WILLIAM BRANGHAM:

It's called the waters of the United States rule, and it has to do with which smaller bodies of water, like streams and wetlands, should be regulated and protected by the federal government under the Clean Water Act.

That question has been litigated in court battles for years.

And so for more on what today's move is all about, I'm joined now by Juliet Eilperin, who's been reporting on this for The Washington Post.

Juliet, welcome back to the NewsHour.

Before we get into the rollback, can you tell me what this rule is really about? And this was, as I understand it, a very big part of Obama's environmental legacy.