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, 16.2 mi from the Pacific Ocean.
 Matilija Creek is Ventura River's most significant headwater and tributary, and adds 17.3 miles 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.
U.S. Route 101 (US 101) in the state of California is one of the last remaining and longest U.S. Routes still active in the state, and the longest highway of any kind in California.
 US 101 was also one of the original national routes established in 1926.
 Significant portions of US 101 between the Los Angeles area and the San Francisco Bay Area follow El Camino Real, the historic road connecting the former Alta California's 21 missions.
Lake Casitas is a man-made lake in Ventura County, California, created by the construction of Casitas Dam on Coyote Creek, two miles (3 km) before it joins the Ventura River.
 Santa Ana Creek and North Fork Coyote Creek also flow into the lake.
 The dam was constructed of earth-fill and was completed in 1959.
 It is 279 ft and was built by the United States Bureau of Reclamation.
 The lake has a capacity of 254000 acre.ft .
 The dam was built as part of the Ventura River Project and was strengthened in June–December 2000 as a seismic improvement to help withstand earthquakes greater than 6.5.
 The project provides drinking water and water for irrigation.
 A secondary benefit is flood control.
 In the center of Lake Casitas is 2 km Main Island, whose peak rises more than 500 ft from the lake surface.
Rancho Santa Ana was a 21522 acre Mexican land grant in present day Ventura County, California given in 1837 by Governor Juan Alvarado to Crisogono Ayala and Cosme Vanegas.
 Rancho Santa Ana was located inland in the Ventura River Valley on the west side of the Ventura River across from Rancho Ojai which was granted in the same year.
 Rancho Santa Ana encompassed present day Lake Casitas and Oak View.
Seaside Park is the traditional name of an event venue located at 10 West Harbor Boulevard in Ventura, California.
 The 62 acre is the home of the Ventura County Fair.
 Trade shows, concerts and other events are held throughout the year at the fairgrounds.
 The beach front site, just downcoast of the mouth of the Ventura River, also includes Surfers' Point, known for its point break that produces distinctive waves.
 Eugene P. and Orpha Foster envisioned a miniature Golden Gate Park when they donated the original 65 acre to the County of Ventura.
 They wanted a beautiful gateway to Ventura, where families could walk and picnic, play tennis, and enjoy family outings.
 Most of the site is now owned by the State of California and is managed by 31st District Agricultural Association.
Casitas Dam is an earthfill dam across Coyote Creek that forms Lake Casitas in Ventura County, California near Oak View, California.
 The dam is located two miles (3 km) above the junction of Coyote Creek and the Ventura River.
 Water from the Ventura River is diverted to Lake Casitas as well.
 The 334 ft structure was built by the U.S. Bureau of Reclamation.
 It is a key part of the Ventura River Project.
 Construction was completed in 1959.
 Lake Casitas has a 254000 acre.ft storage capacity and provides irrigation and municipal uses for the Casitas Municipal Water District.
Emma Wood State Beach is a California State Beach in Ventura County, California.
 It is located on the western side of the city of Ventura and borders the estuary at the mouth of the Ventura River.
Rancho Cañada Larga o Verde was a 6659 acre Mexican land grant in present day Ventura County, California given in 1841 by Governor Juan B. Alvarado to Joaquina Alvarado.
 The grant extended along Sulphur Canyon Creek, east of the Ventura River, between Ventura and Ojai.
 To the west, across the Ventura River was Rancho Cañada de San Miguelito; to the north Rancho Ojai; and to the south and east Rancho Ex-Mission San Buenaventura.
Ventura, officially the City of San Buenaventura, is the county seat of Ventura County, California, United States.
 European explorers encountered a Chumash village, referred to as Shisholop, in Ventura while traveling along the Pacific coast.
 The eponymous Mission San Buenaventura was founded nearby in 1782 where it benefitted from the water of the Ventura River.
 The town grew around the mission compound and incorporated in 1866.
 The development of nearby oil fields in the 1920s and the age of automobile travel created a major real estate boom during which many designated landmark buildings were constructed.
 The mission and these buildings are at the center of a downtown that has become a cultural, retail, and residential district and visitor destination.
Rancho Cañada de San Miguelito was a 8877 acre Mexican land grant in present-day Ventura County, California, given in 1846 by Governor Pío Pico to Ramón Rodríguez.
 The name means valley of San Miguelito.
 The grant extended between the Pacific coast and the west bank of the Ventura River.
 On the east bank of the Ventura River was Rancho Cañada Larga o Verde and to the south was Rancho San Miguel, Rancho Ex-Mission San Buenaventura, and present day Ventura.

Question: What California highway is the longest in the state and is associated with the Ventura River?
Answer: U.S. Route 101