In the years before California’s civil engineers got around to confining the Sacramento River, it often spilled over its banks, inundating huge swaths of the Central Valley. Sometimes the floodwater would stand for a hundred days at a time. The botanist William Henry Brewer, writing in 1862, after a season of torrential rains, described the valley as “a lake extending from the mountains on one side to the coast range hills on the other.” The water was so deep, he reported, that cargo steamers could navigate it. “Nearly every house and farm over this immense region is gone,” Brewer wrote. “America has never before seen such desolation by flood as this has been, and seldom has the Old World seen the like.” Half a century later, to solve the problem, California built a number of flood-control systems, including the Sacramento Weir, a series of forty-eight hand-operated gates placed strategically along the Sacramento and American Rivers. When the waters rose, they would now be shunted into an unpopulated expanse known as the Yolo Bypass, a floodplain roughly equivalent in size to twenty Central Parks.
This winter, for the first time in a decade, and after five years of a crippling statewide drought, the Yolo Bypass is submerged again. Situated at the heart of the Pacific Flyway, a great migratory corridor stretching from Alaska to the tip of South America, the area teems with sandhill cranes, California brown pelicans, and dozens of other bird species. But its estuarine tranquillity is deceptive. In the past five months—the wettest since record-keeping began, in 1895—California has experienced widespread hydrological chaos. In January, after a series of heavy rainstorms, water managers activated the Sacramento Weir, filling the Yolo Bypass. In February, emergency releases from Anderson Lake Dam, in Santa Clara County, flooded hundreds of homes in San Jose. The rain also caused landslides near Big Sur, washing out several roads and bridges and leaving about four hundred people stranded. But it was the near-failure of the dam at Lake Oroville, three and a half hours north of San Francisco, that made the scale of the crisis clear. Oroville is the state’s second-largest reservoir but arguably its most important; it feeds the California Aqueduct, which supplies drinking water to twenty-five million residents across greater Los Angeles and irrigates millions of acres of Central Valley farmland.
Less than a year ago, Lake Oroville was a vivid symbol of the state’s prolonged drought. Aerial images showed a landscape of spider-webbed mudflats and desiccated tributaries as the reservoir fell to levels not seen in almost forty years. Starting in January, though, the lake rapidly filled to capacity. To prevent the water from breaching the dam, engineers began discharging it at a rate of 2.7 billion gallons per hour—about the same flow as at Niagara Falls. The frothing cascade, with its countless bubbles acting as tiny jackhammers, hollowed out a cavernous pit in the concrete spillway. The engineers diverted the flow to an earthen emergency spillway, but the torrent rapidly chewed away at that, too. With the integrity of the dam under threat, close to two hundred thousand local residents were evacuated. (A total failure of the structure, according to one water manager, would have sent a thirty-foot wave tearing through communities downstream.)
Ask most Californians, however, and they’ll tell you that the chaos is in service of a greater good. As of last week, according to the National Drought Mitigation Center, more than three-quarters of the state is out of the drought, with barely one per cent falling into the “severe” category—almost the reverse of the situation at this time last year. Already in 2017, many parts of California have received more than twice their average annual precipitation. The numbers would seem to paint a picture of watery salvation. But Peter Gleick, the chief scientist at the Oakland-based Pacific Institute, told me that one year of heavy precipitation, even a record-breaking one, will not undo the most serious repercussion of the drought: a severe deficit of groundwater. For years, Central Valley farmers have drawn liberally from the region’s aquifers to compensate for reduced supplies from canals and aqueducts. When a large enough volume of groundwater is pumped away, the land can slump like a punctured air mattress. Areas along the valley’s western edge have sunk by nearly thirty feet since the nineteen-twenties, and in some places the local infrastructure—roads, bridges, even the California Aqueduct itself—is at risk. Farmers and municipalities have responded by digging deeper wells, but such measures seem to be prolonging the inevitable. In Tulare County, south of Fresno, where groundwater overdraft has been particularly severe, the number of reported well failures has continued to climb, almost quadrupling since 2014, in spite of last year’s above-average precipitation and this year’s deluge.
Climate change is a significant contributor to the problem. As the Stanford climatologist Noah Diffenbaugh noted in 2015, California’s reservoirs, aqueducts, and canals are vestiges of a cooler, less drought-prone past. The state’s model of water storage is snowpack-dependent, meaning that it works properly only when the bulk of the water in the system is locked up in mountain snow. These days, though, more precipitation falls as rain than as snow, placing stress on the reservoirs. And even though this year has seen record snowpack—a hundred and eighty-five per cent of the average, as of March 1st—California has also experienced dozens of so-called rain-on-snow events, which further hasten the melting. Meanwhile, warmer temperatures are projected to shift the snow line to higher altitudes, dramatically shrinking the over-all size of the state’s snow reservoir. At current rates of warming, the Sierra Nevada could lose a quarter of its snowpack by the middle of the century, according to the California Department of Water Resources.
Sudden swings between drought and flood have been part of California’s climatic history for a long time, Diffenbaugh told me, but those swings now stand to become more extreme. “This is exactly what climate scientists have predicted for at least the last thirty years,” he said. The solution, Gleick said, is to prioritize the aquifers—and quickly, because severe land subsidence can permanently eliminate storage space. “Unless a massive effort is made to both reduce overdraft and to artificially enhance recharge rates, California’s groundwater will continue to decline,” he wrote in an e-mail. Not only are there fewer regulatory hurdles involved in underground water-banking than, say, permitting a new reservoir or desalination plant, but the costs of groundwater storage are far lower than these other options, scientists at Stanford’s Water in the West Program recently found.
Last Friday, the state’s Department of Water Resources reopened the patched-up concrete spillway at Lake Oroville. Hundreds of millions of dollars’ worth of repairs remain, but water managers must make room in the reservoir for the spring melt. At the moment, there is no large-scale engineering system that would allow the huge surge of surface water currently flowing across California to be delivered to the Central Valley’s aquifers. And more to the point, perhaps, the state lacks the sorts of regulations that would make such a system viable; a law passed in 2014 requires that government agencies “achieve sustainability” in how they apportion groundwater, but not until 2040. Ultimately, Gleick told me, California won’t pursue artificial recharge until it can keep better track of who is using what. “It’s like putting money into a bank account that anyone else can withdraw,” he said. “Until it’s monitored, no one will make a deposit.”
*This sentence was updated to clarify the name of the institution affiliated with the groundwater study.
