Does this really happen at the US/Mexico border?

I was watching National Geographic and it showed a sort of doentary about the border patrol and the fight against the drug cartel. I had a show that was going on at the same time so i lost some of the details, but the one thing that caught my ear was when an agent told them about about the cartel's easy escape. Apparently (not sure if i remember this right) all the smugglers have to do is p a river in there car ,get out, cross over to the US side and wait until the agents leave so they can collect the drugs and continue on (again, not sure). So my real question would be who has seen it and what did they really say.