In Southern California, generations of immigrants are creating an evolving definition of "American." Multi-American is your source for news, conversation and insight on this emerging regional and national identity.

Forty percent of the country's nearly 1.3 million Vietnamese immigrants reside in California, concentrated in Orange, Los Angeles and Santa Clara counties, according to a report published Monday by the non-partisan Migration Policy Institute.

The immigrants in those three counties alone make up about a quarter of the Vietnamese population for the entire country.

"That geographic concentration is really fascinating," said Jeanne Batalova, a senior policy analyst at the institute.

Batalova said immigrants' high numbers in California are largely due to secondary migration. When Vietnamese started to arriving in the U.S. in large numbers in the mid-1970s after the end of the Vietnam War, refugee resettlement agencies placed them across the United States.

But, "with time, as social networks and family connections formed in the community, a lot of Vietnamese refugees migrated to a few parts within the United States," Batalova said.

Southern California, Texas and Louisana's Gulf Coast were magnets because of the warmer climates and job opportunities in fishing and agriculture, Batalova said.

Linda Trinh Vo, an Asian American studies professor at the University of California, Irvine, said that a particular draw was the Little Saigon district in southern California.

"People started to realize that there were places where there were temples or churches, markets and restaurants and newspapers and entertainment, particularly those who were limited English speakers ," Vo said.

Vo said it's not a surprise that Vietnamese immigrants settled in California communities where there were already Chinese and Japanese.

"It’s just easier for the refugees to resettle in areas where it was friendly and there was less discrimination," Vo said.

Today, Vietnamese make up the sixth-largest immigrant group in the country, having grown five-fold from about 231,000 in 1980.

But that growth rate has slowed considerably in recent decades. Through the 1990s, refugees came with their families — hence the fast population growth — but newcomers from Vietnam these days do not have refugee status. Rather, they are largely members of the extended family of immigrants in the U.S. Petitioning for a relative to get a green card under the banner of family reunification can take years.

Batalova said that the number of arrivals from Vietnam make up 25 percent of the immigrant population in the U.S., compared to 75 percent who came before then.

Vo said that population growth will slow as it depends on second and third-generation Vietnamese who are having smaller families than their parents.