New York City cannot tap the rivers, bays, and ocean that inhabit, or surround it. It draws water from remote areas north of the City, mainly the Catskill Mountain/Delaware River watershed west of the Hudson River, and the Croton Watershed east of the Hudson River. The water travels from these sources to the city. The movement of this water is known as a 'water transfer,' an activity that conveys or connects waters of the United States without subjecting those waters to any intervening industrial, municipal, or commercial use. The EPA has taken a hands-off approach to water transfers, choosing not to subject them to the requirements of the National Pollutant Discharge Elimination System (NPDES) permitting program established by the Clean Water Act in 1972. Many contend that water transfers can move harmful pollutants from one body of water to another, potentially putting local ecosystems, economies, and public health at risk. Following many lawsuits seeking to establish whether NPDES permits are required for water transfers, the EPA formalized its stance in 2008-more than three decades after the passage of the Clean Water Act-in a rule known as the 'Water Transfers Rule.' Ps sued D. The court granted summary judgment for Ps, vacating the Rule and remanding the matter to the EPA. The court held that the rule represented an unreasonable interpretation of the Clean Water Act, and was invalid under the deferential two-step framework in Chevron. Ds appealed.