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April 2015 P R A C T I C A L W I N E R Y & V I N E YA R D 71 B O O K E X C E R P T region has seen over the past 150 years. The 1861–62 flood, for instance, was the result of a series of large storms that flowed one after another across the Pacific in what climatologists describe as an atmospheric river. These storms slammed the West Coast — from the Mexican border to Canada — for 43 days, bringing triple the average amount of rainfall to the region and leading to unprecedented flooding. Based on the relatively short historic record, water planners have estimated that a flood of this magnitude may recur perhaps every thousand years. However, paleoclimate records spanning the past 2,000 years indicate that floods of this magnitude, or perhaps larger, have recurred every one to two centuries. The largest of these floods occurred during the Little Ice Age, 400 years ago. This event predated the history of mod- ern California, yet it is interesting to note that Native American residents in 1861– 62 appeared to recognize the early warning signs of these storms and the subsequent floods, and they quickly moved out of the floodplains and up to higher ground. Their long history in the region had given them a deeper under- standing that, though the floodplains could provide abundant rich soils and resources, they could only be safely occu- pied seasonally. The Native Americans knew better than to build permanent settlements in California's Central Valley or delta. Future flood risks California's Central Valley Many regions in the western states are vulnerable to flooding today, despite decades of construction of flood control infrastructure. Modern society has pro- moted growth, with cities sprawling out onto the floodplains, deltas and other low-lying areas. Many of these regions are protected by aging levees that have been shown to fail under larger flood events. Other human activities have added to the flood risk. Take, for example, California's Central Valley: viewed from above, it looks like a giant bathtub— up to 400 miles long and 70 miles wide— with the San Francisco Bay estuary and delta as the drain. Most of California's agricul- ture, and much of its future population growth, will occur there. Early accounts from California history describe win- ter floods, like the 1861– 62 event, that turned the Central Valley into an inland sea. Floods have been a natural feature of the Central Valley for millennia, which is why the soils are so rich and fertile. However, flooding on this scale does not mix well with the interests of a mod- ern agricultural system that depends on regularity and controlled water supplies, or with the inland cities that are now replacing them. Over the past century or so, Californians have prevented these cata- strophic floods through massive hydro- engineering projects. Water managers have essentially taken nearly full con- trol of the timing and amount of water that reaches the fertile lands within the Central Valley. The consequence of this transformation of the natural hydrology, combined with groundwater pumping, soil compaction, and the draining of wetlands, has been that much of this vast region has sunk by several feet and, in some areas, by as much as 28 feet. Today, this sunken region may be more susceptible to flooding than it was in 1861– 62. Moreover, the population of California has grown from 300,000 in 1861 to over 37 million in 2012, a number that is expected to double by the year 2050. Room has to be made for this swelling population in the future,