Is this the Bracken Brae bridge in Boulder Creek? Courtesy of Santa Cruz Museum of Art and History (MAH.) Front of postcard:
Bracken Brae bridge. Courtesy of MAH. Undated. (Front of postcard.)
Back of postcard of Bracken Brae bridge. Boulder Creek? Undated.
Santa Cruz Blogazine 
I don’t think this was any of the Bracken Brae bridge unless it preceded the 1905 wood bridge and was slightly south of the current bridge location