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Just the FACTS

Floods in California

  • California is a flood-prone state.
    Most of California is vulnerable to floods. Every county has been declared a flood disaster area multiple times. One in five Californians and more than $580 billion worth of structures (including their contents) are vulnerable. Transportation, energy, water supply, and sewer networks are also at risk of disruption. The state’s many valleys—particularly the Central Valley—are susceptible to rivers overflowing. Lowland coastal flooding is common when high tides coincide with large, storm-driven waves; tsunamis also strike occasionally. Winter storms and intense summer thunderstorms cause flash floods and debris flows, most notably in Southern California, the deserts, and areas recently burned by wildfires. And storms commonly cause local urban flooding because inadequate drainage systems and impermeable surfaces—such as streets and parking lots—lead stormwater to pond quickly and deeply.
  • Flood risk is growing, but funding for flood management is not.
    The number of people and value of properties in California’s flood-prone areas is growing, raising both the economic risk and threat to public safety. Yet the state has a large funding gap for flood management. Traditional flood infrastructure—such as levees and dams—is costly to build, operate, and maintain. At minimum, the state needs to spend $34 billion to upgrade flood management infrastructure. To accomplish these upgrades within 25 years, the state would need to spend $1.4 billion per year—roughly twice the current rate of flood management investments. Bonds approved since the mid-2000s are helping, but they fall far short of meeting funding needs. Flood management is a “fiscal orphan,” with no plan for filling the gap.
  • Risk of flooding is high throughout California

    Figure 1

    SOURCE: California Department of Water Resources and US Army Corps of Engineers (2013). “California’s Flood Future.”

    NOTE: The figure shows population and structures in the 500-year floodplain—the area susceptible to floods so large that they have just a 0.2 percent chance of occurring in a given year. Levees protect much of this area from a “100-year flood,” which has a 1 percent chance of occurring in a given year. Population is adjusted to 2010 levels. Value of structures is based on the depreciated replacement value of structures and their contents in 2010 dollars.

  • Requiring flood insurance is an effective way to manage risk.
    There are less expensive ways to reduce floods than building flood infrastructure. The National Flood Insurance Program makes federal flood insurance available to vulnerable homes and businesses, reducing the economic costs of flooding when it occurs. The federal requirement that homes within the so-called 100-year floodplain (which is prone to more floods) carry insurance is also a disincentive for communities to develop in high-risk areas. In 2007, California passed legislation that doubled the standard of protection for urban areas in the Central Valley.
  • Climate change requires new approaches.
    Many of California’s flood management systems were designed for the hydrological conditions of the past century. California’s climate is changing, with larger winter storms and more extreme high tides. These changes, plus rising sea level, will make many current flood management systems obsolete within decades, requiring major investments in new infrastructure and new approaches to reducing flood risk.
  • Modern flood management can achieve multiple benefits.
    Historically, the goal of many flood projects was to route floodwaters to the sea as quickly as possible to reduce damage. The legacy of this approach—thousands of miles of concrete- or rock-lined river channels—is seen throughout California. The result: communities are cut off from their rivers, significant environmental harm takes place, and, in some cases, the risk of flooding increases. Modern flood management seeks to integrate floods into overall water management goals. This includes taking advantage of floodwaters to restore rivers and wetlands, while reducing the intensity of floods. It can also use floods to recharge aquifers and improve the quality of surface water supplies.
  • Damaging floods are common in California

    Figure 1

    SOURCE: California Department of Water Resources (2013). California’s Flood Future, Attachment C.

    NOTE: The figure depicts floods that caused significant property damage and/or loss of life. Floods named above are the largest and most damaging. Thicker lines indicate floods that crossed over two calendar years.

Sources: For funding estimates, see Hanak et al. Paying for Water in California (PPIC 2014).

Supported with funding from the S. D. Bechtel, Jr. Foundation


Jeffrey MountJeffrey Mount
Senior Fellow



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