LOS ANGELES, OCT. 8 -- Nearly a year after enactment of the new federal immigration law, California's urban economy remains heavily dependent on undocumented alien workers, seriously challenging expectations that the law would provide jobs for unemployed Americans.

Government and labor officials here say a significant loophole in the Immigration Reform and Control Act has left many undocumented workers on the job, with no immediate prospect of detection and deportation.

Although it has been widely reported that some illegal aliens have lost jobs because of their status, the vast majority remain employed. And a new flow of workers across the Mexican border indicates that many still see economic opportunity here.

As much as 80 percent of southern California's 100,000 garment-industry workers may be undocumented, said Miguel A. Machuca, western region organizing director for the International Ladies Garment Workers' Union (ILGWU). He has investigated 14 garment factories in the last three weeks.

But Duke Austin, spokesman for the Immigration and Naturalization Service, said an earlier estimate of 60 percent undocumented workers was "a little high."

He said the law has left significant numbers of illegal aliens at work, "living on the edge" in hope that overburdened INS investigators will not find them.

Despite evidence of a large complement of undocumented workers, several garment contractors have complained of a worker shortage and unsuccessfully sought permission to import sewing-machine operators from the Philippines.

At a meeting here last week with INS and Labor Department officials, contractors were told that they might be able to import workers on seasonal contracts of six or seven months, which most do not want, but should not expect approval of renewable contracts of a year or more.

"I think I'll go sit down and cry for a while," said Garth Ward, president of Fashion Sportswear Inc. and the Garment Contractors Association of Southern California Inc. He said he could use more than 100 workers in his El Monte plant but has found only 60 capable of doing stitching work.

ILGWU's Machuca denied that a garment-worker shortage exists but said one is possible if the thousands of undocumented workers here are forced to leave.

Their unique status rests on a provision added to the law before passage to prevent sudden, catastrophic harm to industries that depended heavily on illegal immigrants, according to Gerry Mackie, special projects director of the Federation for American Immigration Reform (FAIR).

The law grants legal status to any immigrant who can prove residence in the United States since before 1982.

To flush out undocumented workers not covered by this provision, employers were required to demand proof of legal residency from all employes. And employers were made subject to fines or imprisonment if they knowingly employed undocumented workers.

The documentation requirement was amended to exempt employes hired before Nov. 6, 1986, the day President Reagan signed the bill into law. Thus factories, restaurants and hotels nationwide have retained significant numbers of illegal immigrants who arrived after 1981 and do not qualify for amnesty but are not required to prove legal status to their employers, according to INS, business and labor officials.

Even after increasing salaries from the minimum wage, $3.35 an hour, to $4.50, garment-factory managers say they have not attracted new, qualified workers with legal status.

Machuca said the attempt to import workers is a hedge against future shortages, not current ones. The 14 factories he surveyed had few machines without operators, he said, adding that one factory that succeeded in winning Labor Department approval for 40 Filipino workers on a seven-month contract "didn't have any room to put them."

The INS is seeking new appropriations, some of which have passed Congress, to increase the number of investigators from about 600 to 1,600 and of Border Patrol officers from about 3,200 to 4,100, Austin said. They would be used to search for undocumented workers still employed in this country.

Some critics have suggested that even such increases would not be enough to discourage immigration from Mexico while jobs remain available here.

In the agricultural industry, debate rages throughout the West over the new law's impact. Restrictions on hiring undocumented farm workers do not become fully effective until late next year, but growers in Washington state complained earlier in the season that fears of deportation severely cut the number of available pickers.

Last week, however, the state was temporarily glutted with pickers when worried apple growers advertised on radio in California and Mexico, then had to delay the harvest because of unseasonably hot weather.

Washington state Agriculture Department spokeswoman Mary Beth Lang said that cooler weather has come and that the expected apple harvest of 83 million boxes might strain available labor resources.

But supporters of the new law have pointed to the incident as evidence that grower complaints are groundless. "Overall, it is abundantly clear that the allegation of an agricultural labor shortage was a scam," said Richard Estrada, research and publications director for FAIR.