Immigration and the Poverty Rate

Robert Samuelson argues that the U.S. poverty rate has remained as high as it is simply because many poor Hispanics have immigrated here:

Consider: From 1990 to 2006, the number of poor Hispanics increased 3.2 million, from 6 million to 9.2 million. Meanwhile, the number of non-Hispanic whites in poverty fell from 16.6 million (poverty rate: 8.8 percent) in 1990 to 16 million (8.2 percent) in 2006. Among blacks, there was a decline from 9.8 million in 1990 (poverty rate: 31.9 percent) to 9 million (24.3 percent) in 2006. White and black poverty has risen somewhat since 2000 but is down over longer periods.

Only an act of willful denial can separate immigration and poverty. The increase among Hispanics must be concentrated among immigrants, legal and illegal, as well as their American-born children. Yet, this story goes largely untold.

Of course, many of these poor immigrants are nonetheless richer than they were in their country of origin.

I wonder: How much has immigration, via this direct change in the population distribution, affected statistics like median household income?