The Colorado River Basin’s Daunting New Math

The basin’s big reservoirs have fallen to uncharted territory. The forecast isn’t any better.

• Lakes Mead and Powell, icons of 20th-century water engineering in the American West, are in bad shape.
• Both reservoirs are about one-third full and sit today at record lows.
• The federal government, for the first time, will likely declare a Tier 1 shortage on the lower Colorado River, opening a new chapter in the beleaguered river’s history.

By Brett Walton, Circle of Blue – August 13, 2021

Lakes Mead and Powell, icons of 20th-century water engineering in the American West, are in bad shape.

The story of their decline is written into the edges of the receding siblings — it’s evident in the minerals deposited on the rock walls hundreds of feet above Lake Mead, signs of where water once stood, like a white tea stain on nature’s mug. It’s also evident in the sandbars and sandstone arches of Glen Canyon that are reemerging as Lake Powell ebbs. This story, playing out across decades, is deeply consequential for some of the most rapidly growing and intensely irrigated regions of the country; for stressed ecosystems and endangered species; for Native American tribes; for the more than 40 million people who get a portion of their drinking water from the Colorado River.

Both reservoirs, the largest by capacity in the United States, are puddles of their former selves. Each about one-third full, they sit today at record lows, products of the Colorado River’s unforgiving math, in which demand exceeds supply.

Circumscribed in a body of law and policy known as the Law of the River, the math is complex and arcane, setting water deliveries across seven states and Mexico. New numbers will soon be added, altering the equation.

Next week the federal government, for the first time, will likely declare a Tier 1 shortage on the lower Colorado River, opening a new chapter in the beleaguered river’s history and illustrating its vulnerability in a region that is drying because of the buildup of heat-trapping gases in the atmosphere.

The lower basin refers to Arizona, California, and Nevada. This half of the watershed is in a Tier Zero shortage this year, which requires small cuts in water deliveries to Arizona and Nevada. Tier 1 means deeper cuts in water deliveries next year for those two states, especially for Arizona, which will bear the brunt of the shortfall. (To lessen the burden for farmers, who are subjected to these cuts, the state will allow them to pump groundwater and will draw on credits stored in Lake Mead.)

The cuts are intended to reduce strain on Lake Mead, which has fallen 147 feet since the year 2000. The Tier Zero cuts are 192,000 acre-feet for Arizona and 8,000 acre-feet for Nevada. Under a Tier 1 shortage those numbers rise to 512,000 acre-feet for Arizona and 21,000 for Nevada. Mexico will see an 80,000 acre-feet reduction. California does not take any cuts until Lake Mead falls another 22 feet.

Because of what’s happening upstream, the cuts will be less effective next year than had been hoped. The same Bureau of Reclamation report that will trigger the Tier 1 shortage also determines the amount of water released from Lake Powell in the following year. Water from Powell flows into Mead. Because Powell is so low, only 7.48 million acre-feet will be released in 2022. At the moment, assuming no wet winter, that same amount would also be released in 2023. In recent years, the lower basin has grown accustomed to at least 8.23 million acre-feet from Powell (and occasionally 9 million acre-feet), but even at that level of release allocations in the lower basin and Mexico plus evaporation from the reservoir still exceed inflow to Mead. (Voluntary reductions in the last few years lessened demand and bolstered Mead.) In short: with even less water than usual released from Powell in the next two years, Mead is projected to continue to decline, even with the mandatory cuts.

To protect Powell and preserve its ability to generate hydropower, the Bureau of Reclamation is releasing 181,000 acre-feet this year from upper basins reservoirs into Lake Powell. Its goal for this test case is to raise the lake level by 3 feet. How effective will the releases be? Brad Udall of Colorado State University says that the “jury is still out.”

These operational changes are taking place in the face of daunting hydrological changes. The basin is drying out as the planet warms. Udall and colleagues estimate that continued high temperatures could cut the river’s flow by 20 percent by 2050. And that is a conservative estimate, they warn. This year was an example of what that future might look like. A warm, dry spring and parched soils gobbled snowmelt before it reached rivers and reservoirs. Runoff into Powell was the second-lowest on record.

Scientists who study the basin say that it’s tapped out — with demand already exceeding supply and supply on a downward trend, no more water should be taken out beyond what is already used. If recent hydrology is an indication, even current uses need to be reduced.

There is the possibility of new water via cross-state collaborations that would result in more water reuse or desalination plants on the California or Mexican coasts. The Southern Nevada Water Authority and Metropolitan Water District of Southern California, two large urban suppliers, are in the preliminary talks for one such design: a project that would expand water recycling in the Los Angeles area, which would reduce demand for water imported from the Colorado River. These are long-term projects, though, and would take years or decades to bear fruit.

The new math — changes in supply, changes in deliveries — will be on the table starting this fall, when representatives of the basin states begin renegotiating interim agreements that have governed and aligned the operation of Mead and Powell since 2007. Those agreements expire at the end of 2025.

The central question in those discussions, as it always is in the Colorado River basin, will be: how do you get the numbers to balance?