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Last month the Interior Department signed new rules for the Colorado River, and starting in January the three states at the bottom of it, Arizona, California, and Nevada, will take about a fifth less water than they have lately drawn. Lake Mead, the reservoir that feeds all three, sits near an all-time low it set on September 1.

Last Monday's issue told how it came to that, politics and lawsuit included. This one is about the water itself: where it comes from, where it goes, and why the crop that takes the most of it can also give some of it back.

The Colorado's water is snow. It melts out of the Rockies, collects in Lake Powell behind Glen Canyon Dam, and from there the rules send a set amount each year down into Lake Mead, counted in acre-feet (one acre covered a foot deep). For two years that was 7.48 million acre-feet; in April the Bureau of Reclamation, which runs the dams, cut this year's release to 6 million to keep Glen Canyon's turbine intakes under water.

Below Mead the river runs on orders. Hoover Dam releases whatever the farms and cities downstream, and Mexico under a 1944 treaty, ask for, and the lake loses roughly half a million acre-feet a year to evaporation, by Reclamation's own measurements. In most years since 2000 the orders and the evaporation have taken more out of Mead than Powell sent in, about 1.2 million acre-feet a year by the estimate the river's managers have used for a decade, so the lake fell even when Powell released its normal amount. The cut that starts in January, 1.25 million acre-feet a year, is about the size of that gap.

The lakes have floors. At 1,035 feet, four feet below where Mead sat on Friday, twelve of Hoover Dam's seventeen turbines stop; at 950 the last five stop, and at 895, dead pool, nothing comes through the dam. Powell's floors are 3,490 for power and 3,370 for dead pool. When the new rules say they protect "critical infrastructure," they mean those power floors, with a buffer above each.

A team led by the water researcher Brian Richter has counted where the water goes across the whole basin over the twenty years to 2019. Farms take about half of everything the river loses. Of the water people take on purpose, farms use three times as much as everything else combined.

About six in ten gallons of that farm water grow cattle feed, alfalfa and other hay. Alfalfa alone takes more of the river than every other crop combined.

Alfalfa is a perennial. A stand planted once is cut again and again, eight to ten times a year in Arizona's low desert. It drinks accordingly: about five to six acre-feet of applied water per acre a year by University of Arizona estimates, a column about as tall as a person poured over every acre. And it sends a taproot six to twelve feet down when the ground allows.

That root is the quirk that matters. Stop watering an established stand for 45 or 60 days of summer and it does not die. It all but stops growing, holds on with what its roots can still reach, and regrows when the water returns; University of California agronomists say most stands come back to full production, with the low desert's long, hot season the main exception. Richter's team calls alfalfa a "shock absorber" for agricultural production.

The Imperial Irrigation District in California, which is entitled to more of the river than anyone else, pays growers $334 an acre-foot this year to stop watering alfalfa for 45 or 60 days of summer, by the terms on its program page. Every acre-foot they do not take stays in Lake Mead, and under the federal program that funds the payments, the farm cannot take it back later. Last year that kept 222,384 acre-feet in the lake. By the end of this year, the district says, four years of its programs will have added up to 12 feet to Mead.

The farms at the far end of Arizona's canal lost their river water a different way, by standing last in line. The canal, the Central Arizona Project, carries river water uphill to Phoenix and Tucson, and its farm pools went to zero in 2023; Pinal County's growers drilled wells and left fields unplanted. The cut that starts in January reaches the cities' canal water next, while Yuma's oldest fields, on rights that date to 1901, keep drawing straight from the river.

What made all of that conserving voluntary was federal money, and it is running short. The Imperial district said in January that this year's program would be smaller because the federal funding is capped, and about $354 million of that funding has to be committed by September 30 or it goes back to the Treasury. The cut binds on January 1 either way.

The rules and the money decide who leaves water in Lake Mead and whether they are paid for it. The crop decides how much can be left without ruining a farm. What decides how much water there is to argue over is snow, and the next reading of it that matters, the one that sets Glen Canyon Dam's 2027 release into Mead, between 6 and 7 million acre-feet, comes in April.

Anthony

The Luniter Observatory

Research and educational analysis only — not financial advice. Every investment carries risk.