The snowpack in the Sierra Nevada is the deepest it’s been in decades, but those storms that were a boon for Northern California won’t make much of a dent in the long-term water shortage for the Colorado River Basin — an essential source of supplies for Southern California.

In fact, the recent storms haven’t changed a view shared by many Southern California water managers: Don’t expect lakes Mead and Powell, the nation’s largest reservoirs, to fill up again anytime soon.

“To think that these things would ever refill requires some kind of leap of faith that I, for one, don’t have,” said Brad Udall, a water and climate scientist at Colorado State University.

Lake Mead, located on the Arizona-Nevada border and held back by Hoover Dam, filled in the 1980s and 1990s. In 2000, it was nearly full and lapping at the spillway gates. But the megadrought over the last 23 years — the most severe in centuries — has worsened the water deficit and left Lake Mead about 70% empty.

Upstream, Lake Powell has declined to just 23% of full capacity and is approaching a point where Glen Canyon Dam would no longer generate power.

Even with this winter’s above-average snowpack in the Rocky Mountains, water officials and scientists say everyone in the Colorado River Basin will need to plan for low reservoir levels for years to come. And some say they think the river’s major reservoirs probably won’t refill in our lifetimes.

“They’re not going to refill. The only reason they filled the first time is because there wasn’t demand for the water. In the 1950s, ’60s, ’70s and ’80s, there was no Central Arizona Project, there was no Southern Nevada Water Authority, there was not nearly as much use in the Upper Colorado River Basin,” said Bill Hasencamp, manager of Colorado River resources for the Metropolitan Water District of Southern California. “So the water use was low. So that filled up storage.”

Demand for Colorado River water picked up in the late 1990s and early 2000s. The Central Arizona Project, a 336-mile-long water delivery system, brings water from the Colorado River to Arizona’s most populous counties and wasn’t completed until the 1990s. The Southern Nevada Water Authority was created in 1991.

Arizona began starting to take its full apportionment of river water in the late 1990s, and Nevada in the early 2000s. California continues using the single largest share of the river.

“Now the water use is maxed out. Every state is taking too much, and we have to cut back. And so there’s just not enough. You would need wet year after wet year, after wet year after wet year, after wet year. Even then, because the demand is so high, it still wouldn’t fill,” Hasencamp said in an interview.

Climate change has dramatically altered the river. In the last 23 years, as rising temperatures have intensified the drought, the river’s flow has declined about 20%.

Scientists have found that roughly half the decline in the river’s flow has been caused by higher temperatures, and that climate change is driving the aridification of the Southwest. With global warming, average temperatures across the upper watershed — where most of the river’s flow originates — have risen about 3 degrees since 1970.

Research has shown that for each additional 1 degree Celsius (1.8 degrees Fahrenheit), the river’s average flow is likely to decrease about 9%.

In multiple studies, scientists have estimated that by the middle of this century, the average flow of the river could decline by 30% or 40% below the average during the past century.

“The last 23 years are the best lessons we have right now, and they should scare the pants off of people,” said Udall, who has been a co-author of research showing how warming is sapping the river’s flows.

Based on the low levels of Lake Mead and Lake Powell, Udall said, he would estimate that refilling the reservoirs would take roughly six consecutive extremely wet years, with water flows similar to those in 2011.

“We’d need six years like that to refill this system, in a row, based on current operating rules,” Udall said. “And I just don’t see that even being remotely possible.”

The Colorado River Basin very well could get a few wet years, he said.

“We might even get a wet decade. But, boy, the long-term warming and drying trend seems super clear to me,” Udall said. “And a bet on anything other than that seems like water management malpractice, that we have got to plan for something that looks like a worst-case future.”

The Colorado River supplies water to seven states, tribal nations and Mexico. The states are under pressure from the federal government to agree on cuts to prevent reservoirs from dropping to dangerously low levels.

California and the six other states are at odds over how to make the cuts, and have submitted separate proposals to the federal government, with some disagreements centering on the legal system that governs how the river is managed.

Scientists have warned of a coming crisis for many years.

In a 2008 study, scientists Tim Barnett and David Pierce examined the likely flow declines with climate change and estimated there was a 50% chance the usable water supply in Lake Mead and Lake Powell would be gone by 2021. They titled their study “When Will Lake Mead Go Dry?” In research published in 2009, they wrote that based on projections with climate change or even the long-term average flows, “currently scheduled future water deliveries from the Colorado River are not sustainable.”

“Climate change is reducing the flow into the Colorado River system, so the agreements are divvying up more water than exists,” said Pierce, a climate scientist at Scripps Institution of Oceanography. “This drop in reservoir levels is happening because we are abiding by agreements that do not account for changes in water inflow into the system due to climate change.”

There is always the chance of a few extremely wet years with the potential to refill reservoirs, Pierce said.

“It’s just that in the coming decades that likelihood decreases. Our work has estimated that the chance of the reservoirs refilling decreases from about 75% today to about 10% by 2060 if no changes in [water] delivery schedules are made,” Pierce said. “We should be planning for the situation where the hotter temperatures decrease the river flow in the future.”

The capacity of lakes Mead and Powell is gargantuan compared with the capacity of California’s two largest reservoirs, Shasta Lake and Lake Oroville. Lake Mead can store more than 27 million acre-feet of water, and Lake Powell 25 million acre-feet. By contrast, Shasta Lake can hold about 4.6 million acre-feet, and Lake Oroville 3.5 million acre-feet.

The Colorado River supplies, on average, about 25% of the water supplies in coastal Southern California, while the region also gets water from Northern California through the State Water Project, and other sources.

California’s Sierra Nevada snowpack is now about 200% of average at this point in the season, while the snowpack in the Upper Colorado River Basin so far stands at about 140% of the median over the last 30 years.

The bigger snowpack could help the Colorado’s reservoir levels this year somewhat. How much won’t be clear for a few months.

“Absolutely this snow is welcomed. The cold weather is welcome. The real question will be in the spring,” Hasencamp said.

In recent years, hot, dry conditions have led to reduced flows in the river. “That’s what’s been the killer the last few years, is a hot dry spring has taken the snow that’s been there, and it doesn’t make it to the reservoirs,” Hasencamp said.

Daniel Swain, a UCLA climate scientist, said an exceptionally wet decade might someday change things.

“But the problem is, it doesn’t just have to be wetter than average, it would have to be dramatically wetter than the long-term average,” Swain said. And for many years.

Scientists say higher temperatures effectively make the atmosphere “thirstier,” causing more moisture to evaporate off the landscape. Vegetation also takes up more water as temperatures rise, leaving less runoff flowing in streams.

“There is no question that there will be an ongoing downward trend in inflows, but extreme high events are also more likely to occur in the context of climate change, according to the U.S. National Climate Assessment,” said Kathy Jacobs, director of the University of Arizona’s Center for Climate Adaptation Science and Solutions.

Jacobs noted that researchers project atmospheric rivers will become more intense with rising temperatures, and scientists expect more intense extreme storms and periodic flooding.

“I strongly suspect that the dams on the Colorado will be needed for flood control in the future as well as for water supply,” Jacobs said.

As for the future, Jacobs said a great deal depends on whether greenhouse gas emissions are reduced “to net zero in the near term.”

There are workable ways of managing reduced water supplies from the river, she said. “The longer we wait to build more flexible future management schemes, the harder it will be.”