THE challenge the United States faces in controlling illegal immigration across its southern border is evident along the dusty levees of the Tijuana River here. In one section, amid the greasewood and Russian thistle, looms a chain-link fence reinforced with steel matting. A US Border Patrol agent leans against a truck on one side of the fence.
On the other side, 15 feet away, are several dozen Mexicans and Central Americans watching him. The moment the green-jacketed officer moves, even if only a few hundred yards away, the aliens scamper over the fence.
``It's a waiting game,'' says H. Randolph Williamson, a 20-year Border Patrol veteran, bouncing along the top of the levee in a four-wheel-drive truck. ``Someone is stationed here 24 hours. If you pulled the person out, people would just pour through.''
Four years after passage of a landmark law intended to control illegal immigration, the number of people slipping across the US-Mexican border is rising sharply.
Critics are seizing on the numbers to buttress their case that the ``sanctions provisions'' of the law, designed to slow illegal immigration, is not working and should be scrapped.
But supporters argue that the law is doing some good and needs only to be modified.
``The trend is continuing to go up,'' says Doris Meissner, a former top Immigration and Naturalization Service (INS) official now with the Carnegie Endowment for International Peace. ``I think you do have to conclude that the flow is about the same as it was before IRCA (the Immigration Reform and Control Act of 1986).''
Through the first 11 months of fiscal 1990, agents along the southern border arrested 955,000 people trying to enter the US illegally - up 25 percent from this time last year. At least one person is believed to get through for every one arrested.
INS officials expect the year-end total to be around 1.06 million, the highest since 1987. That is well below the record years of 1985 (1.3 million) and 1986 (1.6 million) just before the law was passed. But it represents a significant increase after a three-year decline.
Despite the surge, INS officials don't think the sanctions provision of the law, which was designed to curb the border crossings by penalizing employers who knowingly hire illegal immigrants, has been a failure. They argue that the numbers would be much higher without the law.
They blame much of the increase on the proliferation of forged documents that allow many undocumented workers to circumvent the statute. Under IRCA, employers are supposed to verify the status of new workers by demanding documents such as a resident alien's card or Social Security card. Employers are not, however, responsible for verifying their authenticity.
``We think the message has gotten back to sending nations: `Look, if you make it the US, you can get documentation to get work,''' says Duke Austin of the INS in Washington.
Some researchers see more forces at work than this. They note that as long as economic and social conditions remain tough in Mexico, where the vast majority of the illegals come from, a flow will continue.
``Anything is better than the kind of economic crisis they are experiencing in Mexico,'' says Kitty Calavita, an immigration specialist at the University of California, Irvine.
Talks with some of the illegals along the border here underscore their resolve. One group just arrested by INS agents - seven women (one with a seven-month-old child) and a man - journeyed from the interior of Mexico, where a two-year drought has made agricultural jobs even more scarce than usual.
They were heading for Los Angeles, and carry what belongings they have in a few plastic bags. Will they try to slip across the border again? ``Not here,'' says one. Her compatriots laugh.
TO try to reduce the flow, some lawmakers are calling for more border agents and fine-tuning of the 1986 law. Sen. Alan Simpson (R) of Wyoming, one of its original sponsors, recently introduced a bill that would require more barriers to be built in heavily crossed areas and increase the penalties for those who use or accept fraudulent papers. It would also change state drivers' licenses to make them more impervious to fraud and require they be one of a few IDs workers can use to prove eligibility (17 documents are now accepted).
Three GOP lawmakers from California are pushing for 1,000 Border Patrol agents to be added to the 4,000 now along the border. Others want a telephone verification system, in which employers can check the authenticity of Social Security cards.
Immigrant-rights groups and civil libertarians, however, oppose most of these ideas. They would like to see the sanctions provision abolished. They have long argued - and some independent studies verify - that it has led to discrimination against Latinos and others in the job market: Fearful of being fined, some employers shy away from hiring foreign-looking workers.
Back along the levees of the Tijuana River, a vein of sewage and industrial runoff, the Border Patrol has put up stadium lights, which have deterred some illegals. Most have just moved a short distance west. There, even in the glare of an afternoon sun, dozens are lining up.
Ismael Alvardo-Jiminez, who sells sodas and burritos to those congregating, estimates 3,000 to 5,000 people try to spirit through this small area every day.
``A lot of them know about the law,'' says the vendor, toting a cooler. ``But they count on not getting checked for documents or getting hired out of good luck. Many buy false documents.''
Would anything stop them? He shakes his head: ``They are coming because they need to make a living.''