When he signed an unusual act of Congress rolling back a regulation to protect streams from mining pollution on Thursday, President Donald Trump made good on his promise to ease up on coal mining.

The repeal will mean more greenhouse gas pollution from burning coal. It’s also bad news for scores of little-known imperiled species, such as nearly 50 types of freshwater mussels that live in waters affected by mining.

So far, several of the dozen or so rules being targeted for repeal via the Congressional Review Act, a form of legislative veto of regulations, involve fossil fuel development.

As with so many regulations, the rules being overturned are costly to industry but have far-reaching environmental benefits.

Last year, the Obama administration’s regulatory impact analysis concluded the stream protection rule would have cut carbon dioxide emissions from coal-burning by up to 2.6 million tons a year because of reduced mining, avoiding hundreds of millions of dollars of future damages.

Luke Popovich of the National Mining Association said the rule ignored input from mining states, would have threatened jobs, and that it was “a prime example of a regulation that serves no purpose.”

Tell that to the Ouachita rock pocketbook, the inflated heelsplitter, and the Appalachian monkeyface, all threatened or endangered mussels. Already, mountaintop removal mining—in which coal companies blast off the tops of mountains and dump the detritus below—has damaged some 2,000 miles of streams, according to Appalachian Voices, an environmental group. Federal regulators said the rule would have protected  about 6,000 miles of streams and 52,000 acres of forests over two decades.

“This repeal is ignoring a lot of the recent science that clearly documents a lot of these downstream impacts,” said Emily Bernhardt, a professor of biology at Duke University who has studied the ecological effects of coal mining.

Central Appalachia, where much of the mining that would have been affected by the rule takes place, hosts some of the richest biodiversity in North America. Many endemic species live only within particular river systems or valleys.

The U.S. Fish and Wildlife Service said 171 species proposed or listed under the Endangered Species Act live in areas that would have received greater protection under the stream rule, which was published in December.

“It was really an A to Z look at what do we know now about the detrimental impacts of coal mining, how to minimize and mitigate those impacts,” said Jane P. Davenport, a senior staff attorney at Defenders of Wildlife. The rule would have revised regulations that hadn’t been updated since 1983.

“From the last 30 years of data, we know that the 1983 regulations have not been sufficient to protect water quality, to protect species, to protect ecosystems,” Davenport said. “So continuing to operate as status quo means we’re going to see more streams destroyed as a result of mountaintop mining. It means we’re going to see more species listed.”

One of Bernhardt’s recent papers found that in southern West Virginia alone, coal companies had blasted apart more than 1.5 cubic miles of bedrock and dumped it into 1,544 valleys. An earlier study found that by 2005, 5 percent of southern West Virginia’s land surface had been consumed by mines while 6 percent of its streams had been buried in “valley fill.”

The debris leaches heavy metals and minerals that elevate the water’s salinity for miles downstream.

“There’s very strong evidence, including some of the work we’ve done, that the increase in salinity leads to a dramatic decline in the diversity and the abundance of all kinds of sensitive aquatic insects, which are the base of the food webs in those systems,” Bernhardt said. What’s more, while the mine may open and shut over the course of years, its effects linger. “You basically have valley fills with sometimes up to 100 meters of rock that are going to continue to generate these salts for centuries.”

A memo by a staffer at West Virginia’s Department of Environmental Protection cited evidence in 2009 that such fills had wiped out entire orders of mayflies and stoneflies in some streams. Pollution from surface mining is also implicated in the decline of numerous species of freshwater mussels—the region is home to scores of varieties. Many of the mussels have extremely limited ranges, living within single river systems or only parts of particular streams. Some have already perished, including the green-blossom, turgid-blossom and yellow-blossom pearly mussels, which haven’t been found in decades.

Some species, like the northern riffleshell, have evolved an astoundingly intricate reproductive cycle. The mussels’ larvae develop on the gills of fish, but since the bivalves are blind and sluggish, they’ve had to develop techniques to attract their hosts. In a feat that rivals the best fish-fly designers, they’ve evolved near-perfect facsimiles of the tiny fish on which the larger fish feed. The mussels twitch their lures, complete with false eyes and even mouths, to attract would-be predators. Once the fish bite, the mussels spurt their larvae into the fishes’ mouths before the hosts swim off, infested but unfed.

Last year, the Fish and Wildlife Service listed three aquatic creatures in the Appalachians that are threatened by coal mining. The Guyandotte River crayfish, declared endangered in April, lives among the rocks of creek beds in just a single West Virginia county. The Big Sandy crayfish, listed as threatened, lives in six isolated populations in the upper Big Sandy River watershed. Sediment from coal mining and logging is filling in the crevices between the rocks, robbing the crayfish of their habitat. The Kentucky arrow darter, a 5-inch-long, brilliantly colored fish, has vanished from about half of the 74 streams it once inhabited in eastern Kentucky, leading the service to list it as threatened.

Tierra Curry, a senior scientist with the Center for Biological Diversity, which had petitioned for three listings under the Endangered Species Act, said the conditions that would protect these species would benefit everyone, all the way up the food chain to the people who live in the region.

The rule being overturned would have closed what conservationists say is a loophole to the regulations that allows mines to operate without close scrutiny under the Endangered Species Act.

In 1996, the Fish and Wildlife Service issued something called  a “biological opinion” on the regulations. The document basically told mining companies that as long as they followed the rules, their mines would not unduly harm endangered species, said Davenport, of Defenders of Wildlife. The opinion, conservationists say, served as a blank check for mining companies, allowing them to avoid close scrutiny given to other industries.

The new rule was accompanied by a new biological opinion as well, with updated science that would have required closer examination of impacts on species. While the repeal nullified the opinion, Davenport said that the science contained within it shows that the old rules are inadequate, and that it could still be used as evidence in legal challenges to any mining permits that might harm endangered species. “I think it really opens up the door for litigation,” she said.

Whatever its implications for climate and other environmental damages, the Congressional Review Act has never been tested in court.