The boy told investigators that he and his mother had been swimming in the lake and that he had gotten back into the boat but she had not, according to the Sheriff’s Office.
Sergeant Donoghue said the next day that the boat was found in the northern part of the lake, in an area where the water is about 30 feet deep. Search efforts had been hampered by limited water visibility of less than a foot during the day, and trees and debris under the surface.
The search-and-rescue mission shifted into a search-and-recovery effort as the day wore on. The authorities said they believed Ms. Rivera had accidentally drowned.
The authorities said that Ms. Rivera had previous experience boating on the lake, a reservoir with a campground in Los Padres National Forest, about 55 miles northwest of Los Angeles. The recreational area, which reopened on July 1 with limited activities because of the coronavirus, features boat rentals, kayaking and paddle boarding. It was closed during the search.
On Monday, Sheriff Ayub said Ms. Rivera’s family had expressed gratitude for everyone who assisted in the efforts to find the actress. He also thanked the many agencies that had helped in the search.
Ms. Rivera made her acting debut as a 4-year-old on the 1991 CBS sitcom “The Royal Family.”
She also had guest appearances on other shows including “The Fresh Prince of Bel-Air” and “Family Matters.” But Ms. Rivera was probably best known for her role as a sharp-tongued singing cheerleader, Santana Lopez, on “Glee,” which ran on Fox from 2009 to 2015. The campy show followed the friendships, relationships and everyday dramas of a high school glee club, and Ms. Rivera appeared in 116 episodes.