For more than four decades, the U.S. Environmental Protection Agency (EPA) has controlled discharges of pollutants from point sources through the National Pollutant Discharge Elimination System (NPDES) permit program. Section 402 of the Clean Water Act (CWA), 33 U.S.C. § 1342, prohibits the discharging of pollutants from a point source into a “water of the United States” unless a facility has been granted an NPDES permit that explicitly allows discharges. Under the CWA, a “point source” is any discernible, confined, and discrete conveyance . . . from which pollutants may be discharged.” CWA § 502(14), 33 U.S.C. 1362 While the vast majority of point sources regulated by the EPA are confined and discrete conveyances for water, including pipes, ditches, channels, tunnels, and wells, they also include concentrated animal-feeding operations (CAFOs). EPA regulations define CAFOs as agricultural facilities where animals (other than aquatic animals) have been, are, or will be stabled or confined and fed or maintained for a total of 45 days or more in any 12-month period where crops are not grown. 40 C.F.R. § 122.23. During the past decade, the EPA has vigorously attempted to regulate these large agricultural facilities under the CWA, with varying success. One complication that the EPA faces in its efforts to regulate CAFOs is the CWA’s statutory exemption for “agricultural stormwater discharges.” 33 U.S.C. § 1362(14).