California faces rising flood risk as climate change intensifies, expert tells Assembly hearing

Senior fellow Jeffrey Mount of the Public Policy Institute of California told the California State Assembly at an informational hearing that climate change is increasing the frequency and intensity of major flood events and that the state’s present approach to flood planning is rooted in historical hydrology rather than what climate models project.

Mount said the state faces rising economic risk from flooding and cited modeling that places the scale of a worst‑case event at roughly $1 trillion in damages. He emphasized that ‘‘risk’’ in his remarks referred to the economic consequences of flooding — the probability of large events combined with the cost when they occur.

Mount laid out the basic mechanics: a warmer atmosphere holds more moisture, warmer oceans feed stronger atmospheric rivers, and those rivers are California’s principal ‘‘floodmakers.’’ He described four distinct flood types Californians face — fluvial (riverine), pluvial (rainfall/urban drainage), debris flows and coastal flooding — and said each requires different management approaches. ‘‘We are planning for the past,’’ Mount said, criticizing reliance on historical hydrology to set standards such as the federal ‘‘100‑year’’ flood mapping.

Mount summarized recent losses and near misses, noting that even a relatively mild year produced an estimated $5 billion to $7 billion in damages in 2023, and that models of the 1861–62 ‘‘great flood’’ suggest an event of that scale could become far more likely under current warming trends. He and other witnesses also warned that federal forecasting and modeling capacity is under strain; Mount said some core federal modeling groups face staffing and funding uncertainties that could undermine national forecasting capabilities.

On solutions, Mount stressed a mix of structural and nonstructural approaches: levees, bypasses and dams remain essential, but nonstructural measures such as land‑use planning, hazard mitigation, emergency response and groundwater storage (managed aquifer recharge) are cost‑effective complements. He said groundwater storage is ‘‘the number one opportunity’’ for water management tied to flooding, but that infrastructure and water‑rights and permitting questions limit how much can be recharged during very large storms.

Mount also raised financing as a major challenge. Citing prior PPIC work, he estimated annual statewide needs for flood investment to be in the low billions of dollars — substantially more than current spending — and called flood management a ‘‘fiscal orphan’’ because few Californians pay a dedicated ‘‘flood bill’’ analogous to monthly water service charges. He urged policymakers to update planning and funding to account for a nonstationary climate.

Mount concluded by urging lawmakers to prioritize prospective modeling and to prepare the state for more frequent extreme flood events rather than relying on past patterns.