The justices, in a 5-4 decision, threw out a lower court ruling that held an 1868 treaty confining Navajos to their reservation came with an implied promise that they would have access to water.

Justice Brett M. Kavanaugh said the treaties that established the Navajo reservation did not come with such an affirmative promise.

Justice Amy Coney Barrett cast a key vote for the majority, while Justice Neil M. Gorsuch dissented with the court’s three liberals.

The cases are Arizona vs. Navajo Nation and Dept. of Interior vs. Navajo Nation.

Washington attorney Shay Dvoretsky, representing the Navajo Nation in a losing cause, had argued the court did not need to consider the Colorado River at this stage of the case.

“Today the average person on the Navajo reservation uses just seven gallons of water a day. The national average is 80 to 100 gallons,” he said. “The Nation asks only that the United States, as trustee, assess its people’s needs and develop a plan to meet them.”

For more than a decade, the Navajo Nation has been fighting its water rights claims in federal court. It won a preliminary victory in the 9th Circuit Court of Appeals in 2021, which said it had a claim for breach of trust, noting that the 1868 treaty referred to agriculture.

“The Nation’s right to farm reservation lands … gives rise to an implied right to the water necessary to do so,” the appeals court said. However, it stopped short of deciding whether this included “rights to the mainstream of the Colorado River or any other specific water sources.”

But last fall, the Supreme Court agreed to hear appeals from both the Interior Department and Arizona that sought to toss out the 9th Circuit’s decision.

U.S. Solicitor Gen. Elizabeth B. Prelogar argued the 1868 treaty said nothing about water and established no specific duties for the government related to water. Moreover, the Navajo Nation has been given water rights from two tributaries of the Colorado River, including San Juan River in Utah, she said.

Lawyers for Arizona, joined by the Metropolitan Water District of Southern California, said the high court’s decrees have allocated the waters from the lower Colorado River, and it is too late for lawsuits that seek new rights to the same water.