LOS ANGELES -- An invasive, rapidly multiplying Ukrainian mussel could cost water agencies and ratepayers billions in fouled waterways and pipelines and has prompted the shutdown of the Colorado River Aqueduct for a second time, Southern California's main water supplier said Friday.

Officials from the Metropolitan Water District said Friday that the 10-day shutdown was expected to start July 20 and would not affect any Southern California water deliveries, or diminish supplies.

Spokesman Bob Muir said the invasive quagga mussel that first forced a shutdown of the aqueduct in January after it was discovered had spread further and multiplied faster than hoped.

The mussel has been an expensive problem in the Great Lakes for years, costing billions in cleanup.

Metropolitan delivers water to nearly 18 million Southern Californians in six counties, including San Diego County.

Muir said that when the agency drained the 242-mile aqueduct in January, workers found mussels and microscopic larvae in the first 20 miles of the canal and in densities of about two to 10 per square meter. He said divers and workers had now found the mussel halfway down the aqueduct, and in densities up to 500 per square meter.

He said emptying the aqueduct would kill the mussels by drying them out, and that chlorine "shocks" would be used to control populations when the aqueduct was refilled.

"We understand the chance of eradicating them is relatively low," Muir said. "We're trying to control their spread."

State fish and game officials discovered the mussels -- which are similar to the invasive zebra mussel -- in Lake Mead, one of the main reservoirs along the Colorado River, in January.

Scientists believe the mussel, which is often smaller than a quarter but can grow to slightly larger than a silver dollar, was brought to the United States in the 1980s in the ballast water of transoceanic ships. The mussel is roundish and its color patterns can vary wildly. They reproduce rapidly, have no natural predators, eat voraciously -- robbing other fish and water animals of food -- and attach themselves to submerged surfaces such as piers, pilings, water intakes and screens. It can also attach itself to pipeline and filter openings that would restrict water flow. On top of everything else, the mussel apparently is not good to eat because it accumulates pollution in its tissues up to 300,000 times its environment. It makes water smell and taste bad because it emits phosphorus in its waste.

Fish and game officials have stepped up alerts to recreational boaters and begun monitoring them in the hope of preventing the mussels' microscopic larvae from being spread.

Officials said that, as of last month, nearly 30,000 trailer boats in the Colorado River area had been inspected and only about 2,400 of them had water that needed to be drained.

Gary Eaton, director of operations for the San Diego County Water Authority -- Metropolitan's biggest customer -- said he didn't think water officials were surprised by the spread of the invader. However, he said local officials were worried that the mussel could eventually spread all the way through Metropolitan's system to the water agencies that buy its water. That could mean more cleanup, and more costs affecting more ratepayers.

"Absolutely," he said. "That's why everybody is participating (in education and monitoring)."

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