Rivera went missing on Wednesday after renting a boat with her four-year-old son at Lake Piru, California.
The Ventura County Sheriff’s office has now stated that the effort to find the actress is being considered a “search and recovery operation”.
Officers believe that Rivera drowned, and no foul play is suspected.
Officer Chris Dyer said: “The latest is that we’re switching efforts into a search and recovery operation. That doesn’t change anything in the efforts that we’re putting forth to locate her but it changes the title, in a sense.
“We’re presuming that an accident happened, we’re presuming that she drowned in the lake. There are no signs of foul play, no signs that anything went wrong besides a tragic accident.
“The goal is still to bring Ms Rivera home to her family so they can have some closure.”
According to local reports quoting the sheriff’s department, Nivera rented a boat at Lake Piru at about 1pm local time. She and her son were swimming, and he was wearing a life vest, but she was not.
The alarm was raised after Rivera failed to return the boat, which was found drifting with her young son, Josey Hollis Dorsey, sleeping alone on board.
The 33-year-old star is best known for her portrayal of the cheerleader Santana Lopez in all six series of the musical comedy, Glee.
More recently, she appeared in the TV series Devious Maids, as well as embarking on her music career, releasing the single Sorry in 2013, which featured her then-boyfriend Big Sean.