The Clean Water Act of 1972, one of the most successful and popular of the environmental laws enacted under Richard Nixon, will turn 33 next week. But what should be a celebratory moment is tinged by concern arising from the Supreme Court's decision this week to accept two cases that challenge the law's reach and constitutionality.

The cases could be an early test of the thinking of Chief Justice John Roberts on the scope of the clause regulating interstate commerce, the constitutional basis for Congress's actions empowering federal agencies to protect the nation's waters. Chief Justice Roberts has no track record on the act itself, but the court he now heads has taken an increasingly narrow view of what the Commerce Clause authorizes Congress to do. There is nothing in Chief Justice Roberts's history to suggest a more expansive view.

The better-known of the two cases involves John Rapanos, a Michigan landowner who faces steep criminal and civil charges for filling in protected wetlands on three sites in defiance of the Army Corps of Engineers and the Environmental Protection Agency.

Mr. Rapanos has argued that the Clean Water Act applies only to navigable waterways, like rivers and lakes, and to streams and wetlands directly adjacent to such waters. His properties are 20 miles away from any navigable water, although two are within the Lake Huron drainage system and one runs through a drain to a navigable river.

The United States Court of Appeals for the Sixth Circuit rejected his argument, and rightly so. The language of the Clean Water Act plainly extends protections to all waters. For more than three decades, it has been broadly interpreted as shielding everything from large navigable rivers to seasonal streams and wetlands that, however remote, have some hydrological connection to a larger watershed.

The biggest exception to this persuasive regulatory and legal history was a narrow Supreme Court decision in early 2001 holding that the law could not be applied to certain isolated wetlands if the only reason for doing so was their use by migratory birds. That decision did not, however, directly address the constitutionality of the act itself. Nor did Mr. Rapanos, in the Sixth Circuit case. Now he does, by arguing, in effect, that Congress improperly applied the clause on interstate commerce to regulate - in the words of Mr. Rapanos's appeal - "non-economic intrastate activities like the filling of remote, nonnavigable intrastate wetlands in this case."

In response, the Justice Department argues that comprehensively protecting the nation's waters involves "core federal interests" and is thus well within Congress's purview.

The department is right. At issue here is far more than Mr. Rapanos's land. At issue is the ecological integrity of thousands of miles of small streams and at least 20 million acres of wetlands - waters necessary for, among other things, wildlife, flood control, irrigation and clean drinking water.

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