Is Colorado River going through megadrought?
Basin affected by climate change, scientists say
“We’ve known this was possible for a long time and have planned for it.”
Tom Buschatzke director of the Arizona Department of Water Resources
The water level of Lake Mead, the country’s largest reservoir, has dropped more than 130 feet since the beginning of 2000, when the lake’s surface lapped at the spillway gates on Hoover Dam.
Twenty-one years later, with the Colorado River consistently yielding less water as the climate has grown warmer and drier, the reservoir near Las Vegas sits at just 39% of capacity. And it’s approaching the threshold of a shortage for the first time since it was filled in the 1930s.
The latest projections from the federal government show the reservoir will soon fall 7 more feet to cross the trigger point for a shortage in 2022, forcing the largest mandatory water cutbacks yet in Arizona, Nevada and Mexico.
The river’s reservoirs are shrinking as the Southwest endures an especially severe bout of dryness within a twodecade drought intensified by climate change, one of the driest periods in centuries that shows no sign of letting up.
With a meager snowpack in the Rocky Mountains and the watershed extremely parched, this month’s estimates from the federal Bureau of Reclamation show Lake Mead could continue to decline through next year and into 2023, putting the Southwest on the brink of more severe shortages and larger water cuts.
“What really is starting to emerge is this really long pattern, that we’re in a megadrought in a lot of the western U.S.,” said Laura Condon, an assistant professor of hydrology and atmospheric sciences at the University of Arizona. “It’s kind of like a cumulative impact, that we’ve just been getting hotter and drier and hotter and drier.”
Many scientists describe the past two decades in the Colorado River Basin as a megadrought that’s being worsened by higher temperatures with climate change. While the Southwest has always cycled through wet and dry periods, some scientists suggest the word “drought” is no longer entirely adequate and that the Colorado River watershed is undergoing “aridification” driven by human-caused warming — a long-term trend of more intense dry spells that’s here for good and will complicate water management for generations to come.
Both Lake Mead and the upstream reservoir Lake Powell are dropping. Taken together, the country’s two largest reservoirs now hold the smallest quantity of water since 1965, when Powell was still filling behind the newly built Glen Canyon Dam.
The Colorado River has long been overallocated to supply farmlands and
growing cities from Denver to Phoenix to Los Angeles. And the growing strains on the river suggest that Lake Mead, its sides coated with a whitish “bathtub ring” of minerals along its retreating shorelines, will continue to present challenges as the Southwest adapts to a shrinking source of water.
“There will still be ups and downs and we will have wetter and drier years going forward but overall warmer temperatures mean we should expect a drier basin with less water,” Condon said. “Warmer temperatures increase the amount of water plants use and decrease snowpack. Even if we get exactly the same quantity of precipitation, a warmer basin will produce less streamflow from that precipitation.”
Condon is leading a project that focuses on using artificial intelligence to pull together complex data and models to examine possible scenarios in the Colorado River Basin.
“Warmer climate means we can’t assume the basin is going to respond the same way in the future it did in the past,” Condon said. “These are longterm basin-wide changes and they will require collective action and cooperation. If we consistently have significantly less water in the river, we will need agreements that reflect this new reality.”
Representatives of the seven states that depend on the river met at Hoover Dam in 2019 and signed a set of agreements, called the Drought Contingency Plan, laying out steps to reduce the risks of a damaging crash. Arizona and Nevada agreed to take the first cuts to help prop up Lake Mead, while California agreed to participate at lower shortage levels if the reservoir continues to drop.
The states’ water officials described the deal as a “bridge” agreement to temporarily lessen the risks and buy some time through 2026, by which time new rules for sharing shortages must be negotiated and adopted.
Under the deal, Arizona and Nevada have left some water in Lake Mead in 2020 and 2021. Those reductions are set to increase next year under the “Tier 1” shortage, which the federal government is expected to declare in August.
Arizona is in line for the largest cuts, which will reduce the Central Arizona Project’s water supply by nearly a third and shrink the amount flowing through the CAP Canal to farmlands in Pinal County. Nevada is also taking less water,
and Mexico is contributing under a separate deal by leaving some of its supplies in Lake Mead.
“We have a plan to deal with these shortages,” said Tom Buschatzke, director of the Arizona Department of Water Resources. “We’ve known this was possible for a long time and have planned for it.”
He and other officials say the Drought Contingency Plan never guaranteed the region would escape a shortage, but that it has reduced the odds of Mead falling to critical lows and has pushed back the possibility of more severe shortages and larger cuts. Buschatzke said voluntary conservation measures by the states and Mexico since 2014, plus the initial mandatory cuts over the past two years, have left about 40 feet of conserved water in Lake Mead.
“We would already be in a Tier 2 shortage had that water not stayed in the lake,” Buschatzke said during a panel discussion hosted by the Arizona Capitol Times. “It’s what we can do to slow the reduction in Lake Mead and minimize the depth and length of the shortages.”
