This is the story of a recent U.S. Supreme Court case on the use of cost-benefit analysis at the U.S. Environmental Protection Agency (EPA) for a regulation issued under the Clean Water Act (CWA). The case is Entergy Corp. v. Riverkeeper, Inc., et al. The case was not about the quality of the cost-benefit analysis, nor the fact that EPA conducted one, but whether EPA had CWA authority to base regulatory decisions on cost-benefit. I close with thoughts about an alternative Chevron legal test that acknowledges the state of ecosystem valuation.