Photos taken during the at various Walnut Creek Open Spaces and other natural lands in Contra Costa County
Native clematis vines grow into host buckeye tree, Twin Ponds Loop, Shell Ridge, June 20, 2014.
I think this is Clematis lasiantha. There are several prolific vines of this plant growing high into buckeye trees along the creek that begins at Lower Twin Pond. (It's an unnamed creek in the Walnut Creek Open Space map.)