As droughts strain water supplies across Western states, some cities and farmers have struggled with mandatory cutbacks. Determining who gets cut is decided by the foundational pecking order of Western water: the older your claim to water, created as the country expanded westward, the better protected it is.

When there's a shortage, those with newer water rights have to cut back first, sometimes giving up their water completely before older claims lose a single drop.

It's known as "first in time, first in right." But "first" is a relative term.

"First in time, first in right is kind of laughable, because the ones that were here first were the indigenous people," says Gary Mulcahy, government liaison for the Winnemem Wintu tribe in Northern California.

As the climate gets hotter and further shrinks strained water supplies, Western states are grappling with whether a century-old water system created by white settlers can equitably handle a future of worsening droughts.

Rights to water have long been seen as sacrosanct by many. But after decades of exclusion, Native American tribes are helping lead the charge both in California and on the Colorado River, arguing for overhauling an arcane system they say is inherently racist.

California lawmakers are debating whether to create new authority to rein in the oldest water users, who have long contended their rights can't be constrained by the state. Cities like San Francisco and farming districts with senior water rights are lobbying hard against the bills, saying billions of dollars invested into the water system are at stake.

"The weight of the inequities is really stunning," say Felicia Marcus, a fellow at Stanford University's Water in the West program and a former California water regulator. "Folks are going to need to think about what are the alternatives to cure what might be a historic injustice, while also being aware of the equities of all the communities and people dependent on the system that we do have."

First in time, via a piece of paper on a tree

More than a century ago, San Francisco locked up a pristine water supply. The city was booming in the late 1800s, and officials knew that local supplies wouldn't be enough for the growing population. They set their sights on a river high in the Sierra Nevada mountains, more than 150 miles away.

To tap into that river, the city had to first officially file for a water right.

"It meant you write it on a piece of paper and nail it to a tree," says Steve Ritchie, assistant general manager at the San Francisco Public Utilities Commission.

Thanks to that piece of paper nailed to an 8-inch round oak tree near the Tuolumne River in 1901, San Francisco has enjoyed a stable water supply ever since. During California's most severe droughts, the city hasn't had to make mandatory cutbacks, even when other cities and farms around the state saw their supplies dwindle.

"We and others have invested a lot of money in our systems to make them work based on the principle of first in time and first in right," Ritchie says.

For tribes, being first doesn't mean you have water

For California's Native American tribes, which have largely been excluded from the water rights hierarchy, that focus on the history of settlers' interests rings hollow.

"What we say about the senior water rights holders is they all got their water through murder, mayhem, rape, theft and genocide," Mulcahy says.

The traditional land of the Winnemem Wintu tribe in Northern California was flooded in the 1940s when California built Shasta Dam, creating the largest reservoir in the state. Today, it's one of the most valuable sources of water, supplying farms and cities that stretch hundreds of miles, all the way to Los Angeles.

"We have no water rights," Mulcahy says. "We're the Winnemem Wintu tribe. Winnemem means 'middle water'– middle water people. That kind of tells you our culture, our spirituality is based on water."

California's tribes, like most across the West, were forced to sign treaties with the federal government, giving up their land in exchange for a reservation to live on. But the treaties with most California tribes were never ratified by the U.S. Senate and were lost for 50 years. As a result, the tribes have no federal recognition, giving them little standing to claim water.

"The water rights system absolutely totally needs to change for everybody's right, for everybody's health and well-being, and not just a select few who think that they are the gods of water and they can't be touched," Mulcahy says.

State bills would grant authority over senior rights

California lawmakers are now debating whether to take some steps toward reform. State bills would give regulators more power to investigate the water use of senior rights holders, allow them to order those rights holders to stop using water when there's a shortage, and to increase the fines against those who take water illegally.

The pushback has been swift from senior rights holders, which represent some of California's wealthiest cities and farming areas. Many contend their water use can't be curbed, since their rights were established before California created its regulatory water agency in 1914, the California State Water Resources Control Board.

"We don't think that curtailment should apply to us," Ritchie says. "Water rights are basically a form of a property right. So having the uncertainty that that supply might be cut at some point, that is very troubling."

During California's last two droughts, state regulators struggled to order cutbacks among those with senior water rights, lacking data about how much water was being used and what rights were affected. When water users have defied orders to cut their use, the state's ability to levy fines has been minimal.

"I don't mean to say it's kind of a hot mess, but it's kind of a mess," Marcus says. "We have to figure out how to have a better way of allocating water more fairly according to set-upon rules that everybody can see."

Tribes push for water rights on the Colorado River

Water cutbacks are also contentious on the Colorado River. There, a two-decades long drought is forcing states to face a harsh reality: the future will mean less water for everyone. But tribes on the river have been left out from the very beginning.

The Navajo Nation has been battling with the state of Arizona for decades over getting its water rights clarified on the river. Some parts of the reservation still lack running water, forcing residents to get deliveries by truck. As a federally-recognized tribe, the Navajo Nation has rights to water as part of the "permanent home" the federal government granted with a treaty creating the reservation.

"The issue is that they haven't been quantified and no one really knows what the scope of those rights look like," says Dylan Hedden-Nicely, director of the Native American Law Program at the University of Idaho College of Law.

Last month, the Supreme Court ruled against the tribe, saying the federal government had no duty to support the investment needed to deliver a water supply. Still, after a long-fought battle, tribes are now being included in key negotiations over the future of the river.

In cases where tribes have had their water rights spelled out, they've struck deals to transfer some of that water to alleviate the overall shortage for everyone.

"Those are the types of opportunities that exist if people can get over this historical paradigm that this is a zero-sum game – if you get anything, it's coming out of my hide and therefore I'm going to fight you tooth and nail," Hedden-Nicely says.

As the climate gets hotter, water supplies both on the Colorado River and in California are expected to shrink and become more erratic. With the pressure mounting, the inequities in the system are becoming hard to ignore.

"I think climate change is forcing these conversations that are uncomfortable because the water's just not there," Marcus says. "And we need to figure out what to do."