Almost a week after her disappearance, police confirmed her body had been recovered from the lake.
No other significant factors were listed as having contributed to Rivera's death.
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".
When the boat failed to return on time, its vendor found the vessel drifting in the northern end of the lake with the little boy asleep on board.
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".
After five days, her body was found by authorities with the Ventura County Sheriff Bill Ayub shedding light on what happened in Rivera's final moments.
Rivera, who had experience boating on that lake, played cheerleader Santana Lopez on musical drama Glee for six seasons, from 2009 to 2015.