In the height of a seven-year drought in California, The Agua Caliente Band of Cahuilla Indians (the Tribe) sued the Coachella Valley Water District and Desert Water Agency (Together as “Water Districts”) to secure their right to groundwater stored in the Coachella Valley Aquifer (Aquifer). The Aquifer, like most groundwater resources in California, was severely taxed during the drought. This forced California to respond by passing the Sustainable Groundwater Management Act (SGMA), the first groundwater regulation in the State’s history. SGMA requires “sustainable groundwater management” for all basins by creating Groundwater Sustainability Plans (GSPs). These plans are, in effect, stakeholder negotiations on basin management. Basin adjudications will likely occur if these negotiations break down. In Agua Caliente Band of Cahuilla Indians v. Coachella Water District, the Ninth Circuit became the first federal court to expand the definition of Winters rights to include groundwater. Winters rights are federally reserved rights to water to help sustain the primary purpose of a federal reservation. The expanded definition of Winters rights increases the bargaining power of tribes, as stakeholders, in the GSPs and any possible basin adjudication. This decision greatly impacts California and other states in the Ninth Circuit. It would also have major implications for Arizona if Arizona had not been managing groundwater in much of the state since the 1980s. Additionally, Arizona recognized a tribal right to groundwater in Gila III in 1999. Agua Caliente affects the future of water supplies by broadening the definition of federally reserved rights to include tribes’ right to groundwater. This Comment recommends that private and public stakeholders across the West follow Arizona’s lead with respect to water planning in the Ninth Circuit’s jurisdiction by using settlement agreements with tribes to secure contested supplies of groundwater.