Flooded road near Camino Del Este in Mission Valley. Photo via @NWSSanDiego Twitter

Californians have suddenly turned their attention from drought to flooding

The future likelihood of a series of huge atmospheric rivers in California, a so-called ARk storm scenario, seems to be a certainty. Atmospheric rivers channel moist tropical air towards the West Coast, where mountains condense it to rain and snow. Over the last few weeks, California has suffered through a sneak peak of its devastating potential.  

Opinion logo

In late December of 1861, weeks of snow and rain from a huge ARk storm caused flooding from Oregon and Idaho to Mexico. The new settlers did not listen to the Indigenous peoples of California who knew that winter meant moving away from the river.

State government had to be temporarily relocated from Sacramento to San Francisco. The California Supreme Court made the move permanently. The Central Valley became an inland sea, and flooding was severe in Southern California. One percent of the state’s population died. 

These megastorms occur about once every 150 years. Climate change will intensify them.

Flood control reservoirs already line the Sierra Nevada foothills, including Shasta, Oroville, Folsom, New Melones and others. Some reservoir space is emptied each fall to make way for potential oncoming floods, reducing the value of the reservoir for hydroelectric generation, water supply, recreation and cold water storage for fish. Reservoir operators can minimize (but not eliminate) dumping valuable water if no major storm is predicted.  

But Sierra Nevada and similar Southern California flood control reservoirs like Prado and Seven Oaks cannot store enough floodwater to sufficiently reduce the effects of atmospheric river megastorms. The reservoirs will fill, but continuous flood flows will pass through as if the reservoirs were not there.

Gigantic new flood control reservoirs could theoretically be built. But the costs would be in the tens of billions of dollars, and the reservoirs would serve little purpose for decades since they would have to be emptied at the start of each flood season. It’s unlikely that the Legislature or Congress would invest in such a flood control system. 

Indeed, at least $3 billion in levees and floodwater bypasses are needed just to prevent major flood damage in the Central Valley from storms that are expected to occur much more often than megastorms.

Can flood waters be diverted into “off-stream” storage reservoirs for later use? Not really. The proposed giant Sites Reservoir could divert only a small percentage of the water expected in the Sacramento River in even moderate flood events. The value of such reservoirs is largely in their water supply benefits.

Still, much can be done to prepare.

First, California needs to increase investment in flood plain acquisition and expansion, and prevent the urbanization of flood-prone areas. Staying out of harm’s way is the best idea. 

Flood water bypasses help protect the Sacramento Valley and can recharge groundwater. The San Joaquin Valley urgently needs a similar system. It’s likely too late to build them in highly urbanized parts of Southern California.  

Second, property owners who are at risk only from a megaflood should be encouraged to purchase flood insurance. For property outside the “100 year” flood zone, it would be a small annual investment to cover the damage that is bound to occur.  

Third, locally managed evacuation drills should be held in areas where the flood risk is highest, such as Sacramento and areas near the Los Angeles and Santa Ana Rivers. A megastorm will require evacuation of millions of people in the Central Valley and parts of the Bay Area and Southern California. The public needs to be prepared.

Californians have spent billions of dollars to prepare for earthquakes and catastrophic wildfires. Recent quakes and fires are often on our mind, and leaders are reacting appropriately.  

But Californians have largely forgotten the death tolls and huge property losses of previous deadly floods, and even larger floods are likely to come. They will affect all Californians, and require greater investment in flood preparation, insurance and evacuation planning.

Gerald Meral is the director of the California Water Program at the Natural Heritage Institute. He has previously served as deputy director of the California Department of Water Resources, deputy secretary of the California Natural Resources Agency, and a staff scientist at the Environmental Defense Fund. The author wrote this for CalMatters, a public interest journalism venture committed to explaining how California’s Capitol works and why it matters.