TIJUANA, Mexico — Under a gleaming midday sun, in the fetid drainage canal that is the Rio Tijuana, Valentin Gomez plunged a hypodermic needle of crystal meth just under a friend’s collarbone.

The man’s eyes rolled back in ecstasy, or relief. Which one was hard to tell, as the drug did its work and he was in no condition to chat.

This is how Mr. Gomez passes his days along with many of the hundreds of others living in hovels, lean-tos and simply on the pavement in the canal — a catch basin for the drug addicted and the deported, sometimes both, just shy of the United States-Mexico border.

It is known as “El Bordo,” a Spanglish version of “border.” For many living here, it may as well be limbo.