Groundwater is an essential water source for millions of Americans, and it is invaluable for those in the western United States. It is stored in underground aquifers, geological formations of permeable rock and pore space, which lie under every state in the country. But states have no uniform standard for how they manage the withdrawal of groundwater, and even states with somewhat similar legal regimes for water use allow users to extract groundwater differently. These disparities create a resource-use environment that is ripe for conflict. An interstate groundwater dispute, however, had never been decided in the U.S. Supreme Court until November 2021. For the aquifer at issue in Mississippi v. Tennessee, the Court endorsed equitable apportionment—a legal doctrine that it had historically applied only to interstate surface water disputes—in a myopic decision. The doctrine is ill-suited to emerging groundwater conflicts in the West not only because it is less feasible for a hard-to-measure resource like aquifers, but also because its emphasis on total consumption is unsustainable for the region’s arid future. The more suitable doctrine of interstate nuisance focuses on current use and works backward to determine how states can sustainably use a water resource while doing minimal harm to each other. Current and emerging conflicts in the West, including those involving the Snake Valley and Ogallala Aquifers, would benefit from the application of interstate nuisance.