Building a fence would solve the illegal immigration problem
A border fence wouldn’t affect immigrants who enter legally and overstay their visas
Building a fence along the entire Mexican would be impractical and ineffective, and it would not affect immigrants who enter the United States legally and stay here after their visas expire. Ralph Basham, who was appointed by President George W. Bush as the commissioner of Customs and Border Protection in 2006, has written that fencing off the entire Mexican border was “one of the dumbest ideas I heard” while serving as CBP commissioner. Basham continued:
It is critical to recognize that fencing (even with barbed wire, electrification, and possibly a moat filled with alligators) is not a solution, it is only a tool. There's a fundamental misunderstanding about what a physical barrier--even the triple-layer fencing in San Diego--actually does or doesn't do for the agency charged with building fencing and securing the border. All it really does is buy you time where a crosser could otherwise quickly escape or assimilate. None of the fencing is impenetrable. People will eventually dig under it or cut through it or go over it, but it gives you enough time to respond and apprehend them..
Furthermore, The Wall Street Journal reported in 2013 that 40 percent of undocumented immigrants in the U.S. entered the country legally, but overstayed their visas. A full border fence would not affect them.