LAWMAKERS, MOSTLY but not only Republicans , are seeking to undermine the twin foundations of Environmental Protection Agency authority: the Clean Air Act and the Clean Water Act . In both cases, Congress should back off.

The agency badly needs clearer rules on which bodies of water it has the power to regulate. This clarification has been needed since a pair of Supreme Court rulings last decade left murky the definition of “waters of the United States,” a core Clean Water Act term. Environmental groups complain that this uncertainty has led agency personnel to hold off on enforcing the act in places where it almost certainly has jurisdiction. Everyone can agree that the EPA can regulate the Chesapeake Bay and the large rivers that feed into it. But what about the streams that lead into those rivers? The wetlands and ponds adjacent to them? More isolated headwaters that are hydrologically related? The EPA claims authority over all of these.

Many Republicans condemn that claim as a lawless and virtually limitless power grab — “The Obama EPA is trying every scheme they can think of to take control of all water in the United States,” Sen. John Barrasso (R-Wyo.) said — rather than a good-faith effort to lawfully reassert federal control over significant waterways. It’s true that the agency’s plan would expand the scope of Clean Water Act regulation. But it also would expand the number of exceptions. Besides, the proposal is contingent on an extensive scientific review confirming the ecological need to regulate upstream. The EPA reckons that the benefits of resource preservation and pollution control would offset the added regulatory costs. It’s past time for this definitional line to be drawn, and lawmakers’ objections aren’t convincing enough to justify forcing the EPA to start over. If lawmakers don’t like the call the EPA is making, they should clarify the terminology themselves.

Then again, having an unconvincing case almost certainly won’t stop lawmakers from trying to end another EPA initiative — cutting greenhouse-gas emissions. The agency last month announced rules governing emissions from existing power plants, the largest sources of planet-heating carbon dioxide in the country. The plan would slash power sector emissions by 30 percent by 2030, based on 2005 levels. EPA regulation is not the best way to achieve emissions cuts. It is cumbersome and will be subjected to legal attacks. But the United States needs a serious national greenhouse gas policy. Unless the critics endorse a better idea — such as a sizable carbon tax — simply handcuffing the EPA will be counterproductive.

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Signs are posted near the gold miners' camp along the Salmon River near Riggins, Idaho July 3, 2014. Citing states' rights, miners were using portable pumps and hoses to collect gravel and sand from the streambed of a stretch of the federally protected Salmon River, which is closed to suction dredging and other mining by the U.S. Environmental Protection Agency to protect imperiled fish (Jim Urquhart/Reuters)

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