Dorsey says his son, now nine, told him he was worried about getting into the water - and that Rivera had said, "don't be silly!".
Dorsey said he immediately got into his car and drove 145 miles to Lake Piru, where Rivera and their son had been swimming.
"Something he's said over and over is that he was trying to find a life raft, and there was a rope, but there was a big spider on the rope, and he was too scared to throw it," Dorsey told People.
Speaking to People, the 41-year-old actor said that "the last thing she said was his (her son's) name, and then she went under, and he didn't see her anymore".
Local police said they believe that after saving her son, Rivera did not have enough energy to save herself.
