In 2017, the Agua Caliente Band of Cahuilla Indians affirmed their legal right to water in a landmark victory in the Ninth Circuit Court of Appeals. In an exercise of its sovereign authority, the Tribe then implemented a permit system to regulate use of the groundwater underlying its reservation. But local and state water agencies already have a conflicting regulatory framework in place. In the past, courts have resolved similar water management disputes by applying a complicated framework based on who is regulated and where the regulation takes place.

But this outdated approach leads to divergent outcomes and often does great harm to Tribal interests. Courts should instead recognize a presumption of exclusive Tribal regulatory authority over all onreservation water resources. This approach safeguards Tribal health and welfare while providing sorely needed predictability to Tribalstate regulatory disputes over water. States can be confident that their interests will be adequately accounted for because Tribes have a proven track record of equitably regulating water resources, and there are plentiful opportunities for state-Tribal cooperation.