How much water flows down the Colorado River? The right answer is more important than ever

• The seven states on the Colorado River are spending millions of dollars to accurately measure how much water flows downstream and how much is taken out along the way.
• Quantifying the river's flow is critical as states work on plan to divide up water in times of shortages. Upper river states are trying to avoid giving up water, leaving shortages to the lower river.
• Legally, the upper and lower river states should split the river equally, but that has become impossible, in part because there are so few ways to store water on the upper river.

With their funding source under review by the Trump administration, states and the federal government are continuing a mission to better understand how much water flows in the Colorado River, and how much of that water gets used before it reaches Arizona. 

As the possibility of legal battles on the Colorado River grows, competing states could use water data to back up their arguments, including claims that Arizona should bear the most water cuts in future shortages

The Upper Colorado River Commission — a body that represents the four states in the upper Colorado River basin — is in its third year beefing up the measurement of stream flows, water consumption by crops, and water diversions that its states use to regulate their water use. Though the Trump administration is reviewing the federal funding designated for the projects, the commission says it has continued its work.

In 2023, the commission and its member states (Colorado, Utah, New Mexico and Wyoming) began installing and re-activating stream gages, eddy covariance towers and other measurement technology that will ultimately cost around $50 million. The new and re-activated systems measure the water consumption of hayfields, hookup river level gages to the internet, and count the cubic feet of water that run down ranchers’ and farmers’ diversion ditches. 

“One of the critical paths to successfully managing water in an uncertain future is having the best available information to guide decisions," the commission's executive director, Chuck Cullom, said in a written comment.

So far, the commission has spent close to $40 million, with the largest share going to measuring the water upper basin users divert from the Colorado and its tributaries. The commission has also sought contractors to integrate all the information from these tools and create an online data portal for public view.

The new data will help the Upper Basin fine-tune its water management, but it could also play a role in lawsuits between Colorado River states if ongoing negotiations break down. A lawsuit could drag on for years, partly because states would interpret the new data differently, at a time when some experts say the river needs shared understanding and basin-wide action.

Quantifying the river's flow is also critical in the short term, as the seven states try to reach an agreement on how to divide up the water during times of shortages. The current plan expires in 2026, but the states have been unable to reach a consensus on most of the key issues. And the agency that oversees the river's operation, the Bureau of Reclamation, is still without a commissioner, further slowing work on a new deal.

How do you measure the water in the Colorado River?

Measuring water use has challenged Upper Basin water managers for decades. A network of almost 70 major creeks and smaller rivers feeds into the Colorado before it reaches Arizona. Cities and farms divert water from more than 20,000 points along those tributaries. Plants and trees also take their share, and so do the soil and atmosphere. The amount nature provides, and the amount ecosystems and people consume, changes each year depending on the weather and human development.

When the river hits Arizona, water dynamics become much easier to measure. Nearly all the water in the river system has naturally collected in one channel, and there are far fewer points of diversion (though those diversions are larger). Some Lower Basin tributaries like the Gila River are not included in the overall measurement of the river for interstate legal purposes, a difference in accounting that Upper Basin states commonly say is unfair.

Over the last century, governments and scientists have developed an immense network of sensors, gates, and webpages to communicate the vast, ever-changing reality of the Upper Basin to the millions of people who rely on that river every day.

Across four states, federal and state officials monitor and operate more than 125 snow measurement sites, 325 real-time streamgages, 20,000 water diversions, 22 large dams, nine eddy covariance towers, and four high orbit satellites that transmit data from those sites to data centers and ultimately computers. There are at least four federal departments involved in this network, along with the four state governments and thousands of water users. 

“We feel responsible for providing this real time, reliable public information,” said Matt Ely, director of the US Geological Survey’s Colorado Water Science Center, which operates streamgages throughout the state of Colorado.

The streamgages, some of which are more than a hundred years old, transmit data to satellites or cellphone links every 15 minutes. The data is then bounced back to a database called the National Water Information System, where it then becomes visible to the public through the internet.

The data from the gages and related systems is a bedrock resource for communities in the Upper Basin. Rafting companies and anglers check the flows every morning before they hit the river, and state water officials use the meters to decide who gets water every day. 

“I took a fly fishing course once, and the instructor said, ‘The first thing you do before you head out is you look at the USGS streamgage.’ I didn’t say anything, but that was a point of pride,” Ely said in an interview.

