Downtown Walnut Creek is home to the Northern California Family Center where, judging by the sign on the door, they also have a runaway hotline.
From their website:
The Northern California Family Center is a non-profit counseling center that has been serving the mental health needs of Contra Costa County since 1978. Our staff consists of dedicated professionals who provide experienced clinica care and assessment for a wide range of personal and family problems. Our Crsis Home Program serves the runaway youth population of Contra Costa County. Our Long Term Foster Care Program serves children who are foster children who are waiting for possible reunification with their parents, or maybe removed from their parents due to abuse or neglect. We serve children and adolescents, individual, parents, couples and families
Located at 1621 N. Broadway, across the street from the downtown Walnut Creek library, you need to go to the end of a side walkway to the very end to get there. It is not easy to find. They were closed when I went there, not sure whether it’s closed permanently or not. Their main office is in Martinez.