One day after Earth Day’s 50th anniversary, environmentalists had more than just the holiday to celebrate. Instead of gutting the Clean Water Act as many had feared, the U.S. Supreme Court handed down a surprisingly measured decision in County of Maui v. Hawaii Wildlife Fund, largely preserving the jurisdictional reach of the iconic law. Yet in the wake of the decision, the courts and the U.S. Environmental Protection Agency (EPA) will almost certainly continue their decades-long interpretive tussle. At issue in County of Maui was the scope of the Clean Water Act’s authority over discharges conveyed to navigable waters through groundwater rather than directly through a “point source,” such as a pipe, channel, or some other discrete convey...