over 5000 B.C.E. has been documented in nearby coastal sites. These prehistoric sites may contain middens, milling stone sites, large villages, cemeteries, and tool making sites. The diversity of natural resources along with the temperate climate with a long growing season produced a lengthy archaeological record of human activity along the coast. Calleguas Creek and the Santa Clara River were populated with many Native American villages as evidenced by archaeological sites such as the Calleguas Creek Site that was listed on the National Register of Historic Places in 1976. Several sites have also been documented at Mugu Lagoon. The numerous archaeological sites in the adjacent Santa Monica Mountains also demonstrate the long history of human habitation. Many of the sites are located adjacent to permanent water sources as the presence or absence of water is a crucial predictor of site location in Southern California. Many of the archaeological sites on the plain have been disturbed by erosion, farming, gophers, bulldozers, and other cultural and natural sources of disturbance. Spanish period (1782 to 1822) Spanish explorers made sailing expeditions along the coast of southern California between the mid-1500s and mid-1700s. In the 18th century, Spain began the colonization and inland exploration of Alta California. They established a tripartite system consisting of missions, presidios, and pueblos. Mission San Buenaventura was founded in 1782 next to the Ventura River, upcoast from the Santa Clara River. The Oxnard plain was used for grazing herds of livestock which required thousands of acres. The traditional way of life of the Chumash people became increasingly unstable and unsustainable on the Oxnard Plain with the introduction of these animals. They also experienced further disruptive contacts through the increasing number of Europeans and Americans that visited the California coast looking for pelts from fur-bearing animals such as sea otters, and trade in hides and tallow beginning in the 1790s. The destruction wrought by the livestock and shortages of wild plants that they used for food may have made the missions appear to be the only viable alternative to a disintegrating way of life. At its peak in 1816, the mission had over 41,000 animals including 23,400 cattle, 12,144 sheep. The 4,493 horses constituted one of the largest stables of horses of the California mission sites. The Chumash culture, including political and social relationships between communities, trade, and inter-village marriage patterns, could not be sustained as more and more Indians