The death of Guadalupe Cortes, 26, was ruled accidental from drowning with hypothermia -- low body temperature that can produce unconsciousness -- as a secondary cause, coroner's spokeswoman Deputy April Skalland said.
A toxicology test for drugs or alcohol in her system will take about four weeks to complete, Skalland said.
Cortes drove her 1999 Jeep to a parking lot in Highlands Park at 8500 Highway 9 near Ben Lomond on Sunday to join two male friends for a swim in the river that runs along the park's eastern border.
"She'd taken some of her clothes off to go swimming," Skalland said.
The water in the river was cold, as low as about 45 degrees, she said.
At around 12:30 p.m., when the two men got out of the water to leave, Cortes told them she wanted to remain behind in the water, Skalland said.
One of the two men reported her missing at 9 a.m. the next morning and deputies from the Santa Cruz County Sheriff's Office search and rescue team went to look for her.
At about 3:30 p.m., deputies found Cortes' partially clothed body in the river about an eighth-mile from where the men last saw her, Skalland said.
The sheriff's department is asking anyone who witnessed or has information about this incident to call the investigations division at (831) 471-1121 or leave an anonymous tip at (831) 454-5995.