Atmospheric rivers are relatively narrow bands of moisture that ferry precipitation across the Pacific Ocean to the West Coast and are key to California’s water supply.

They are commonly referred to as the “Pineapple Express” because of their origins in tropical regions. While atmospheric rivers are necessary to keep California’s water reservoirs full, some of them are dangerous because the extreme rainfall and wind can cause catastrophic flooding and damage. Their presence has been likened to the West Coast version of the hurricane hazard posed to the southeastern United States.

With the dual threats of aging levees and anticipated rising sea levels, floodplains — low areas along waterways that flood during wet years — are increasingly at the forefront of many public policy and water issues in California.

Adding to the challenges, many floodplains have been heavily developed and are home to major cities such as Sacramento. Large parts of California’s valleys are historic floodplains as well.