The Ventura River, located in western Ventura County in southern California, United States, flows through an eponymous narrow valley with steeply sloped sides. The smallest of the three major rivers in Ventura County, the Ventura River main stem begins at the confluence of Matilija Creek and North Fork Matilija Creek, from the Pacific Ocean. Matilija Creek is Ventura River's most significant headwater and tributary, and adds to the length of the river's main drainage. San Antonio Creek, the river's second most significant tributary, joins the river from the east halfway to the ocean; Coyote Creek enters the river from the west and Cañada Larga Creek from the east a few of miles downstream. In its final stretch, the Ventura River flows through the Ventura River estuary, which extends from about the 101 Freeway bridge to the Pacific Ocean.