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Climate and Environment

Get Ready — Winter Wonderland Of Snow Heightens Risk Of Bigger Spring Floods (Landslides, Too)

Geese walk along a concrete embankment alongside a river
Geese walk along the L.A. River bank after the recent rains.
(Samanta Helou Hernandez
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The climate crisis is not only bringing hotter summers, but it's also making our mild California winters more dangerous.

This year’s snow and rain is great for our water supply, but too much of it poses dangerous flood risks. As global carbon pollution changes the climate and causes more intense storms and severe swings in weather, this is a big challenge for adaptation and keeping communities safe.

A Changing Water Cycle
  • Maybe you remember the concept of the water cycle from elementary school: when water from lakes, rivers and the ocean evaporates, it condenses to form clouds, then falls back to the earth in the form of rain or snow. 

  • But as our climate heats up, water on the landscape, such as mountain snow and rainfed streams and lakes, evaporates more quickly and dries out soils faster. That leaves less water for humans and the animals and plants that rely on that surface water, and also leads to a “thirstier” atmosphere, which then dumps that evaporated water in the form of increasingly intense storms and atmospheric rivers. 

    • That’s why the climate crisis is making California’s drought-to-deluge cycle even more extreme. 

    What causes floods?

    Floods happen when there’s heavy rainfall for a short burst or sustained over a period of time. As the climate crisis makes our atmosphere thirstier, major rain events are becoming increasingly intense.

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    Floods also happen when snow melts too fast — a big worry as the climate crisis drives increasingly wild weather swings and shortens “shoulder seasons” such as Spring.

    This all happened not too long ago: in March 2017, a record season of rain and snow followed what was previously the worst drought in California’s recorded history (sound familiar?).

    That March, record amounts of melting snowpack in the southern Sierra Nevada threatened to flood the Owens Valley, prompting former L.A. Mayor Eric Garcetti to declare a state of emergency. The Sierra Nevada mountains have seen the biggest snowpack gains in recent storms, leading to much of the range being completely out of drought conditions for the first time since Jan. 2020.

    Currently, snowpack levels in the southern Sierra Nevada are 231% of “normal” for this time of year, according to the California Department of Water Resources. In March 2017, the day Garcetti declared the state of emergency in the Owens Valley, snowpack levels were 165% of normal for that time of year.

    While it’s still too soon to tell how severe it’ll be, there’s also a chance of another major atmospheric river and serious temperature increase in the middle of March, said Alex Tardy, a meteorologist with the National Weather Service. That potentially warm system could quickly lead to floods.

    “In this case, we’re concerned about two things: a rapid warmup that could melt snow too quickly and then also rain on top of that or maybe an atmospheric river that can also accelerate and greatly increase the flooding and the runoff,” Tardy said.

    Tardy said this year is even more extreme than 2017, which also broke records.

    “We're seeing these variables from year to year, month to month that are more extreme, and that might be our main indication [of climate change],” he said. “It's not the total number of events, it's not necessarily the type of events, but the severity and the magnitude of some of these events, whether they're dry or wet."

    Floods aren’t the only risk: As mountains and foothills get more and more saturated with water, the potential for landslides grows.

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    And with more rain on the way, mudslides, especially in burn scars, are possible.

    Unprepared infrastructure

    Flood infrastructure, such as the paved-over L.A. River, is generally designed based on the idea of the 100-year flood, which is supposed to have a 1 in 100-year chance of occurring (or 1% chance of occurring per year). But climate change is making these big floods more likely to occur. For example, in the Santa Ana River watershed, which spans from the San Bernardino mountains to Huntington Beach, the likelihood of a 200-year flood event is expected to at least double by the end of the century, according to the federal Bureau of Reclamation.

    A recent study from UC Irvine found that nearly 1 million people in L.A. live in areas that would be significantly affected by a 100-year flood.

    “The Los Angeles River, the San Gabriel River, Compton Creek, Dominguez channel — some of these channels are no longer able to contain what we would call a 100-year flood,” said Brett Sanders, lead author of the study and a civil and environmental engineering professor at UC Irvine.

    While Sanders and his team found nearly 1 million people at significant flooding risks, the Federal Emergency Management Agency (FEMA) says there are only 30,000 Angelenos at risk. That presents a problem for insurance coverage and aid in the case of a major flood, which has happened in L.A. and will happen again — only with a lot more people in the crosshairs and far more powerful storms and flooding driven by climate change.

    And the impacts are unequal. The UC Irvine study found predominantly Black communities in the L.A. Basin are at the highest risk — 79% more likely than white Angelenos to be exposed to waist-high flooding in a 100-year event. Latino communities were 17% more likely to be exposed to severe flooding.

    What's next

    Prepare for the next storm by reading some tips here.

    Climate Emergency Questions
    Fires. Mudslides. Heat waves. What questions do you need answered as you prepare for the effects of the climate emergency?

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