The most recent – Jones Tract, in 2004 – illustrates a danger that worries people throughout California. The delta supplies much of the state’s fresh water. But when Jones Tract flooded, it “swallowed” enough water to reverse the flow of the delta, sucking in salt water from San Francisco Bay and tainting the delta’s fresh water. The harvesting of fresh water was halted for 10 days.
In a worst-case scenario, levee failures during a storm could cause many islands to collapse at once, and the delta could “gulp” in enough salt water to shut down freshwater supplies to 20 million Californians for months.
Ongoing subsidence worsens the risk. “The more volume you have below sea level, the more [saline] water can flow onto those islands,” says Dr. Deverell. “That’s one of the driving concerns.”
Subsidence is not just a California problem. Wetland drainage has caused some parts of Florida to deflate by three feet or more. And New Orleans could not have been devastated to the same degree by hurricane Katrina in 2005 were it not for 150 years of drainage that caused parts of it to sink as much as 20 feet.
In many instances, the problem has been papered over by a montage of historic myopia, denial, and mythology, say some experts. Consider the granddaddy of cases, the Netherlands.