The Biden administration announced this week that it’s forming an interagency working group to address the worsening drought in the West and support farmers, tribes and communities affected by shortages. Federal officials said with water allocations at historic lows in areas like the Colorado River Basin, there is an urgent need to minimize the impacts of the drought and develop long-term plans.
“Water is a sacred resource,” Interior Secretary Deb Haaland said in a statement. “This Interagency Working Group will deliver a much-needed proactive approach to providing drought assistance to U.S. communities, including efforts to build long-term resiliency to water shortages.”
A warmer watershed, a shrinking river
Scientists have found that the Colorado River is sensitive to rising temperatures as the planet heats up with the burning of fossil fuels. In one study, scientists determined that about half the trend of decreasing runoff in the river’s Upper Basin since 2000 was the result of unprecedented warming.
In other research, scientists estimated the river could lose roughly onefourth of its flow by 2050 as temperatures continue to rise. They projected that for each additional 1 degree C (1.8 degrees F) of warming, the river’s average flow is likely to drop by about 9%.
The past year has been especially harsh. Ultradry conditions intensified across much of the West, with extreme heat adding to the dryness throughout the Colorado River watershed. According to the National Weather Service, the past 12 months were the driest on record in Utah, Nevada, Arizona and New Mexico, and the fourth-driest in Colorado, where much of the river’s flow originates.
Lake Powell now stands just 36% full. The reservoir typically gets a boost in the spring and summer as the river swells with runoff from melting snow. But this winter, the snowpack peaked at 88% of the long-term median and has since dropped to 71% of the median. The dry soils in the watershed are soaking up some of the melting snow like a sponge, leaving less water running into the Colorado and its tributaries.
The amount of water that will flow into Powell from April through July is now estimated at just 38% of average.
Water researchers Eric Kuhn and John Fleck said their analysis of the latest federal numbers points to some alarming possibilities. The two — who coauthored the book “Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River” — wrote in separate blog posts that a careful reading of the data in the 24-month study, which only goes out to March 2023, shows the projections point to bigger troubles at Mead and Powell later that year.
Fleck wrote that the “most likely” scenario would put the level of Mead at an elevation around 1,035 feet at the end of September 2023, which would trigger larger cuts for Arizona, Nevada and Mexico, as well as California’s participation in reductions.
“I’m talking about the midpoint in a range of possible outcomes,” Fleck wrote. “A run of wet weather could make things substantially better. But a run of dry weather could make them worse.”
Kuhn wrote that the assumptions in the government study “do not fully capture the climate-change driven aridification of the Colorado River Basin.” He said the projections suggest Lake Powell could drop in 2023 to “a level that is troublingly close to the elevation at which Glen Canyon Dam could no longer generate hydropower.”
While California’s supplies aren’t affected under a first-level shortage, the state’s water managers say they’re prepared for possible cuts if Lake Mead continues to drop.
“The conditions we’re seeing this year highlight the threat of climate change and the drying trend we’re seeing on the river,” said Jeffrey Kightlinger, general manager of the Metropolitan Water District of Southern California. “We must continue to work collaboratively as we begin longer-term discussions on how to address the river’s supply imbalance.”
A growing body of scientific research has documented how changes in climate, mountain snowpack and river flows have played out in the Colorado
“Heat waves and drought are both strongly driven by increases in temperature, which is really where we have the greatest confidence in those types of predictions.”
Katrina Bennett hydrologist
River watershed over the past few decades.
Across the West, snow has traditionally stored a vital portion of the water, gradually melting and releasing runoff in the spring and summer. But that’s changing with higher temperatures. Researchers from the University of California, Irvine, found in a study last year that the western U.S. has experienced longer and more intense “snow droughts” in the second half of the period from 1980 to 2018.
“The main issue is the snow drought everywhere in the entire West, including Arizona, Utah, California, Colorado,” said Amir AghaKouchak, a professor in UC Irvine’s Department of Earth System Science. “When the snow is below average, it means low-flow situations in summer, drier soil moisture. And drier soil moisture increases the chance of heat waves.”
The upshot, he said, is that “we have to prepare for a different hydrologic cycle, basically.”
With higher temperatures, more snow has been melting earlier in the year. Scientists recently examined 40 years of data from snow monitoring sites across the western U.S. and Canada and found increasing winter snowmelt at a third of the sites.
Other researchers have discovered that the dry periods between rainstorms have grown longer on average across the western United States during the past 45 years. Scientists with the U.S. Department of Agriculture and the University of Arizona found this trend throughout the West in their study, but they saw the most extreme changes in the desert Southwest, where rainstorms have been happening much less frequently. The average dry period between storms in the desert Southwest has gone from 31 days to 48 days, an increase of about 50 percent since the 1970s, the scientists found. Annual precipitation declined by about 3.2 inches in the region over that period, a much larger decline that the West as a whole.
“In the desert Southwest, we were averaging around 10 inches and now we’re averaging around 7 inches,” said Joel Biederman, a hydrologist at USDA’s Southwest Watershed Research Center in Tucson. “That’s much more impactful when you consider that the amount in our region is smaller to begin with.”
