Stillwater Sciences was contracted by the California State Coastal Conservancy to describe historical riparian vegetation distribution and composition and analyze the primary physical factors responsible for shaping riparian vegetation conditions in the Santa Clara River mainstem (mouth to Ventura/Los Angeles County line) and major tributaries (Piru, Sespe, and Santa Paula creeks). The report characterizes the likely historical (pre‐1900) extent and form of riparian vegetation within the lower Santa Clara River to provide a potential reference condition for restoration planning, as well as the relationship between selected physical processes and the distribution of riparian plant species within the study area. The analysis is used to elucidate linkages between physical processes and the establishment and maintenance of riparian vegetation and assist in predicting how riparian vegetation may establish and persist on restored floodplains within the study area.