The Trump administration is scrapping protections for America's streams and wetlands, repealing Barack Obama's Waters of the United States regulation.
The move will dismantle federal protections for more than half of wetlands and hundreds of small waterways in the US.
The White House says the change will be a victory for American farmers.
But critics say the change will be destructive - part of Mr Trump's wider assault on environmental protections.
Under the new regulations, landowners and property developers will be able to pour pesticides, fertilisers and other pollutants directly into millions of miles of the nation's waterways for the first time in decades.
The administration's new rules replace the Waters of the United States regulations put in place during the Obama administration. Mr Trump vowed as soon as he took office to repeal the regulations.
The president has angered environmental activists and conservationists since he took office by siding with the agriculture and mining industries in rolling back environmental protections. Speaking at the American Farm Bureau Federation's annual convention in Texas on Sunday, the president called the existing waterways rules "disastrous".
"That was a rule that basically took your property away from you," he told the assembled farmers.
The White House says that farmers will be a primary beneficiary of the change. Some farmers rejected the protections, claiming they were too broad and required the industry to go to great lengths to protect small bodies of water on their properties.
But the administration's own data shows that real estate developers and those in other non-farming industries are poised to reap the greatest rewards, by applying for permits to develop on previously protected waterways, the Associated Press reported.
According to the data, real estate developers and other business sectors outside farming take out substantially more permits than farmers for projects impinging on wetlands, creeks and streams.
The new rules are already facing court challenges from environmental groups and Democratic-led states. "This will be the biggest loss of clean water protection the country has ever seen," Blan Holman, a federal water policy specialist at the Southern Environmental Law Center, told the New York Times.
"This puts drinking water for millions of Americans at risk of contamination from unregulated pollution. This is not just undoing the Obama rule. This is stripping away protections that were put in place in the '70s and '80s that Americans have relied on for their health," he said.
The scrapping of these waterways regulations is part of a much broader environmental rollback directed by the president, who has appealed for electoral support from America's beleaguered mining and farming communities.
Since taking office, Mr Trump has slashed regulations on oil and gas development, weakened fuel emission standards for automobiles and proposed changes to the Endangered Species Act - a law credited with keeping hundreds of species from going extinct.
Earlier this month, while announcing other changes to environmental policy, Mr Trump criticised existing "job-killing regulations", while insisting the US would maintain "world-class standards of environmental protection".
"We have some of the cleanest air and cleanest water on Earth," he said.
The US is the world's second largest emitter of greenhouse gases. A report found carbon dioxide emissions rose by 3.4% in 2018 - the largest spike in eight years - after three years of decline.