On February 21, his first day on the job, Justice Samuel Alito settled into one of the nine high-backed chairs at the Supreme Court to hear Rapanos v. United States and Carabell v. the U.S. Army Corps of Engineers--a pair of cases that, though not as well publicized as Jose Padilla's antigovernment petition nor as high profile as federal wiretapping, will probably eclipse their importance. Bundled together, the cases ask the justices (and Alito in particular, a projected swing vote) to declare whether national agencies can patrol the soggy patches of earth between dry, developable land and federally protected wetlands. A decision in favor of the plaintiffs could put most of the nation's watery habitats at risk.

"The tension," summarizes Margaret Strand, a wetlands lawyer in Washington, D.C., "is that protecting biodiversity translates into regulating private property"--pitting two American precepts, federalism and environmentalism, against each other. That tension originated with the 1972 Clean Water Act, which tasked the Environmental Protection Agency with preserving all "navigable waters," later defined as "waters of the U.S.," against unpermitted discharges. At the time, wetlands protection was just an appendage to the broader water safety effort, but recognition of the biome's ecological value inspired courts to uphold broad-based government power over the next 20 years. They could do so because "almost anything a little bit squishy might count as waters of the U.S.," says Don Carr, another lawyer in Washington.

Enter John Rapanos and June Carabell, two Michigan landowners who were denied development permits despite a 20-mile distance from the nearest navigable water and a berm blocking drainage, respectively. Over decades of court battles, the petitioners have marshaled a crusade against EPA jurisdiction they claim goes too far. One brief on their behalf traces government power to "remote desert washes hundreds of miles from the nearest navigable waters"--far beyond what they think Congress intended with the act. Instead Rapanos lobbies for federal jurisdiction limited to "navigable-in-fact" waters (commercial channels) and their adjacent wetlands. But such an interpretation would leave 90 percent of protected wetlands up for grabs, according to Richard Lazarus, a Georgetown University law professor.

States are unlikely to replace federal enforcement because only half have wetlands programs, explains Jon Kusler, president of the Association for State Wetland Managers. And "there are pressures in local communities for land development" that keep district guidelines from being as rigorous, Strand adds. Carabell's experience is a good example: whereas Michigan authorized a permit for condominiums on the land in question, the federal government asserted jurisdiction and denied one.

Relying on state laws becomes an even bigger problem with downstream pollution: discharge between differently regulated states presages a domino effect on the entire watershed. "All [polluters] have to do is [dump] far enough upstream," reasoned Justice David Souter during oral arguments; "it will eventually get in the navigable water."

The petitioners, however, claim that their lands are just too detached to cause that kind of trouble. And a 2001 Supreme Court decision offers precedent for their case, finding that "hydrologically isolated" wetlands belong under state jurisdiction. Federalists such as Robert Pierce, a former Army Corps of Engineers official who once enforced the EPA's laws, believe that the government's permit denials are often off target. According to Pierce, states may be able to take up the slack, but perpetuating government programs is "a major economic drain" and a "waste" of money that could be used elsewhere.

Yet with natural wetlands in constant decline, government inefficiencies may be a price worth paying. "You can't protect the nation's biological integrity without protecting these waters," notes Sandra Postel, director of the Global Water Policy Project. And one only has to recall Hurricane Katrina--gathering strength where wetlands once buffered the levee system--to measure the costs to society.

Nevertheless, federalist watchdogs cling to Rapanos (which may be decided this month) as an opportunity to curb Washington's power. "This case isn't about losing wetlands or saving wetlands," says Pierce, who has conceded that a government rollback might cause environmental harm. "This is about putting the federal government where the Constitution says it should be. In my mind, that's more important."