State officials use the network to decide who gets water during dry times. When the water level gets too low to provide for everyone, they pick up the phone, or hop in their trucks, and tell water users with the lowest-priority water rights to shut down.

“Some folks aren't extremely excited to be shut off,” Colorado State Engineer Jason Ullmann said. “It’s a difficult job, because the (water officials) have to make that decision, and each year is completely different based upon the snowpack we receive.”

The data also indicates how much water Arizona will get each year, as Arizona’s water allocation relies on the amount of water that flows out of the Upper Basin.

Still, the system is not perfect, and officials are filling gaps. Until recently, Colorado had no strict requirements for measuring water diversions in some parts of the Colorado River basin (diversion records were kept through more informal or situational methods by local officials). Now, the state is expanding water measurement devices into more corners of the state and establishing rules everywhere, using money from the same Biden-era law that is funding the UCRC project.

Utah is also expanding the number of its measurement sites that can transmit data remotely, according to Utah State Engineer Teresa Wilhemsen.

"The expansion of water measurement and monitoring in the Upper Division States is improving the information available to water managers and users to adapt to changing water supplies," Cullom said.

Why the new tech matters for Arizona

All of this data — and the way it’s interpreted — has gained special importance in the last two years as Colorado River basin states negotiate who should bear the brunt of an ongoing megadrought and long-term unpredictability in water supplies brought on by climate change.

The negotiations have been strained, but states have met to discuss their positions as recently as mid-March, according to a river commission spokesperson. If the talks break down, the data could be critical in understanding the reason for the dwindling supplies in the Colorado River, and how much of a cut Arizona should take in dry times.

Flows in the Colorado River have dropped 20% over the last century. Scientists believe a warming climate will have an overall drying effect on the river system, though it will happen through dramatic swings of intense drought and intense wetness.

Legally, the Upper Basin and the Lower Basin (Arizona, California, and Nevada) should split a fixed, equal amount of Colorado River water, but that split has become physically impossible as water levels swing and drop. Someone has to take less than they are entitled to — a “cut” — and the states are debating who that should be. 

In their first climate-related argument, the Upper Basin states claim they should not have to take administered cuts because climate and geography have constrained the river, not them. The Upper Basin has pushed this argument in two forms.

In their first argument, Upper Basin states say that because their geography and climate already curtails their water supply in dry years, they shouldn't have to take additional cuts. Unlike the Lower Basin states, the Upper Basin can’t rely on large reservoirs like Lake Powell and Lake Mead in dry years, which is why they have state officials who cut off water users frequently. Upper Basin states say they already take these nature-caused "involuntary cuts" each year.

“When you are a headwaters state and all your water comes from snowpack and follows gravity throughout the state, you don’t have a lot of choice and sometimes it sucks,” Colorado's negotiator Becky Mitchell said at the annual Colorado River Water User’s Association conference in December.

In their second climate-related argument, the Upper Basin says it cannot be held responsible for what climate change does to the river. According to the 1922 Colorado River Compact, which divides the river between the two basins, the Upper Basin states must not “cause” the river to be depleted below a certain amount. If climate change depletes the river to the point that downstream states like Arizona don’t get their promised amount of water, the Upper Basin says it isn’t at fault. 

Data is critical in making both arguments work, according to John Fleck, a professor of water policy at the University of New Mexico. At this point, Fleck said the data isn’t totally clear cut. 

New data has already complicated the Upper Basin’s argument about water shortages in dry years. In 2014, the Upper Basin states began pushing the federal government to update its methods for calculating those states’ agricultural water consumption on the Colorado River. Under the old methods, the data did not show that Upper Basin states use less water in dry years, taking “involuntary cuts.” Rather, it indicated that thirsty farm fields used slightly more water in dry years. The new methods were supposed to clear that up, but the story is still complicated.

Colorado has pointed out that in the new consumption data, the Upper Basin has used 200,000 acre-feet less in the five driest years than the five wettest. The basin used 3.8 million acre-feet in dry years and 4 million in wet ones. But the data also challenges the Upper Basin’s narrative in some way. Two of the Upper Basin’s five highest-use years are also among the five driest. In addition, the basin seems to use the same amount in average years that it uses in dry years (3.8 million acre-feet).

