(NewsNation) — Spread out across already water-scarce parts of Arizona, New Mexico and Utah, the Navajo Nation is facing a siphoned water supply that is leaving nearly one-third of its families without running water to drink, clean or flush their toilets.

Spanning more than 16 million acres and three states, the Navajo Nation Reservation is the largest in the country. Thirty percent of families in the Navajo Nation live without running water, at times traveling several miles to secure water to bring home and meet basic needs.

“They have to haul water from a spigot that’s quite a distance from their residence or get it in other ways,” said Melody McCoy, a staff attorney for the Native Americans Rights Fund‘s Supreme Court Project. “It’s amazing. They really don’t have access compared to the rest of the population.”

The Supreme Court will hear oral arguments Monday in a case that could determine if the Navajo Nation has access to the already diminishing supply from the Colorado River.

Navajo people say they were promised adequate water as part of a more than 100-year-old treaty with the United States. The tribe now says the U.S. breached that treaty, causing many to go without reliable water access.

But the U.S. Department of the Interior claimed water rights — particularly from the lower and mainstream of the Colorado River — were never explicitly agreed upon. Several states, including Arizona, Nevada and California, also assert the Colorado River’s water supply is already limited, and that allowing the Navajo more access would only worsen that strain.

A spokesperson from the department declined to comment Friday, citing ongoing litigation.

The tribe’s fight for water rights has been heard by several courts throughout the years, and the “basic human rights of hundreds of thousands of Navajos” hang in the balance, according to their court filings.

The Navajo said their rights to water access are established by the 1800s-era treaty that established the Navajo reservation as the tribe’s “permanent homeland.”

“(The treaty) promised that, by giving up ‘nomadic life’ outside the Reservation, the Navajos could return
to their ‘farming’ way of life on part of their ancestral home, with government-provided ‘seeds and agricultural implements,'” the tribe wrote in its court brief.

Decisions about how to distribute rights to the Colorado River are complicated by drought conditions that have caused the water supply to dwindle.

Last summer, the U.S. Bureau of Reclamation issued a warning to seven western states that rely on the Colorado River for water when it dropped to historic lows. Those states were encouraged to find ways to save the river before it completely dries up.

Because so many rely on Colorado River water, several other organizations, including additional tribes, are keeping a close watch on the case, and what it could mean for future water distribution.

“It’s really the Navajo Nation’s case, of course, but there’s been a number of tribes and amicus groups that have supported the Navajo on this, knowing the potential implications for tribal interests are at stake,” McCoy said.