A Trump administration proposal to reduce the scope of the Clean Water Act would exclude more waters than at any other point in the past 50 years. But it also left open the possibility of going even further.
Administration officials last week unveiled their plan to define “waters of the U.S.,” a frequently litigated term that delineates which waters and wetlands are regulated by the 1972 law. In the proposal, EPA and the Army Corps of Engineers sought to establish a bright-line test for determining how often rivers and streams must flow in order to be covered.
It suggests including only rivers, streams and other waterways that flow at least for the duration of the “wet season.” The proposal also floats an alternative approach: exclusively regulating perennial waters and wetlands.
Perennial waters, which flow continuously every day of the year, include iconic rivers like the Mississippi, the Shenandoah and the Columbia. But despite their charismatic nature, perennial waters account for less than half of rivers and streams in the continental U.S., according to multiple studies.
Applying the Clean Water Act only to those waters would be “a very significant change,” said Ciaran Harman, an associate professor of landscape hydrology at Johns Hopkins University. It would exclude a vast network of freshwater intimately connected to the watersheds that millions of Americans depend on for drinking, irrigation and economic activity, several hydrologists said.
“There are so many places where that basically takes a lot of the landscape off the map,” Harman said. “That would be a very significant change.”
The Clean Water Act aims to restore and protect the health of the nation’s waters. EPA and the Army Corps are charged with implementing it, including by requiring companies and landowners to obtain permits prior to polluting or altering a “water of the U.S.”
In 2023, the Supreme Court ordered both agencies to take a much narrower approach, asserting they could regulate only “relatively permanent waters” and wetlands connected to them via a “continuous surface connection.” Trump administration officials say their proposal would adhere faithfully to the ruling, Sackett v. EPA.
“What we’re looking for post-Sackett is a clear, simple, prescriptive rule that will withstand the test of time,” EPA Administrator Lee Zeldin said last week.
Since the 2023 ruling, most ephemeral waters likely already lost protections. Those are waters that are exclusively fed by rainfall and snowmelt and run dry for the vast majority of the year.
Despite their short-lived nature, ephemeral waters can have a big impact on water quality, especially in the Southwest, southern Texas and even parts of the coastal mid-Atlantic. A 2024 study in Science found that on average, 55 percent of the water in larger rivers in the continental U.S. originated in an ephemeral stream.
Hydrologists are concerned that the latest WOTUS rewrite could exclude even more critical headwaters, leaving them more vulnerable to pollution.
“It doesn’t make any sense to treat [ephemeral waters] differently from a scientific point of view,” said Peter Raymond, a biogeochemistry professor at Yale University who co-authored the Science study. “Ephemeral and perennial streams are connected. They might not be connected all the time, but they could be connected often.”
Climate change and state policies
The agencies’ effort to define “relatively permanent” waters is emerging as perhaps the most critical — and contentious — part of the rule.
Once finalized, the definition will affect not only the number of rivers and streams regulated, but also wetlands. That’s because wetlands must physically touch a jurisdictional surface water and contain surface water themselves, at least for the duration of a typical wet season, in order to be regulated under the draft rule.
That change would leave over 80 percent of wetlands in the continental U.S. unprotected by the Clean Water Act.
It’s not clear exactly how many streams would be “in” or “out” under the wet season standard, especially as climate change continues to alter precipitation trends, hydrologists said. The agencies’ regulatory impact analysis did not include an estimate.
“What’s the wet season? Ask 100 hydrologists and you’re going to get 100 answers,” said Adam Ward, who heads the department of biological and ecological engineering at Oregon State University.
Because of the vast number of small streams in the U.S., scientists are still mapping them and trying to understand exactly how often they flow, Ward said. In addition, the historical average is no longer a reliable predictor of precipitation or river and stream flow for the future, Ward said.
“How does a rule that’s based on climate from the mid-20th century manifest in this really rapidly changing and dynamic 21st century we’re living in?” he said. “In my cursory read, there is really nothing in the rule that lays this out.”
The proposal does not reference climate change, which President Donald Trump has called a “con job” and a “hoax.” The agencies assert that the wet season refers to the period when water is reasonably expected to flow in a given region, based on average precipitation as observed year after year.
As an alternative to the wet season concept, the agencies said they considered defining “relatively permanent” waters as those that flow for a minimum number of days per year, such as at least 270 days east of the Mississippi River and at least “30 or 60 days” in the West.
Another alternative considered was the perennials-only approach, which the agencies said could potentially be simpler to implement. It would also, however, probably receive the most pushback from environmentalists and some Western states. Nonetheless, the draft rule seeks comments on the idea.
One legal expert said he doesn’t expect the agencies to ultimately go that route.
“If you limit the definition to only cover perennial streams and exclude intermittent or seasonal streams, that on its face would appear to be inconsistent with Sackett,” said Duke McCall, a partner at Morgan Lewis who has represented oil companies, utilities and industry associations in environmental litigation. “I think there’s a recognition that would be challenging given the language of the Supreme Court’s decision.”
Even if the administration maintains its proposed definition of relatively permanent waters, hydrologists say that alone would exclude many smaller streams that provide crucial flood control and pollution reduction benefits.
EPA’s own researchers concluded in a 2008 study that ephemeral and intermittent streams account for 59 percent of streams nationwide, excluding Alaska, and perform “the same ecological and hydrological functions as perennial streams by moving water, nutrients, and sediment throughout the watershed.”
Trump administration officials have noted that states can still regulate those waters on their own. Still, as of now, 27 states do not have surface water protections that supplement the federal Clean Water Act, and 28 lack their own permitting requirements for filling in wetlands, the agencies estimated.
A few states, like Wisconsin and Colorado, established or strengthened wetlands protections after Sackett. Others, including Indiana and North Carolina, instead opted to make their state-level regulations narrower to align with the WOTUS definition.
“A rapid change in the federal level [regulations] doesn’t mean states have the time or desire necessarily to step in and fill that void,” Ward said. “There’s not a barrier to states doing more, but there’s a real risk that states don’t maintain the status quo and that they drop in response to dropping federal protections.”
