Think invasion of the boat-motor-snatchers; think fields gasping for irrigation; think slimy creatures choking drinking-water pipes; think hydropower sputtering to a stop; think of ''Jaws'' the size of thumbnails threatening Lake Powell.

The possibilities sound creepier than an environmental horror flick, ever since wildlife officials confirmed this week they've found mussel larvae at Lake Powell.

"These things are incredibly scary," said Larry Dalton, Utah's new Aquatic Nuisance Species Coordinator.

"They may represent the single largest impact to the aquatic environment that I've seen in 36 years."

Thursday, four government agencies - the Glen Canyon National Recreation Area, the Utah Division of Wildlife Resources, the Arizona Fish and Game Department and the U.S. Fish and Wildlife Service - announced they found three microscopic larvae in water samples taken from Wahweap Marina and near the Glen Canyon Dam.

The agencies are not sure how widespread the mussels might be or if there is an established population. They don't know how the mussels might change the Lake Powell ecosystem or spread to other premiere Utah recreation spots, such as Bear Lake and the Jordanelle Reservoir.

In fact, the DNA and microscope tests that were used to identify the mussels do not tell for sure whether the Lake Powell mussels are quaggas or their close cousins, zebra mussels.

The state has estimated it will cost $17 million a year to deal with quaggas if they spread statewide, not including the impacts of fishing. The fear has prompted the creation of Dalton's month-old position, which will eventually oversee an agency with about 40 people, and an emergency expenditure by the state of about $1 million through next spring.

"If we wait a year," he said, "it will be too late."

Once the mussels are established, they are virtually impossible to eradicate.

Utah officials have kept a wary eye on mussel developments for years, Dalton noted.

One study showed Great Lake facilities spent $69 million over seven years to deal with mussels. A Canadian report referred to on the Web site 100thmeridian.org described a price tag of more than $172 million because of mussel infestations at eight hydropower stations, 86 municipal plants and 67 industrial facilities.

"It's just been killing those guys back there," said Dalton, "so we have been worried."

Then, in January, quagga mussels were found downstream on the Colorado River, in Lake Mead. "That," he said, "put us on high alert."

In late June, quagga mussels were found on a boat at Wahweap Marina that had been in infested waters. It was decontaminated before the boat was allowed to launch.

Water samples were taken from the two Lake Powell locations on July 19 and 30. Three of the five samples showed "extremely small" numbers of larval mussels. A good contrast is Lake Mead, where the infestation is believed to have started two years ago and where similar samples contain hundreds or thousands of mussels.

Baby mussels are about the size of a grain of sand and take about a year to reach full size. Adults are the size of a thumb nail.

All spring, the government agencies with roles at Lake Powell have pushed mussel prevention. Signs posted around the lake urge boaters to inspect their crafts if they have been in other waters in the past 30 days and to use 140-degree pressure washers to clean up the boats, plus a five-day period of drying, before launching into Lake Powell.

State Sen. Curt Bramble, a Provo Republican who owns a houseboat on Lake Powell, said it was "inevitable" that quagga mussels would come to Utah.

He said the findings announced Thursday "could be a serious concern," far more important than its affect on recreation.

Yet, he added: "I'm not sure the research is in and what the ramifications will be."

Kitty Roberts, superintendent of the Glen Canyon National Recreation Area, noted that that the search for better answers is already under way.

"Additional samples have been collected from Lake Powell and are being analyzed for quagga and zebra mussels," she said. "In the coming weeks, more samples will be collected from various location around the lake to determine if mussels are present in other areas."

Dalton said a biologist donning diving gear had inspected 150 boats Thursday. There are another 1,000 to go.

fahys@sltrib.com

Quagga quagmire

A freshwater mollusk indigenous to the Dnieper River in Ukraine has invaded waterways of the United States, altering the food chain and rapidly colonizing on hard surfaces, such as the hulls and outboards of watercraft.

* Range: Found in January in Lake Mead, downriver from Lake Powell. First U.S. sighting was in the Great Lakes in 1989.

* Treatment: Chemical controls pose environmental risks; chlorination has been applied routinely. Biological controls are being researched.

* Prevention: Recreationists are urged to clean their watercraft to curb spread of the mussel.