The myth originated, when an unlikely, yet possible, rumor was spread about an old dam engineer, who was fixing a filter in the dam vent. Apparently, he was pulled into the vents and crushed by the machines. The vent's valve was shut at the time and so no water, nor machinery, could have sucked him through. To make the story less believable, they say that when they opened the valve, the water streaming down was tinted red with his blood.
Another myth about the Los Santos Dam is about a woman, who was stabbed on top of the dam, and was thrown into the water. Myth hunters have taken a submarine (via Cargobob) to the dam and have been searching for the body ever since they found out, however, nothing has been found as of yet.