By Paul Burke and Jane Wagner-Tyack

Californians are experiencing an El Niño period, the effects of which will vary depending on where we live. One possible effect is flooding, and tens of thousands of Californians live in regions where protections against flooding are inadequate. For one thing, California's dams are aging. CalMatters reported in 2024 that of over 1500 dams across the state, 42 are restricted in how much water can be held in their reservoirs. They have an average age of about 100 years and need repairs.

Construction rules for buildings allow 1% risk of flood damage every year, 26% risk during a 30-year mortgage. Inside the 1% line on flood maps, mortgages require flood insurance, which is wise anywhere and also covers mud flows. But risks are bigger and much more widespread. The UK (p. 2) and Holland (p. 49) design for floods with a chance of 1 in 1,000 per year.

People elevate buildings to avoid floods and mold. We reduce floods by holding upstream flows in stormwater basins, beaver ponds, reservoirs, and by speeding downstream flows. Managed aquifer recharge (Flood-MAR) on farms and open land, which is intended to replenish overdrafted groundwater basins when high-flow events occur, will also reduce unwanted flooding.

Inland, the main risk is rain. The Great Flood of 1861–1862 made the Central Valley a 300-mile inland sea. Floods this big are expected every 140-200 years. Coastal risks include storm surges, waves and sea level rise. Coastal property, including highways and railroads, disappears unless there are wave-absorbing seawalls. The Coastal Commission disallows seawalls for property built since 1977.