Authorities have launched an investigation after a stretch of Hertfordshire river important to wildlife was bulldozed.
The works were labelled a "tragedy" after the mile-long stretch of a protected river's banks was flattened.
The Herefordshire Wildlife Trust (HWT) said it hopes for a prosecution over the damage to the River Lugg, which it called a "crime against the environment".
The trust said all bankside and riverside habitats have been "completely obliterated" after the river and its banks were "bulldozed, straightened and reprofiled into a sterile canal".
A potato and cattle farmer has come forward claiming he cleared the land to prevent flooding at nearby properties.
However experts have claimed the works will make flooding worse downstream and put wildlife at risk.
Farmer John Price, 66, claimed local homeowners had become frustrated waiting for authorities to tackle the blocked river after their homes were damaged in last year's floods.
So he had taken matters into his own hands, he told The Telegraph.
Mr Price, whose farm neighbours the river on a farm in Kingsland, near Leominster, admitted to the newspaper he had bulldozed and cut trees down - but claimed he acted with permission from local authorities.
He said he had watched the river all his life, claiming "no one knows this river better than myself. I have always looked after the river."
He also claimed he had the support of the village and parish council, and had been asked to stop the erosion because he was the land owner adjoining that stretch of the river.
Mr Price told The Telegraph: "It was up to the Environmental Agency to look after these rivers but they don't do any work and haven't got any money to do the work because they spend it all on clipboards."
He added: "I have not pushed any trees out and I haven't knocked any trees down I have only cleared what ones came down in the flood."
The Environment Agency (EA) said it had launched an urgent investigation.
West Mercia Police confirmed the force is assisting the probe.
The HWT said wildlife affected by the damage includes crayfish, otters and salmon, lampreys and dragonflies and a host of rare river wildlife at the Site of Special Scientific Interest.
The trust said the River Lugg would be a "test case" for the Government's commitment to strengthen wildlife protection, and it hopes for a prosecution.
HWT chief executive Helen Stace said: "A large stretch of one of the UK's most important rivers, the Lugg, has been devastated, with dire consequences for wildlife and water quality downstream - this is a tragedy.
"The bankside trees are all grubbed out and burnt, the river gravels have been scraped away, and the beautiful meanders of the river have been straightened and reprofiled.
"As former leader of the English Nature rivers team which notified the Lugg as a Site of Special Scientific Interest, I know this river to be one of the most enchanting tree-lined reaches, with immense value for wildlife.
"This is a crime against the environment. Swift action needs to be taken and we want to see the authorities investigate the matter swiftly.
"We expect this case to be dealt with in a serious and robust manner and any resulting prosecution should act as a deterrent to prevent anyone committing this type of crime ever again.
"We will also be calling for restoration of the river to its natural channel."
HWT said it believes bulldozing part of the river is a contravention of the Wildlife and Countryside Act and the Environmental Permitting Regulations 2016 and should have required consent from the Environment Agency, as well as planning consent.
Craig Bennett, chief executive of The Wildlife Trusts, said: "The Government has promised to transform our environmental governance as we leave the EU and for the UK to be a world leader in environmental protection.
"Devastating incidents such as this one will be a vital 'test case' as we come to the end of the transition period for the Government's ambition to develop a 'world-leading' environment enforcement structure.
"Unfortunately, the Government has so far failed to live up to this standard, with poor resourcing leaving Natural England unable to properly monitor and protect our most important wild places - Sites of Special Scientific Interest.
Andrew Nixon, conservation senior manager of HWT, said: "The actions taken on this stretch of the river will have far-reaching consequences, both for this stretch of river and floodplain and further downstream.
"Removing all bankside vegetation and scraping out the riverbed and banks will cause a huge increase in the speed the water moves through the river and increases the flood risk downstream.
"With no stabilising vegetation, any heavy rainfall and rise in river level and speed will mean massive erosion of the banks, with a great amount of soil washed into the river along with agricultural pollutants such as phosphates and pesticides.
"This soil smothers the riverbed for miles, destroying fish spawn and invertebrates that inhabit it."
Herefordshire Council is considering if there has been a planning rules breach.