Although no one knows exactly when it will happen or precisely how severe it will be, the volcanic eruption geologists believe is building in the Eastern Sierra near Mammoth Lakes could seriously jeopardize thirsty Los Angeles' largest source of water.
Two-thirds of all the water used in the nation's second-largest city comes from a far-flung network of streams, reservoirs and aqueducts that stretches for hundreds of miles down the backbone of the Sierra Nevada.
A recent swarm of earthquakes near Mammoth Lakes, believed to be caused by magma fracturing rock or pressurizing liquid four miles underground, has heightened concern that it is only a matter of time before the region experiences another violent episode in its long and tumultuous volcanic history. And that raises troubling questions about the potential impact of another eruption on the city's chief source of water.
"We know it would be bad," said Winston K. Wu, a geotechnical engineer with the Los Angeles Department of Water and Power. "Our water supply up there will not be usable depending on the nature of the eruption."
But when and precisely where such an eruption will occur is uncertain. "The magma is still five to six kilometers below the surface," Wu said. "You just don't know. It could be years, decades, centuries away."
Geothermal activity along Hot Creek south and east of Mammoth Lakes has been causing water quality problems for years by introducing arsenic and other chemicals into the water supply. Extensive treatment is required at several points downstream to reduce the level of the toxic element in the city's drinking water.
Despite mounting evidence of seismic unrest and volcanic precursors in the area, including a more than 2-foot rise in the earth's surface during the past 18 years, it was only last week that DWP geologists recommended forming a committee to update the department's contingency plan "in regards to this ongoing threat of a volcanic eruption."
In fact, Assistant General Manager James F. Wickser acknowledged that the DWP's emergency plan for the Owens Valley anticipates a major earthquake, but not a volcanic event. He promised that the plan will be revised.
"Clearly the volcanic threat was not on the horizon in the early 1970s," said Wickser, who is in charge of the city's water system. "It could have dramatic effects, far more pronounced than an earthquake."
