Rosie, who was a type 1 diabetic, was slipping in and out of consciousness when the woman found her near the Lone Oak picnic area at about noon today, Kahn said.
After finding the girl, the woman ran to the picnic area and flagged down an officer, Kahn said.
Police have not yet had a chance to talk to the teen to find out why she went to the park, but Kahn said it didn't appear that any foul play was involved.
Rosie reportedly left her house on Ardmore Drive at about 8 a.m. Wednesday to ride her bicycle to school at Berkeley High School, but never arrived.
She is believed to have instead ridden into Tilden Regional Park, police said.
Rosie's parents reported her missing Wednesday evening and Kensington police officers spent the night looking for her.
“It was an extraordinary effort by Sgt. Rickey Hull, who spent 27 hours as the incident commander and wouldn't go home until he found her,” Kahn said.
Chief of police Greg Harman also stayed up all night looking for Rosie, Kahn said.
Kahn also thanked the Contra Costa County Sheriff's office volunteer search and rescue team, which launched a full-scale search. Kahn said their efforts were particularly appreciated because Kensington's police department is made up of only about 10 officers. The search and rescue team gave them the manpower to cover much more ground.