I have looked up the video of the hoover dam construction, and before they built it the River Colorado would overflow into (and created) the Salton Sea. With all the publicity the Salton Sea has been getting lately, and the possible volcanic activity below it, then not so far away is the San Andreas Fault, I do think it could be a chain-reaction type scenario that would be assisted by HARP. This is the absolute worst case of course.
