“The cause of death is drowning, and the manner of death is accident,” the Ventura County’s Medical Examiner said.
The 33-year-old, best known for playing cheerleader Santana Lopez in Glee, went missing during last Wednesday’s boat trip with her son at Lake Piru.
After a long search, divers found her body on Monday, prompting an outpouring of love and sympathy from her co-stars.
Rivera was first reported missing last Wednesday, after she had rented a pontoon boat with her four-year-old son at Lake Piru, north-west of Los Angeles.
The boy told police she pushed him back into the boat before disappearing beneath the surface.
On Monday, Ventura County Sheriff Bill Ayub suggested the lake’s currents could have caused difficulties.
“The idea perhaps being that the boat started drifting – it was unanchored – and that she mustered enough energy to get her son back onto the boat, but not enough to save herself,” he said.
The child was found alone and asleep in the boat by a leasing agent after it was not returned. He was wearing a life jacket, but his mother was not.
Rivera’s body was found exactly seven years after her Glee co-star, Cory Monteith, was found dead in a Vancouver hotel room of an overdose of heroin and alcohol.