The US Supreme Court on Monday ruled in favor of  New Mexico, ending the latest chapter in a decades-old water dispute involving the Pecos River. The conflict involves how water is apportioned between New Mexico and Texas under an interstate compact first approved by Congress in the mid-twentieth century.

Texas v. New Mexico centers around the Pecos River, which originates in north-central New Mexico—near Santa Fe—and flows into Texas, emptying into the Rio Grande. In 1948, the two states authored the Pecos River Compact with the goal of providing for the equitable apportionment of the use of the river’s water by both states and to “remove causes of present and future controversies.” Congress approved the contract the following year.

In 1988, the court appointed a River Master to review “the extent of New Mexico’s obligation to deliver water to Texas” under the terms of the compact. Additionally, the court adopted the River Master’s Manual, which instructs the River Master on how to calculate New Mexico’s water obligations to Texas. Specifically, the manual provides that “when water is stored at the request of Texas” in a New Mexico facility, “then New Mexico’s delivery obligation will be reduced by the amount of reservoir losses attributable to its storage,” i.e. evaporation.

Tropical Storm Odile hit the Pecos River Basin in 2014 and caused heavy rainfall. In an attempt to prevent flooding, Texas requested that New Mexico temporarily store water from the river that would otherwise free-flow into Texas, making sure to specify that the water “belonged to Texas.” Months later, Texas asked New Mexico to release the stored water. New Mexico obliged, released the water, but discovered that a “significant amount of water (approximately 21,000 acre-feet) evaporated.”

After years of confusion, the River Master issued New Mexico delivery credit for the evaporated water in 2018. Texas contested. It argued that it should have received credit for the evaporated water and filed a motion for review with the Supreme Court. Siding with New Mexico, the court emphasized that Texas is not entitled to the credit because the water was stored at Texas’s request. “That result,” wrote the court, “is both legally accurate and entirely fair.”

Justice Brett Kavanaugh delivered the opinion of the court, with Chief Justice John Roberts, and Justices Clarence Thomas, Stephen Breyer, Sonia Sotomayor, Elena Kagan and Neil Gorsuch joining. Justice Samuel Alito filed an opinion concurring in part and dissenting in part. Justice Amy Coney Barrett, the court’s newest addition, did not participate in the consideration or decision of the case. Oral argument took place on October 5.