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The Trump administration today stripped protections from most wetlands and millions of miles of streams, ignoring warnings from its own scientists that these modest waterways are essential sources of clean water and wildlife habitat.

The 1972 Clean Water Act made it illegal to drain, fill in, or pollute the “waters of the United States” without a permit, but exactly which waterways that broad definition included has been the source of legal disagreements ever since. The Obama administration sought to clarify matters in 2015 with its Clean Water Rule, sometimes called WOTUS for Waters of the United States.

Real estate developers, farmers, and others complained that the Obama rule, which encompassed about 60 percent of the nation’s waters, was too strict, requiring them to seek costly permits for routine operations. Last September, the Trump administration threw the new rule out and reverted to an earlier interpretation of the act. 

Today’s replacement, called the Navigable Waters Protection Rule, further loosens regulations. Prior to the Obama rule, the U.S. Environmental Protection Agency and the Army Corps of Engineers decided on a case-by-case basis whether to issue permits for activities affecting ephemeral streams and isolated wetlands. Under the Trump rule, landowners need not even seek a permit for those activities.

Lakes, steady-flowing streams, and other major water bodies remain protected under the new rule. However, the changes mean a permit is no longer needed to drain or discharge pollution into ephemeral streams—those that flow only after precipitation. The same goes for wetlands and ponds that don’t directly abut or have a regular surface connection to a larger, protected water body. Despite appearing disconnected, these isolated wetlands and sometimes-dry streams nevertheless play an important role in the integrity of watersheds, scientists say, as well as providing important bird habitat. 

President Trump teased the changes earlier this week at the American Farm Bureau Federation’s annual meeting, and EPA Administrator Andrew Wheeler formally announced them today at a gathering of the National Association of Home Builders.

“EPA and the Army are providing much needed regulatory certainty and predictability for American farmers, landowners and businesses to support the economy and accelerate critical infrastructure projects,” Wheeler said in a press release.

Environmental groups sharply disagreed with Wheeler’s explanation that the rule “strikes the proper balance between Washington and the states.” Instead, the loosened regulations are “an affront to the health and safety of hundreds of millions of people across the country who depend on rivers and streams for clean water,” said Bob Irvin, president and CEO of the nonprofit American Rivers, in a statement. 

“We cannot overstate how far this sets us back when it comes to protecting our water,” said Chris Wood, President and CEO of Trout Unlimited. “You can bet on gravity every time and whatever is in our headwaters will ultimately end up in our own backyards. Headwaters and wetlands are some of the most important components to our network of streams and rivers.” 

Even the EPA’s own Science Advisory Board that “aspects of the proposed rule are in conflict with established science.” That includes a of 1,200 scientific studies the EPA conducted in 2015, which concluded that even small, ephemeral streams and wetlands outside of floodplains shape the health of downstream waters. The letter also noted that irrigation canals, no longer covered by the Clean Water Act under the new definition, are frequently the source of E. coli contamination in vegetables.

The water bodies no longer protected by the new rule include some of the most important bird habitat on the continent, advocates say. They include prairie potholes, seasonal wetlands used so heavily by waterfowl that the part of the northern Great Plains where they’re found is sometimes called “America’s duck factory.” Other seasonal wetlands in the Everglades, where wading birds like Wood Storks and Roseate Spoonbills hunt fish for their nestlings, also lose protection. So do ephemeral desert streams, sometimes called arroyos or washes, that may flow only occasionally but nonetheless provide essential habitat for vulnerable species like the endangered Southwest Willow Flycatcher. 

Just over half of the nation’s wetlands now fall outside the scope of the Clean Water Act, according to government estimates Those same figures show that about 18 percent of U.S. streams are ephemeral and therefore no longer protected. In parts of the West that proportion is far greater. Roughly two-thirds of California’s streams are ephemeral, according to by Trout Unlimited. In Arizona, it’s nearly three-quarters. 

“Even if a water body only flows after rain or when we have snowmelt, those are incredibly important bird habitat that now could be drained and destroyed more easily,” says Julie Hill-Gabriel, vice president for water conservation at the ���Ϲ���. 

Important as they are now, these waterways will become still more valuable in a future altered by worsening climate change, Hill-Gabriel adds. Conserving water in arid regions of the country will grow increasingly urgent in drought years to come, she says, and draining wetlands means losing their sponge-like capacity for storing flood water as a warmer atmosphere yields more punishing downpours. 

The law will take effect 60 days after it is published in the Federal Register, but in the meantime, as with Trump's repeal of the Obama rule, the rollback is certain to see numerous legal challenges from Democratic attorneys general and environmental groups.