Sea level rise—driven principally by melting continental ice and expansion of the oceans as they warm—is a significant threat for those living along the coast. Adapting to increases in coastal erosion and flooding—and managing the broad impacts of sea level rise on the state’s water systems—will be expensive and disruptive for millions of Californians.

A recent study by the US Geological Survey took a sobering look at future risks from rising seas. The authors describe how storm surge, very high tides (popularly known as king tides), and large waves during storms can amplify the effects of sea level rise. They simulated coastal erosion and flooding over the next two centuries and found there could be dramatic increases in damages during modest storms, and catastrophic flooding during rare, large storms. These impacts are projected to appear by the middle of the 21st century, at a scale likely to significantly affect the state’s economy.

Beyond costly efforts to protect against coastal erosion and ocean flooding, coastal areas will also need to manage increased inland flooding. Large storms can amplify the effect of rising seas by bringing greater storm surge, particularly if these coincide with king tides. Storm runoff flowing down coastal creeks will meet rising waters caused by storm surge and tides, exacerbating flooding. This issue is particularly acute in parts of the Bay Area (including much of Silicon Valley) and low-lying areas in Southern California. Major new efforts to manage runoff and protect existing homes and businesses will be needed.

Sea level rise will also affect water management in other ways. One area is wastewater treatment. Throughout coastal California and particularly in the San Francisco Bay Area, wastewater and stormwater treatment takes place in facilities that are currently at or near sea level. The Bay Area alone is home to 30 such facilities that are vulnerable to damage by rising seas and storms.

Water supply will also be affected. Many coastal aquifers will see increases in salinity as sea levels rise and storms push saltwater farther inland. But the most significant impact on supply is likely to be associated with changes in the Sacramento‒San Joaquin Delta.

The Delta lies at the head of the San Francisco Bay estuary and the confluence of the Sacramento and San Joaquin Rivers. More than 25 million Californians and 3 million acres of farmland rely on the Delta for a portion of their water supplies. These exports are managed by large state and federal projects that pump water from the southern Delta.

Sea level rise affects the Delta in two ways. First, to keep its waters fresh enough so that cities and farms can use it, a certain amount of fresh water has to flow out of the Delta and into the San Francisco Bay. If the outflow is insufficient—whether due to drought, reduced inflows from upstream water use, or high export pumping—tides push saltwater into the Delta, making its water unusable.

The second way sea level rise will affect the Delta concerns the 1,100 miles of levees that line its channels. Levees hold water back from Delta islands, whose lands are now well below sea level due to sinking soils. Failure of these levees—particularly in the western Delta—can lead to changes in the way tides circulate water in the Delta. These changes amplify the increases in salinity from sea level rise alone; together, they put additional pressure on water supplies.

Addressing the many effects of sea level rise on California water management requires extensive planning and costly investments in infrastructure. The US Geological Survey study shows that these impacts are not in some distant future, but are likely to become significant in just a few decades. The good news is that state and local officials are starting to work on this problem, which will take many years to address.