As for the climate change argument, Fleck said negotiators will need more than data to distinguish between the effects of climate change and the effects of human water consumption. If climate change dries the river, thirsty crops could require more irrigation, blending human use and climate into a combined drain on the river.

“This is one of the huge scientific uncertainties,” Fleck said.  “There’s an unanswerable question … Let’s say my field takes three acre feet of water for alfalfa this year, and it used to take two and a half. Is that climate change, or is that me? I mean, I'm the one taking more water, but I have to because of climate change. And you know, we'll just argue about that.”

Entering the ‘Red Zone’

If states can’t reach an agreement about how to distribute cuts, the Supreme Court may wade into these undefined and thorny data questions, an outcome public figures in the basin frequently say they want to avoid.

“Trials and litigation are expensive in terms of dollars, resources, and trust,” said Jeffrey Wechsler, a litigator focusing on water law at the New Mexico-based law firm Spencer Fane at the water users association event.

Conflicts over data analysis can make those trials costlier, Wechsler pointed out. In a 1995 Supreme Court case between Kansas and Colorado over water allocations in the Arkansas River, the states fought for more than 100 days just to determine which hydrologic model the court would use for measuring water use. Wechsler estimated that a lawsuit between states over the Colorado River could run five to seven years before the actual trial even begins.

“And along that same timeframe,” said New Mexico Deputy State Engineer Tanya Trujillo in the same event, “the hydrology is shifting, operations haven’t ceased, and people haven’t stopped planting crops or drinking water in their homes.”

The engineers and water experts at the Bureau of Reclamation, the federal agency that manages the Colorado River’s largest dams, have signaled they are unlikely to make their own reading. In releasing their alternatives for the next set of operating guidelines for managing the river, the Bureau did not include an option that would place cuts on the Upper Basin, as Lower Basin states have proposed.

The Bureau has also not firmly stated how or whether it would “enforce” the Colorado River Compact, and it has not analyzed the potential consequences of compact enforcement — something Lower Basin states have repeatedly asked the agency to do. That enforcement would require the federal government to interpret the compact in relation to the Upper Basin’s climate argument, drawing legal resistance. The Bureau may also be losing time and effectiveness as the Trump administration has still not appointed a commissioner to lead it.

With the Upper Basin hardening its arguments, producing more data for both sides to interpret in their own ways, the federal government declining to intervene and interpret the compact for the states, or otherwise threaten to “enforce” it in some way, the states could be headed toward the litigation nightmare described by Wechsler, and that litigation nightmare could arrive soon. 

Official projections from the Bureau of Reclamation show that flows from the Upper Basin could drop below the amount required to satisfy allocations to the Lower Basin and Mexico as soon as 2027, depending on changes in dam operations.

“Given the current stalemate between the Upper and Lower Division States over how the reservoir system should be operated, it means the potential for basin-wide litigation is now in the ‘Red Zone,’” Fleck wrote in a January blog post. 

A brighter alternative

Instead of fueling conflict, the data could help states collaborate on basin-wide reforms, argues Mark Squillace, a law professor at the University of Colorado. Squillace has argued that states should simultaneously enact reforms to their water laws to discourage waste and maximize efficiency, stretching water supplies to serve more users. Doing so, he has written, would require a consensus around how to legally quantify water consumption, something new measurement technology could help water managers do.

“My view is that no state wants to get out front on efforts to conserve water, because they don't think it's fair that they should do those conservation measures when another state is not doing them,” Squillace said. “And so if the states were able to reach agreement about changing their laws in ways that would better conserve water, then everybody would have, I think, an equal sort of incentive to do it.”

Referencing the late sociologist Elinor Ostrom, Fleck pointed out that data can be as useful for collaboration as it is for conflict.

“Ostrom argues that you don't need a centralized government to make decisions for you and impose solutions—that it works better if the management regime emerges from the people who are using the water,” Fleck said. “And one of the things Ostrom thought was crucially important for the success of these water management regimes was a shared understanding of the numbers, a shared understanding of the measurement and quantification of the resource.”

Austin Corona covers environmental issues for The Arizona Republic and azcentral. Send tips or questions to austin.corona@arizonarepublic.com

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust.

Sign up for AZ Climate, our weekly environment newsletter, and follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook and Instagram.