standards, including on water quality.

Amy Mount, of the Greener UK environmental coalition, said: “It is promising that ministers want to set higher environmental standards post-Brexit, but their success will depend greatly on the strength of this new watchdog.”

An Environment Agency spokesman said: “Water quality is better than at any time since the Industrial Revolution thanks to tougher regulation and years of hard work by the Environment Agency and others. There are still many challenges to overcome, but we will not be scaling back on our ambitions.”