A diversity of aquatic and terrestrial organisms populate Sonoma Creek and its riparian zone. Winter-run Chinook salmon ("Oncorhynchus tsawytscha"), Delta smelt ("Hypomesus transpacificus") and steelhead ("Oncorhynchus mykiss") are the most prominent fishes. Anadromous fish movements in Sonoma Creek have been studied extensively not only in the mainstem Sonoma Creek, but in some of the tributaries. These investigations have demonstrated a historical decline in spawning and habitat value for these species, primarily due to sedimentation and secondarily to removal of riparian vegetation since the 1800s.