Economists, in evaluating the long-term benefits in public works projects, have traditionally held that long-term benefits will contribute very little to the total present worth of the benefits accrued over the life of the project. It is shown, however, that the long-term social cost resulting from projects that cause water pollution cannot be neglected or assumed away in the host of practical cases where ‘amenity’ demands for good water quality in a project’s area of influence are significant.