A large clonal patch 30 feet in diameter growing under an open canopy in a riparian forest. California grape is covering perhaps half of the branches, but the plant seems to be festooning its stems onto the grape vines for support.

Leaves between 2 and 5cm wide with 4-6 cross veins (a bit more veins than the Jepson eFlora gives in the key to Cornus:
http://ucjeps.berkeley.edu/cgi-bin/get_IJM.pl?key=10016.