What's needed to discourage illegal immigration into the United States has been known for years: Enforce existing law.

Amazingly, that is happening now – to some degree. This trend may already be shrinking the flood across the Mexican border and have a modest positive impact on job prospects for "native born" Americans during the present economic slump.

Immigration prosecutions reached an all-time high in March, reports the Trans­actional Records Access Clearinghouse (TRAC), a data research and distribution group at Syracuse University in New York. Using data from the Justice Department, it calculates that prosecutions were up 49 percent from February and 72.7 percent from March of last year. This highly unusual surge is filling up US detention centers and jails.

March prosecutions numbered 9,360. That's small compared to the estimated 12 million undocumented immigrants in the US. Nonetheless, "It's working," says Mark Krikorian, executive director of the Center for Immigration Studies, a Washington think tank that would like immigration levels reduced considerably.

The hike in prosecutions stems from an expansion of "Operation Streamline" last year by the Department of Homeland Security (DHS). Under the effort, undocumented aliens caught by border guards are no longer simply steered into "air-conditioned buses," as Mr. Krikorian puts it, and driven back across the border to try crossing again. Instead, they are charged with crimes and detained.

The most common charge is "reentry of a deported alien." But there are at least nine other crimes, including entry of an alien at an improper time or place. The result is detention until trial, usually before US Magistrate Courts. A typical sentence is one month, and then "removal."

That time under detention, DHS hopes, will deter these aliens from trying again and discourage others from even trying. Border crossings have plunged, especially in areas where those caught are put into lockups. Border patrol apprehensions along the Mexican border were down 17 percent to 347,372 between October 2007 and March 2008, compared with the same period a year previous.

In addition to the border measures, immigration officials have stepped up well-publicized raids on meatpacking firms and other companies hiring undocumented workers. States, including Arizona, also have been cracking down on employers of illegal immigrants, a crime often harder to prove in court than illegal border crossing.

Krikorian guesses that in the past, 800,000 to 900,000 illegal immigrants successfully entered the US every year, and about 400,000 left voluntarily or were deported each year – a net growth of about 500,000 illegal immigrants a year.

If current moves to restrain illegal immigration trim that growth by 100,000 to 200,000 immigrants, it should have some effect on the nation's labor supply, notes University of Chicago economist Jeffrey Grogger. He's coauthor of a paper calculating that a 10 percent increase in the supply of a particular skill group caused by higher immigration prompted a reduction in the wages of similarly low-skilled black men by 4 percent between 1960 and 2000, lowered their employment rate by a huge 3.5 percentage points, and increased their incarceration rate by almost a full percentage point.

So, presumably, fewer low-skilled immigrants could gradually induce more work for low-skilled native Americans.

The weaker economy and labor market should also prove less of a draw for immigrants, mostly undocumented ones, over the next year or two, cutting the flow by "several hundred thousand" per year, reckons a new study by four economists with Goldman Sachs, a prominent Wall Street investment bank. That would reduce labor-force growth by 0.2 to 0.3 percentage points compared with the growth rate in the past few years – and thus the potential for greater economic growth. The Goldman Sachs economists would welcome an increase in the flow of immigrants as a way to absorb the excess inventory of homes troubling the housing industry, and mitigate the "incipient pressures on the federal budget due to the impending retirement of the baby boom generation."

But a study by Northeastern University's Center for Labor Market Studies in Boston attributes the "unprecedented" levels of legal, illegal, and temporary immigration as a factor underlying the "devastation" in the job scene for America's teens and young adults over the past seven years. That's especially the case for males with no schooling beyond high school and youths from low-income families. Summer seasonal jobs as a proportion of all jobs are at the lowest level now in the past 30 years.