As President Donald Trump signed one of his first executive orders, he was surrounded by smiling, clapping homebuilders, farmers and other supporters eager to see the new president rein in a 45-year-old law protecting the nation’s waterways.

The Environmental Protection Agency has “truly run amok” by saving “nearly every puddle or every ditch on a farmer’s land,” Trump told the gathering. Calling it a “massive power grab,” he added, “if you want to build a new home … you have to worry about getting hit with a huge fine if you fill in as much as a puddle – just a puddle – on your lot.”

But these so-called puddles and ditches, according to scientists across the country, are fundamental to the nation’s drinking water supplies and to wildlife, including many rare animals and plants.

In February, Trump ordered his EPA administrator, Scott Pruitt, to throw out a 2015 rule that protects seasonal streams and small isolated ponds, which would shrink the reach of the 1972 Clean Water Act by narrowing the definition of “waters of the United States.”

In its place, Trump asked for a new rule that, in effect, would eliminate protection for tens of thousands of wetlands from coast to coast. What’s at stake? Seasonal pools hidden in California grasslands and Maine forests, mountain streams in Arizona, freshwater marshes along the Gulf Coast, cypress swamps in Florida, glacial prairie potholes pockmarking the Midwest and more.

Based on the U.S. Fish and Wildlife Service’s National Wetlands Inventory, at least 20 million acres of vernal pools, potholes, salt flats, creeks and other isolated wetlands would be put at risk if the EPA follows Trump’s directions.

And as many as 60 percent of U.S. streams would lose protection, according to the agency’s own calculations. These headwaters and ephemeral streams and creeks contribute to the drinking water supplies of more than 117 million Americans.