The Santa Clara River flows approximately 100 miles from its headwaters near Acton, California, to the Pacific Ocean south of the city of Ventura. Flowing east to west through a beautiful valley formed between the Santa Susana Mountains and the Transverse Ranges, the river crosses lands with many uses and local governments. Threats to the ecological health of the river include urban development, channelization, oil spills, and stormwater runoff pollution. The river supports many populations: the human communities which dot its banks, plus a great variety of flora and fauna. Extensive patches of high-quality riparian habitat, totaling over 4,000 acres, are present along its banks. The Santa Clara River was selected by American Rivers as one of the nation's ten most endangered rivers for 2005.
The Santa Clara is Southern California's last major river system that remains in a mostly-natural condition. There are few levees and only one diversion dam. The river channel retains its dynamic nature. For most of its length, it flows through natural and agricultural landscapes, including some of the best remaining riparian woodland in the southland - a region that has lost all but 5% of its historic river woodlands to farms and urbanization. In contrast, the Los Angeles and Santa Ana Rivers, which rival the Santa Clara in size, were long ago largely converted to concrete channels.
The biological resources of the Santa Clara River are impressive. The riparian forest next to the river is home for a host of bird species, including the endangered least Bell's vireo. The unarmored three-spine stickleback, a small endangered fish, inhabits the river's upper reaches. The river estuary supports the western snowy plover, least tern and tidewater goby, all federally listed as endangered. Overall, 14 resident bird species along the are listed as endangered or of special concern; and 6 plant species are endangered or candidates for listing.
Of special note is the importance of the Santa Clara River to the long-term survival of the federally-endangered southern steelhead. This iconic fish, which can reach two feet in length when fully grown, migrates upstream from the ocean to several river tributaries to spawn, similar to other salmonids. These tributaries include primarily Santa Paula Creek and Sespe Creek, but other smaller creeks as well. Upstream migration in Piru Creek is currently blocked by the Santa Felicia Dam, constructed in 1954. The Freeman Diversion facility on the main stem remains a major impediment to upstream fish migration; as of 2015, modifications to the facility to allow fish passage are being evaluated and construction is anticipated to follow over the next few years.