We conduct a benefit-cost analysis of a relicensing agreement for two hydroelectric dams in Michigan. The agreement changed daily conditions from peaking to run-of-river flows. We consider three categories of costs and benefits: producer costs of adapting electricity production to the new time profile of hydroelectric output; benefits of reductions in air pollution and greenhouse gas emissions; and benefits of improved recreational fishing. The best estimates suggest that the aggregate benefits are more than twice as large as the producer costs. The conceptual and empirical methods provide a template for investigating the effects of an environmental constraint on hydroelectric dams.