When it floods in California, the culprit is usually what’s known as an atmospheric river—a narrow ribbon of ultra-moist air moving in from over the Pacific Ocean. Atmospheric rivers are also essential sources of moisture for western reservoirs and mountain snowpack, but in 1861, a series of particularly intense and prolonged ones led to the worst disaster in state history: a flood that swamped the state. The megaflood turned the Central Valley into an inland sea and washed away an estimated one in eight homes.

What would happen if the same weather pattern hit the state again? Los Angeles Times reporter Louis Sahagun explains in a new report that one dam in Los Angeles County is critically unready for such an event. If the dam fails, it could affect up to a million people in a dozen cities.

And UCLA climate scientist Daniel Swain explains why disastrous flooding events like the one in 1861 are not only becoming more likely as the planet warms, but may have already been a more frequent occurrence than previously thought.