The death of the "Glee" star was also considered an accident.
"The results of the autopsy are consistent with drowning and the condition of the body is consistent with the time it was submerged. No traumatic injury or pathological process was identified during the autopsy," said the office in a press release.
Rivera, 33, disappeared last week from Lake Piru in Ventura County in southern California. She went to the lake that afternoon and rented a pontoon boat with her 4-year-old son, authorities said.
His body was found Monday morning.
Authorities said there was no indication that drugs or alcohol had played a role in his death, but toxicological tests would be carried out.
"She was our friend"
After learning of his death, Rivera's former co-stars and friends paid tribute to social media.
On Tuesday, the creators of Fox's "Glee" said in a statement that they were "heartbroken" by the death of the actress, who played Santana Lopez on the program for six seasons.
In a statement, Ryan Murphy, Brad Falchuk and Ian Brennan recalled that Rivera was no stranger to the series when she was chosen for the show. In fact, it only had a few lines.
"But it didn't take us more than an episode or two to realize that we had been fortunate enough to find one of the most talented and special stars we would ever have had the pleasure of working with," said their communicated.
"Naya could play, she could dance, and she could sing (could she ever sing!) She could nail a joke as well as she could crush you with an emotional scene. She was a joy to write, a joy to to lead and a joy to be around. "
The statement also recognized the significant connection that viewers of the show shared with the character of Rivera, who was a lesbian. They said that the "humor and humanity" that she brought to her on-screen relationship with "Heather Morris" Brittany was "her greatest" Glee "legacy."
"It was one of the first times that an openly lesbian high school relationship was seen on network television and Naya understood what 'Brittana' meant for the many young women who saw themselves represented on television for the first time ", the statement said.
"Naya has always ensured that Santana's love for Brittany is expressed with dignity, strength and with pure intentions. Naya has always been moved by the girls who contacted her to tell her how love of Santana and Brittany affected them. "
Their statement added, "Naya was more than just an actor on our show - she was our friend."
Murphy, Falchuk and Brennan said they were "in the process of" establishing a college fund for Rivera's son.
CNN's Sarah Moon, Cheri Mossburg and Amir Vera contributed to this report.
