Peter Annin is director of the Mary Griggs Burke Center for Freshwater Innovation at Northland College in Ashland, Wis., and author of “The Great Lakes Water Wars.”

Lake Mead, America’s largest reservoir, is a lifeblood for 25 million people in the Southwest. But for the third time in six years, it’s about to hit a record low. Water levels have fallen more than 140 feet since 2000, leaving the reservoir only 36 percent full.

Today, Mead is rimmed by a broad white bathtub ring marking how far water levels have fallen during the Colorado River’s 22-year megadrought. The Bureau of Reclamation says the new low record will be set on Thursday, a sober climate-change milestone.

Mead has always managed to bounce back from prior lows. But the Bureau’s latest 24-month forecast shows the reservoir stubbornly staying in record territory through year’s end. That has never happened since Hoover Dam was built in the 1930s.

Hence, the federal government is expected to declare an unprecedented “shortage” on the river this summer, prompting mandatory water cuts, especially in Arizona. “This is a day we knew would come at some point … and we’ve been preparing for this moment for at least a couple of decades,” said Ted Cooke, general manager of the Central Arizona Project, which delivers Colorado River water to 80 percent of the state’s population.

Cooke and others continue to push for more conservation. Perhaps no conservation effort is more telling than the water cops in Las Vegas. During a ride-along last month, I watched water waste investigator Perry Kaye nab four homeowners for illegal lawn-watering in just 50 minutes. Nevada is a weird place. Prostitution is legal. Weed is, too. But midday lawn-watering can bring an $80 fine.

Vegas doesn’t want the money. Officials would rather pay homeowners to swap their grass for desert landscaping. “If the only person walking on that grass is the person pushing the lawn mower, it should go,” declared Colby Pellegrino, with the Southern Nevada Water Authority. Last week Nevada even passed a law banning grass, in some areas, by the end of 2026.

That has attracted national attention, but it’s not enough to prevent the looming cutbacks under pre-negotiated, hard-fought drought-planning agreements. Under those plans, Arizona, Nevada and California agreed to predetermined water cuts if Lake Mead dropped to certain trigger points. The reservoir is expected to stay below the “Level 1” trigger through year’s end.

But there is a more ominous stat in the Bureau’s 24-month Lake Mead projection. Data for October 2022 shows the reservoir dropping below elevation 1,050 — the drought agreements’ next tier of pain — and staying there for months, potentially prompting even more severe cuts.

The drought hurts energy production, too. Less water means less power generation from the mighty turbines in the bowels of Hoover Dam. Today’s low water has already curbed electric production by 25 percent. If Mead drops below 1,050, it will impact the grid far beyond the river. President Biden has yet to nominate a Bureau of Reclamation commissioner to navigate this unprecedented era.

Not surprisingly, the megadrought has spawned a series of preposterous, expensive and polarizing water-diversion proposals, such as using trains to ship water from Minnesota; diverting water from the Mississippi River to the Southwest; or building a sprawling, federally subsidized “interstate water system” so that the East can support unsustainable water practices in the West.

The vast majority of Colorado River water goes to agriculture, including water-slurping cotton and alfalfa. Rather than build an expensive interstate water system to take water from Minnesota and Mississippi, wouldn’t it be better to use the existing interstate highway system to transport cotton from Mississippi and alfalfa from Minnesota?

And is this even the best political strategy? At the rate things are going, the Southwest may well need congressional drought relief. But proposing unrealistic and controversial diversions that roil half of Congress does not seem like the best way to create a sympathetic audience.

Fanciful water-diversion schemes have been around for half a century. They are enormously controversial, especially in the Great Lakes region. These proposals all rely on the same false foundation. “We ship oil around the world, so why not water?” Answer: Because ecosystems don’t depend on oil for survival. Water diversions bring serious harm to originating watersheds, which is why the Great Lakes region banned them in 2008.

The smartest Western water managers are investing in technologies that provide a diverse supply of drought-resistant local water options. California has 12 seawater desalination plants, and six more have been proposed. Meanwhile, Scottsdale, Ariz., has built one of the largest potable water recycling facilities in the country, which turns sewage into drinking water. Los Angeles has pledged to recycle 100 percent of its sewage by 2035. “Water reuse,” says Patricia Sinicropi, executive director of the WateReuse Association, “is the future of water management.”

If that’s the future, then Hoover Dam is the past. That expansive white ring circling Lake Mead is a reminder that the harrowing water struggles of the past will likely pale in comparison with those of our climate-changed future, testing water officials like never before.