The Trump administration is taking steps to roll back an Obama administration policy that protected more than half the nation's streams from pollution.
The Environmental Protection Agency and the Army Corps of Engineers on Tuesday outlined a process for rescinding a 2015 regulation that defines which waterways are covered under the Clean Water Act.
President Donald Trump issued an executive order in February ordering EPA to review the rule, which farmers, fossil fuel companies and property-rights groups have criticized as too strict. Environmentalists counter that the rule is essential to protecting water for human consumption and wildlife.
The agencies say they'll move immediately to withdraw the existing measure as an interim step, then undergo a broader review of how far the federal government's jurisdiction over the nation's waters should reach.
The rule is key in enforcing pollution limits in places like the Chesapeake Bay watershed, by expanding the definition of what waterways are subject to federal authority.
Maryland environmentalists expressed concern that progress cleaning up the Chesapeake Bay could be reversed if the rule is revoked.
"Clean water is essential to MD $9.5B outdoor economy," the Maryland League of Conservation Voters wrote on Twitter. "Eliminating the Clean Water Rule risks that & public health."
Chesapeake Bay Foundation Vice President Kim Coble said that some states have their own laws and regulations extending environmental protections to headwater streams and wetlands.
"But that could change," potentially setting back bay cleanup progress that has reached a critical point, she said. "It is far cheaper to prevent pollution in the first place, than clean it up downstream."
The American Farm Bureau Federation, whose members include the Maryland Farm Bureau, has argued the rule is a federal overreach.
"The rule defines terms like 'tributary' and 'adjacent' in ways that make it impossible for farmers and ranchers to know whether the specific ditches, ephemeral drains or low areas on their land will be deemed 'waters of the U.S.'" the federation wrote on its website.
Baltimore Sun reporter Scott Dance and the Associated Press contributed to this article.