The actress, 33, is presumed to have died at Lake Piru in southern California on Wednesday and a search and recovery operation is under way.
Mystery surrounds how Ms Rivera came to disappear and an adult-sized life jacket was found on her rented boat, while her son Josey Hollis, who is safe and well, was wearing one.
Ventura County Sheriff’s Office senior deputy Miguel Gonzalez said authorities are keeping an open mind and have not ruled out Ms Rivera hitting her head before she drowned.
He told press: “Unfortunately, until the body is recovered we definitely won’t know what actually happened with her, whether it be something medically or if she did have any trauma prior to going down beneath the water.
“And until we recover the body we probably won’t find the answers to those questions.”
Ms Rivera disappeared at Lake Piru, about 56 miles north-west of central Los Angeles. CCTV from the dock shows her boat departing at about 1pm and it was later found drifting in the northern area of the lake at around 4pm, where the focus of the search remains.
Cadaver dogs have joined the operation, police said, while technology including sonar scanning is also being used.
The search is complicated by perilous conditions below the lake’s surface, with visibility as low as 1ft in some areas, while debris including trees are a constant danger to rescuers.
Rivera is best known for starring as a high school cheerleader in Glee.