The author says that immigration is a 'powerful stream, one that can only be addressed at its source." What is the meaning of this statement?

I think the author is using a metaphor of a stream or river to describe the raw power of immigration from Mexico. The author feels that to address the huge issues involved, governments must go to the source of the stream beginning in Mexico. Here one can find all the reasons, potential, fears and dreams that power this stream of people flowing into America.