Via the Reason blog: The 1976 U.S. Supreme Court case of U.S. v. Martinez-Fuerte established an exception to the Fourth Amendment, allowing the federal government to establish roadblock checkpoints within 100 miles of U.S. borders to stop people and search for illegal immigrants and smuggling.
The ACLU notes that 190 million people live within 100 miles of U.S. borders, providing this helpful map. (Although Lake Michigan is entirely within U.S. boundaries, by treaty Canada is allowed full navigation rights to the lake--so it's not clear if that 100-mile boundary would actually be as in the ACLU's map around Lake Michigan.)
There are currently 33 checkpoints in operation within the boundary area. Here's some video footage of one of them in Arizona:
(Via Checkpoint USA, which has numerous videos of interactions at one of these temporary checkpoints.)