CLAY, Circuit Judge. Plaintiffs, Kentucky Waterways Alliance, Sierra Club Cumberland Chapter, Kentuckians for the Commonwealth, and Floyds Fork Environmental Association, appeal the district court’s grant of summary judgment in favor of Defendants, Stephen L. Johnson, in his official capacity as Administrator of the United States Environmental Protection Agency (“EPA”), the Commonwealth of Kentucky, the Kentucky Coal Association, Associated Industries of Kentucky, the Kentucky Chamber of Commerce, and the Kentucky League of Cities, on Plaintiffs’ challenge, brought pursuant to the Administrative Procedures Act (“APA”), 5 U.S.C. § 701 et seq. (2000), of the EPA’s approval, under § 303(c) of the Clean Water Act, 33 U.S.C. § 1313(c) (2000), of Kentucky’s regulatory implementation of its Tier II water quality antidegradation rules. For the reasons set forth in parts I, II, and III-A of this opinion as well as for the reasons expressed in Judge Cook’s concurring opinion, we AFFIRM in part and REVERSE in part the district court’s opinion and order, VACATE in part the EPA’s approval of Kentucky’s Tier II antidegradation rules, and REMAND the matter to the EPA for further proceedings consistent with these opinions.