Children who live near a beauty spot say they are concerned the river through their village is "dying" after conducting water quality sampling.
Pupils from Dedham Church of England Primary School in Essex asked a local climate action charity to help them investigate pollution in a stretch of the River Stour popular with swimmers, paddleboarders and tourists.
Year Four teacher Emily Keeley said the children were "really quite disheartened to find it was so polluted" but were passionate about raising awareness of the issue.
"If we don't look after our world, there's no planet B and if we don't fix it, then there will not be the river and no animals in here," said nine-year-old Florence.
"We should tell people that the river is dying and we need to help it."
Noah, also nine, added: "If no one does anything about it, then the river will just go and no one will be able to go in," adding that it would no longer remain a nationally-important landscape.
The pupils are planning to write to the Conservative MP for the area, Bernard Jenkin, asking for a storm drain to be installed on the nearby A12 to stop chemicals draining into the river.
Immortalised by painter John Constable, Dedham sits in an Area of Outstanding Natural Beauty (AONB) on the Essex-Suffolk border, and has reportedly recently seen an increase in issues including litter and bad parking.
Miss Keeley said the Year Fours had been looking at environmental issues "in a global context" but the water sampling had "brought it home to Dedham".
"It's a beautiful place and just from visiting on a day trip, you might not necessarily know that there are all these underlying issues," she said.
"I think these guys are so passionate and enthusiastic about everything we've learned this half term, and they're so keen to do things even beyond the classroom, so it's been fantastic to see them get so engaged with it."
The Environment Agency said it respected the "citizen science" work of PACE Manningtree and welcomed the new Water (Special Measures) Act, which is aimed at improving the regulation of water companies in England and Wales.
"The Act will give us, as regulator, more power to protect our precious water quality and resources, hold water companies and others to account and ensure the polluter pays," it said.
"A number of factors can affect water quality. It is certainly not just about water companies. Weather, animal poo, road-run-off and littering are all to blame."
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