Editor's Note: This story was initially published in Western Water Notes, a Substack newsletter published by independent environmental reporter Daniel Rothberg.

The headwaters of the Muddy River, a tributary of the Colorado River, take their form with water invisible to us. They start with groundwater. The Muddy River, as it charts its course to Lake Mead, relies on groundwater-fed springs as its primary supply. And for the last two decades, those springs have faced the threat of groundwater overuse.

Like so many rivers in the dry Southwest, the Muddy River is stretched thin. For more than a century, it has been divided up with some water rights “vested” (meaning they predate Nevada’s statutory scheme for regulating water use). Las Vegas currently has rights to a portion of the river, using it to boost its Colorado River supply. The springs feeding the Muddy River support an ecosystem that includes the rare Moapa Dace, an endemic species that despite many threats to its habitat over the years, persists today.

The river was allocated first. Then the groundwater feeding it was allocated separately and later. And in a story that repeats itself over and over across the West, the state of Nevada issued more rights to use groundwater than there was water to go around — because if too much groundwater is pumped, it could deplete the springs feeding the Muddy, with cascading effects on people and wildlife that rely on water being there.

But it was not at all clear how Nevada water law would handle this kind of issue. For years, groundwater and surface water were managed as separate sources, despite the fact that the science showed they were connected. In addition, the state made choices about water rights based on boundaries of aquifers that were drawn in the 1960s. The maps, however, often did a subpar job of showing how water actually flowed beneath the surface, as science and the understanding of groundwater has evolved since then.

This all came to a head when Coyote Springs (which planned to move forward with a master-planned community outside Las Vegas) looked to pump more water from the area. The state and the Southern Nevada Water Authority, which controls water rights on the Muddy River, raised concerns. It led to years of proceedings and a ruling from the state, Order 1309, that recognized the changing science of how water flowed in the area, known was the Lower White River Flow System, and how much was there.

The order did not take action but it offered a baseline:

• It recognized the connection between the effects of groundwater pumping on the flows of the Muddy River. That groundwater and the river here are connected.
• It found the decades-old basin boundaries that the state used to allocate water rights in the area did not capture the full extent of the Lower White River Flow System. It found that groundwater moved in what some call a “superbasin.”
• It determined that the available amount of water to pump was about 8,000 acre-feet, far lower than the approximately 30,000 permitted rights the state had issued in the past.

Taken together, the order was poised to change the status quo for how water might be managed in an area (and other areas) where many varied interests had water rights: the Southern Nevada Water Authority, Coyote Springs, NV Energy, Republic Services, Lincoln County Water District, the Apex Holding Company, the Moapa Valley Water District, the city of North Las Vegas, The Church of Jesus Christ of Latter-Day Saints.

Multiple parties appealed the ruling (or at least certain parts of it), and a district court judge ruled in 2022 that the state engineer, Nevada’s top water regulator, exceeded his authority. In a unanimous order Thursday that could have far-reaching implications for how water is managed in Nevada, the Supreme Court concluded the opposite.

The Supreme Court order affirms broad authority for the state over groundwater.

The court found that the state “has authority to conjunctively manage surface waters and groundwater and to jointly administer multiple basins and thus was empowered to issue Order 1309.” The justices are effectively saying that state regulators have the ability to view surface water and groundwater as a single source and can redraw the map to reflect how groundwater actually flows under the surface. That’s a huge deal.

The justices pointed to multiple parts of the state’s statutes, emphasizing at times the doctrine of prior appropriation, a guiding concept of Western water law. The doctrine states that those who received water first in time are first to receive water in times of a shortage. The justices said statutes give the state authority to prevent groundwater pumping from conflicting with “vested” river rights, such as those along the Muddy.

And if the science shows that groundwater and surface water act as one source, the state should be able to act upon the data, regardless of how past maps were drawn.

“If the best available science indicates that groundwater and surface water in the [Lower White River Flow System] are interrelated and that appropriations from one reduces the flow of the other, then the State Engineer should manage these rights together based on a shared source of supply,” Justice Patricia Lee wrote in the ruling.

Why this matters: It might sound technical and boring, but this decision could have major ramifications for how water is managed in the Great Basin in the 21st century.

For one, it gives Nevada’s top water regulator the clear authority and legal certainty in managing the depletion of groundwater in Nevada. It also recognizes that rights are conditions and the state is able to adjust its decisions as the science evolves — if the use of water is going to harm the water rights of others or the public interest.

From the Amargosa River outside of Las Vegas to the Humboldt River in the north, there are clear connections between groundwater pumping and its effect on surface water flows. This decision supports the state engineer’s authority to do something about it. All eyes are on the Humboldt River, where irrigators in Lovelock have long raised concerns about groundwater pumping for agriculture and mining upstream.

It’s noteworthy too as it resolves, at least for now, an ongoing tension that played out in the Legislature last year. Lawmakers attempted to clarify the state’s ability to follow the best-available science and recognize hydrologic connections. The effort did not make it very far, amid opposition from business groups and developers. Today, the Nevada Supreme Court said that the existing law already gave the state that ability.

The order also includes notable language around the role of the state in balancing wildlife protections and the “public interest.” In addition to emphasizing “vested” rights and prior appropriation, the court also emphasized the role of state regulators in determining whether water applications are “detrimental to the public interest.”

There remain many major unanswered questions. The ruling outlines what the state can do but it does not contemplate how the state should managing groundwater and surface water together. What does conjunctive management actually look like?

And it’s important to note that the lawsuit is not over. The parties in the case must go back to the district court. Now that the question of the state’s authority is cleared up, parties must argue that the state relied on substantial evidence in making its decision.