Biederman and his colleagues focused on changes that have been measured and didn’t attempt to parse the influences of natural variations and climate change.
A separate analysis of climate data over the past 30 years by the National Oceanic and Atmospheric Administration shows the nation’s “normals,” or averages, have shifted dramatically in a decade, growing wetter in the central and eastern U.S. and drier in the Southwest while climate change has pushed temperatures higher. Another group of scientists at Los Alamos National Laboratory recently looked at how interconnected extremes influenced by climate change — from floods to droughts and heatwaves — are expected to intensify in the future in the Colorado River Basin. They found these sorts of concurrent extreme climatic events “are projected to increase in the future and intensify” in key regions of the watershed.
“Heat waves and drought are both strongly driven by increases in temperature, which is really where we have the greatest confidence in those types of predictions,” said Katrina Bennett, a hydrologist and lead author of the study. “Droughts are projected to increase as we move ahead in the future.”
Preparing for shortages
The Colorado River and its tributaries provide water for cities, tribal nations and about 4.5 million acres of farmland from Wyoming to the U.S.-Mexico border. About 70 percent of the water diverted in the United States is used for agriculture, flowing to fields of hay and cotton, fruit orchards and farms that produce much of the country’s winter vegetables.
Arizona gets an estimated 36% of its water from the Colorado, and a large portion of it flows through the CAP Canal to cities, farms and tribal lands.
The state’s plan for managing the shortages involves deliveries of “mitigation” water to help temporarily lessen the blow for some farmers and other entities, as well as payments for those that contribute water. The state and CAP approved more than $100 million for these payments, with much of the funds going to the Colorado River Indian Tribes and the Gila River Indian Community for water they contributed.
Managers of Arizona water agencies said they’re prepared for the cutbacks under an initial Tier 1 shortage in 2022 and 2023, as well as the growing risk of larger reductions under a Tier 2 shortage. They announced plans for a livestreamed meeting on April 29 to discuss preparations for the shortage.
In a first-level shortage, cities’ water supplies are protected under the state’s plan. That includes the 10 cities that belong to the Arizona Municipal Water Users Association, which supply nearly 3.5 million people.
Warren Tenney, the association’s executive director, said in a blog post that the shortage comes as no surprise and “does not mean a shortage at our tap.”
“Valley cities have been preparing for a shortage on the Colorado River for decades, so there will be no immediate impact to our municipal water supplies,” Tenney wrote.
Cities have been preparing, Tenney said, by investing in infrastructure, storing water and developing conservation programs “that have created a water-efficient culture.” He pointed out that many cities offer customers rebates to incentivize conservation, offering cash to help pay for removing grass, converting to desert landscaping and installing efficient irrigation equipment. Other states are taking different approaches. In Las Vegas, for example, water officials are supporting a proposal in the Nevada Legislature to ban unused grass along roads and on medians.
In Arizona, water officials say grass has been banned alongside public roads and on medians in the state since the 1980s following the adoption of the state’s groundwater law. Since then, when government agencies have designed landscaping along roads, they’ve been required to put in low-water-use plants.
“It has been the policy of the state of Arizona for more than 30 years that we don’t put grass on medians, generally speaking,” said Cynthia Campbell, Phoenix’s water resources adviser.
In places, there are still strips of grass on private land in front of businesses, Campbell said. And there are some “grandfathered-in” exceptions in parts of Phoenix, but if the grass dies in those places, she said, it has to be replaced with drought-tolerant plants.
As the city has grown, more yards have been filled with xeriscaping rather than lawns. Phoenix officials estimate that in the 1970s about 80% of singlefamily homes had yards mostly covered with grass, and that number has shrunk to less than 10%, Campbell said.
The City Council convened a committee in 2019 to discuss water conservation measures and approved a dozen of its recommendations, such as plans for using “water budgets” in city landscaping contracts, expanding water audits of homeowners associations and subsidizing water-saving irrigation controllers for customers. These sorts of measures, the committee said, could help move Phoenix toward a goal of reducing per-capita water use.
Preparing for less reliable water supplies from the Colorado River, the city last year began building a pipeline project that will enable the city to move water from other sources to north Phoenix in case there are eventually cuts. The city also has supplies from the Salt and Verde rivers, and the goal of building the pipeline is to make that water available.
“We need to make sure that we are ready with our infrastructure to be able to move water,” Campbell said. With the conditions on the Colorado River changing year to year, she said, the city has to be able to “adjust quickly and make the changes that we need to make so we can continue to be resilient.”
This year, the Bureau of Reclamation issued a report to Congress analyzing the effects of climate change on the Colorado River and other rivers in the West. It said the years from 2000 through 2019 were the driest 20-year period for the river in more than a century of records and, based on records from tree rings, one of the driest periods in the last 1,200 years. Those 20 years were about 2 degrees Fahrenheit warmer than the average of the 20th century.
The report also cited the rapidly growing population of the Southwest. It said challenges “arise from the likelihood of continued population growth and the significant uncertainty regarding an adequate future water supply.”