MEXICALI, Mexico — Arrests of illegal migrants trying to cross the southern U.S. border have plummeted to levels not seen since the early 1970s, according to tallies released by the Department of Homeland Security last week, a historic shift that could reshape the debate over immigration reform.

The Border Patrol apprehended 327,577 illegal crossers along the U.S.-Mexico border in fiscal year 2011, which ended Sept. 30, numbers not seen since Richard Nixon was president, and a precipitous drop from the peak in 2000, when 1.6 million unauthorized migrants were caught. More than 90 percent of the migrants apprehended on the southwest border are Mexican.

The number of illegal migrants arrested at the border has been dropping over the past few years but appears to be down by more than 25 percent this year.

Experts say that Border Patrol apprehensions are a useful marker for estimating the total flow of illegal migrants, though imprecise because the U.S. government has no idea how many are not caught. But coupled with census and labor data from both countries that show far fewer Mexicans coming to the United States and many returning home, it appears that the historic flood of Mexican migration north has slowed dramatically.

“We have reached the point where the balance between Mexicans moving to the United States and those returning to Mexico is essentially zero,” said Jeffrey Passel, a senior demographer at the Pew Hispanic Center, whose conclusion was shared by many migration experts.

 Read More at The Washington Post By Nick Miroff and William Booth, The Washington Post