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When the Levee BreaksLast August, Dettinger made one of his periodic forays to the many streambeds running through Yosemite National Park. This time he brought his 10-year-old daughter Annie along and the two caught a glimpse of the future she probably will experience as an adult. While he waded through creeks to take readings with his flowmeter, Annie trailed close by with a homemade net. She rushed around among pools of evaporating water normally fed by snowmelt into late summer. The hot weather and diminished flow were leaving large trout stranded in shrinking ponds they couldn't escape. Annie scooped up the fish and dropped them into the main stream to give them a chance at survival. The diminishing streamflow witnessed by the Dettingers during one particular field trip does not mean in itself that global warming has come to the Sierra Nevada. Nearly all climate models suggest, however, that the scene is likely to become a more typical sight a decade from now. In the Sacramento delta, the effects are subtler but are there nonetheless.  
The California Bay-Delta Authority, a state government agency to which Dettinger is a consulting scientist, takes salinity readings at stations extending inland from San Francisco Bay up the Sacramento and San Joaquin rivers. For the past decade, salt concentrations at all stations have gradually ticked upward, a consequence of increasing amounts of freshwater being pumped out for use in Southern California, aggravated by recent changes in snowmelt timing. Because winter storms are dumping more rain and less snow in the Sierras, the precipitation is leaving the mountains in torrents as soon as it falls rather than dripping gradually away through spring and summer as snow melts. The rainwater surges oceanward in winter but is gone by spring. Opposing the force of the rivers are rising seas, which encroach further up the delta the higher they get. Scripps has become in Dettinger's words the state's "go-to" entity when officials need up-to-date climate change data, especially when it comes to sea-level rise. Recent projections, according to Scripps climate researcher Dan Cayan, show that sea levels will likely rise by at least 0.5 meters (19 inches) and may rise by more than 1 meter (39 inches) by the end of the century. Even at the lower end, this is so much greater than the historical rate that it will be very difficult for ecosystems to adapt. People will have their hands full as well. "Sea-level rise will pose the greatest threat during episodes when storms coincide with high tides during an El Ni o condition," said Cayan. "And the same storms that pump up sea levels can also produce floods, another threat to delta levees." The effect of the rivers's losing battle with the ocean are multiple and complex. Delta water bound for Southern California is pumped from the delta west of Tracy, Calif. If salt water reaches that point, the water would be virtually unusable without significant treatment. It would have effects on the delta's wetland ecosystems, natural purifiers of water, in ways scientists can scarcely predict. Sea level rise would also add stress on delta levees, which serve to hold back the sea. The levees, built to a lower standard than those in New Orleans that failed during Hurricane Katrina, could cause similar devastation if they collapsed. Bay-Delta Authority analyses suggest this is not an "if" situation but a "when," with a likelihood of failure occurring within 60 years. Besides the flooding that would inundate the state capital in such a scenario, Southern California would be cut off from its Northern California water supply for 18 months another conservative estimate. In response, state officials are considering options ranging from the construction of more dams to the digging of a peripheral canal that bypasses the delta completely. Whatever project is chosen, the course of nature suggests that the state has 25 years to get something in place. "Even if we knew what to do, we'd be cutting it close," Dettinger said. Next page: Is the source of last resort tapped out? |
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