THE SUPREME COURT agreed this week to hear a pair of environmental cases that will provide an early test of Chief Justice John G. Roberts Jr.'s approach to federalism -- that is, the balance of power between the national government and the states. The cases involve the protection of wetlands, which may seem like a subject far from the high constitutional theories of the chief justice's confirmation hearings. But it isn't. Wetlands protection presents a delicate question of federal constitutional law: How broad is the federal government's power to regulate interstate commerce, and is it broad enough to protect sensitive ecosystems that might exist entirely within a single state?

A few years back, a closely divided high court hinted at a disturbing answer. It struck down a federal rule allowing the protection of isolated pools of water in Illinois that migratory birds were using as nesting sites. The Clean Water Act gives the government authority over the "waters of the United States," but because these pools were not connected to any navigable waterway, the court considered them beyond the scope of the statute. To read the law more broadly, the court ruled, could present a serious constitutional problem by asserting federal jurisdiction beyond what the Constitution allows.

This decision was wrong, and it opened a can of worms that the lower courts have struggled with since. How isolated does a wetland have to be before the feds have to take a pass? Current federal rules interpret the "waters of the United States" to include, in addition to major waterways, the wetlands "adjacent" to them. But the government interprets "adjacent" to include those wetlands that exchange water with larger bodies even if they are somewhat physically remote.

This may sound like a stretch, but it makes sense. How, for example, could the federal government protect a watershed such as the Chesapeake Bay if it could not prevent people from dumping chemicals into wetlands that feed it? Yet one federal appeals court interpreted the law to require that wetlands be literally adjacent for the national government to have jurisdiction. Looming over the dispute is the broader question of whether Congress has the power to protect wetlands at all.

It isn't just wetlands protection that depends on the answer. For the court to block federal power here, it would have to further narrow the law, or strike it down as beyond the congressional power over commerce. Either course would be a dangerous attack on the ability of Congress to enact meaningful legislation in areas where national power depends on the Constitution's commerce clause. One way or another, the case will illuminate whether the new chief justice will bring a fresh and more sensible approach to these questions.