A federal court ruled Friday that President Obama’s regulation to protect small waterways from pollution cannot be enforced nationwide. In a 2-1 ruling, the Cincinnati-based Court of Appeals for the Sixth Circuit delivered a stinging defeat to Obama’s most ambitious effort to keep streams and wetlands clean, saying it looks likely that the rule, dubbed Waters of the United States, is illegal. “We conclude that petitioners have demonstrated a substantial possibility of success on the merits of their claims,” the judges wrote in their decision, explaining that the Environmental Protection Agency’s (EPA) new guidelines for determining whether water is subject...