The Santa Clarita Valley is part of the upper watershed of the Santa Clara River in Southern California. The western portion of the valley was part of the 48,612-acre Rancho San Francisco Mexican land grant. Located in Los Angeles County. The valley's main population center is the city of Santa Clarita which includes the neighborhoods of Canyon Country, …The Santa Clarita Valley is part of the upper watershed of the Santa Clara River in Southern California. The western portion of the valley was part of the 48,612-acre Rancho San Francisco Mexican land grant. Located in Los Angeles County. The valley's main population center is the city of Santa Clarita which includes the neighborhoods of Canyon Country, Newhall, Saugus, and Valencia. Adjacent unincorporated communities include Castaic, Stevenson Ranch, Val Verde, and Valencia.