ESCONDIDO - Escondido officials extended a hastily called boat ban on Lake Wohlford from five to six days Wednesday, as state, regional and local officials expressed shock at how fast a tiny, but expensively invasive mussel has raced through California waterways.

The quagga mussel is a fingernail-sized Ukrainian shellfish that required billions of dollars to clean up in the Great Lakes and along the East Coast. It has been running Southern California officials ragged since January, and local water officials revealed Tuesday that it has been found at San Vicente Reservoir in Lakeside.

Hours later, Escondido city officials said they may have found quaggas in Lake Dixon, and they were temporarily banning boating at Lake Wohlford in the hope of keeping the pest out of that reservoir.

San Diego County Water Authority officials said they're creating a group to develop a plan to control the mussels, which attach themselves to everything from docks, boats and engines, to pipelines, valves and other equipment and could foul pumping stations and waterways.

The mussel has also been discovered in Riverside's Lake Skinner, but not in the massive Diamond Valley Lake Reservoir near Hemet.

The quagga was first discovered in California's water supplies in January in reservoirs of the Colorado River, which helps supply drinking water to nearly 18 million Southern Californians in six counties, including San Diego. Officials say they think the shellfish may have been spread to the river's reservoirs by contaminated water in boats.

Ever since, water and wildlife officials have been scurrying to find and kill the mussel and its water-current-riding microscopic larvae in the hope of keeping it from gaining a foothold in Southern California

But despite those efforts, which included twice draining and super-chlorinating the 242-mile Colorado River Aqueduct, the mussel has spread all the way south to San Diego County.

Stunned officials, meanwhile, say the shellfish's numbers are increasing tenfold every two months.

"If somebody were to put this scenario together for the way they've spread in Southern California, we would have said, 'No way,' " Mike Guisti, a senior biologist with the California Department of Fish and Game, said Wednesday. "There's no way I would have believed it."

Several sources have reported that officials in the Great Lakes region and on the East Coast have spent billions of dollars to clean up quagga mussels - which have no native predators to keep their populations in check. In Michigan's massive Lake Huron, the quagga has changed the ecosystem, eating so much phytoplankton that it is starving out salmon and other fish there.

Regional water officials said Wednesday that it was impossible to tell how much it could cost to clean it up here. Metropolitan Water District, Southern California's main water supplier, said that it had already spent $2.28 million, mostly in chlorine, to fight the quagga.

"It's going to be expensive, no question," said Rick DeLeon, head of Metropolitan's quagga control program.

But DeLeon said that most of the damage the mussels created in the Great Lakes was to hydroelectric plants.

DeLeon and Gary Eaton, director of operations for the San Diego County Water Authority, said that the mussel would not be able to reproduce enough to choke off the large water pipelines that deliver Southern California's life-sustaining imported water supplies.

"We have large-diameter pipelines," Eaton said. "But we have pumps and they would cause us additional maintenance headaches."

Meanwhile, officials in San Diego registered different reactions to Tuesday's quagga discoveries. City of San Diego officials, who operate Lake San Vicente, said they did not plan to increase boat inspections or ban boating, but would wait for the Water Authority's group plan.

But Escondido officials immediately shut down boating at Lake Wohlford, even though no other reservoirs had apparently done so. Escondido officials said they ordered the ban in part because they feared people who fished in San Vicente could spread the mussel to Wohlford in their still-damp boats.

Escondido ranger Jay Cowan said he discovered what could be quagga mussels at Lake Dixon in a dive to check underneath the lake's dock. State Fish and Game officials are expected to examine the mussels today. Escondido also operates Wohlford. Dixon gets its water in part from Water Authority pipelines delivering Colorado River water. Wohlford gets water from Lake Henshaw.

Cowan said Water Authority officials showed city and water agency personnel photos of quaggas and the damage they could do earlier this year. He said one photo was of a beach on the East Coast where the tiny crustaceans were piled high - "like broken glass" - along the waterline.

"We're kind of spooked," Cowan said. "I mean, if these quaggas are half as nasty as they're telling us, its something we really don't want in our lakes."

Water agencies in Southwest Riverside County said they were worried, too. But unlike San Diego County, which has 20-plus reservoirs that could be affected, Riverside's Western, and Eastern, municipal water districts do not operate any reservoirs. Eastern spokesman Peter Odencrans said that the mussel could still get into the agency's systems through its connections with Metropolitan's pipelines.

"At this point, we haven't found anything in our system," he said. "We're just keeping our fingers crossed."

Meanwhile, officials rejected suggestions that maybe state and regional agencies were not moving fast enough to fight the mussel - a fight that has now moved from the hope of eradicating the mussel to learning how to contain it.

Cowan said the state Department of Fish and Game had done a great job of sending information to boaters throughout the state.

DeLeon said Metropolitan had already built stations to inject thousands of gallons of larvae-killing chlorine into its aqueduct system.

Part of the problem, officials said, is that the quagga seemed to be reproducing even faster than they expected.

"Everybody is moving as fast as we can, but it's moving faster," said Guisti of the Department of Fish and Game.

- Contact staff writer Gig Conaughton at (760) 739-6696 or gconaughton@nctimes.com.