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California’s deadly floods won’t break the megadrought

Atmospheric rivers are dumping rain on California. That’s not a good thing.

A person carrying an umbrella hunches underneath it as they cross the street under what looks like heavy rain. Cars with their windshield wipers and headlights turned on wait in the background.
A pedestrian crosses the street in San Francisco, California, on January 4, 2023. A series of atmospheric rivers has hit the state in the last two weeks, bringing torrential rain, winds, and flooding.
Liu Guanguan/China News Service/VCG via Getty Images
Neel Dhanesha is a reporter covering science and climate change at Vox. Prior to Vox, Neel was an editorial fellow at Audubon magazine and an assistant producer at Radiolab.

A “river” more than 100 miles wide is gushing through the air high above California, bringing with it heavy rain, winds, and snow. It’s the third in a series of weather systems known as atmospheric rivers — long, heavy columns of water vapor in the sky — to hit the state in the last two weeks.

It’s already proven deadly: Two people have died as a result of the storms, including a toddler; roads have flooded or been hit by mudslides, forcing evacuations; and more than 180,000 Californians lost power. On Wednesday, California Gov. Gavin Newsom declared a state of emergency ahead of the storm’s arrival, and the city of San Francisco ran out of sandbags for the second day in a row as residents rushed to protect their homes from the possibility of flooding.

Once the storm passes, there will be little respite: Another atmospheric river is forecast to hit the state this coming weekend and next week, bringing even more flooding.

California is looking drenched at the moment, but for the past two decades, it’s been suffering through a megadrought of the kind that hasn’t been seen in more than 1,000 years. The drought threatens the region’s agricultural industry and ordinary citizens alike, putting livelihoods at risk and raising concerns about what the future of life in the West might look like.

Which might, understandably, raise a simple question: Can all this rain, despite the suffering it brings, help alleviate the drought?

The simple answer: Unfortunately not. A flood during a time of drought is a double disaster.

Reason 1: Too much water all at once

As we wrote last August, droughts and floods are something of a vicious cycle. It takes time for water to soak into soil, and having multiple storms hit in quick succession is something like overwatering a potted plant: The soil simply can’t take any more water. Eventually the rain turns into floods, which further erode the soil and bring the risk of downed trees, which can take out power lines and damage buildings; a 2-year-old child was killed this week when a redwood fell on a mobile home in Sonoma County.

“We are in the middle of a flood emergency and also in the middle of a drought emergency,” said California Department of Water Resources (DWR) Director Karla Nemeth in a media briefing on Wednesday. “This is an extreme weather event and we’re moving from extreme drought to extreme flood. What that means is a lot of our trees are stressed, after three years of intensive drought, the ground is saturated and there is significant chance of downed trees that will create significant problems.”

In non-drought conditions, tree roots act a bit like sponges, soaking up water from the soil. But droughts make tree roots less sponge-like, which means they can’t soak up as much water right away. That also makes the roots weaker and the trees more susceptible to falling during extreme flooding.

If the rain had been spaced out over a series of months, it might have helped with the drought by filling reservoirs over time, said Noah Diffenbaugh, a climate scientist at Stanford University’s Water in the West program. The soil would also be less saturated, allowing for more water to soak in more slowly, replenishing groundwater wells and reducing the chance of flooding.

Instead of collecting in reservoirs or soaking into the ground, the water has nowhere productive to go. So it floods.

Reason 2: Too little water altogether

Expecting these extreme rain events to alleviate the drought is a bit like racking up thousands of dollars in debt over the course of months and only receiving one or two paychecks at the end of the year.

“Most people wouldn’t say the problem’s been solved because of one normal monthly paycheck,” Diffenbaugh said. “A normal year of rainfall would not break the drought. In fact, even one wet year wouldn’t necessarily break the drought.”

California’s “megadrought” designation is a recognition that the state has been through a series of drought years with relatively few wet seasons. Breaking the drought would require multiple years of above-average rain and snowfall.

As things stand, the atmospheric rivers hitting the state have filled smaller reservoirs to capacity, while major reservoirs still sit mostly empty.

The smaller reservoirs filling to capacity isn’t exactly good news: Those reservoirs are used for flood control as well as storage, which means the risk of flooding increases since there’s nowhere else for the water to go. The water can’t be diverted to the large reservoirs, either, since the system that’s used to move water around the state isn’t designed for quick, heavy events like these atmospheric rivers, and building out a system that can do so will take massive investments of time and money.

Reason 3: Climate change is making snowpack melt earlier

The reservoirs are also only part of the water puzzle in California. Just as important is snowpack, or the accumulated snow on mountains, which acts like a natural water-storage system and provides about 30 percent of the state’s water.

The atmospheric rivers are bringing snow to the Sierra Nevada mountains, but the snow line is moving higher and higher as climate change intensifies, meaning there’s less snow overall, and the snow doesn’t last as long as it used to.

A snow survey conducted by the DWR earlier in the week showed the storms that hit California in December brought a significant amount of snow with them, but the question is whether that snow will last through the year. A 2022 January snow survey came back with the seventh-highest measurements on record for that location, but by April 1, much of that snow had disappeared, leading to the third-lowest measurements on record for the same spot. Losing that snow early means it won’t be available during the summer months, when the water is most needed.

Atmospheric rivers like the ones hitting California this winter are going to keep hitting the state. As climate change continues to transform the water realities of the West, Diffenbaugh said, the state will likely have to retool its infrastructure to capture more stormwater from those events and reduce its reliance on the steadily disappearing snowpack.

“We have a lot of water infrastructure systems that are really sophisticated,” said Diffenbaugh. “Part of the challenge is updating those really well-developed systems to be resilient to and prepared for the challenges that this new climate provides.”