The death certificate also confirmed Rivera was buried at Forest Lawn Memorial Park in Los Angeles last Friday.
An autopsy conducted earlier this month had already confirmed that the actress had accidentally drowned in the lake during a boat trip with her four-year-old son, Josey, who was found alone on the vessel hours later.
Rivera's body was located in the same area where she went missing, concealed by shrubbery.
The Ventura County Medical Examiner's Office said the findings from an autopsy released today determined "the cause of death is drowning and the manner of death is accident".
"The autopsy findings are consistent with a drowning and the condition of the body is consistent with the time that she was submerged," a statement from the Medical Examiner's Office said.
Rivera's four-year-old son Josey told investigators his mother boosted him onto their rented boat after they had been swimming together before she "disappeared".
"No traumatic injuries or disease processes were identified at autopsy," the statement added.
"There is no indication from the investigation or examination that drugs or alcohol played a role in the decedent's death."
Police believe Rivera, "mustered enough energy to get her son back onto the boat but not enough to save herself," Ventura County Sheriff Bill Ayub said after her body was found.
