Complex sentence: It was created in 1948 when the owners of the Vail Ranch constructed the 132-foot high Vail Lake Dam, which has been owned and operated by the Rancho California Water District since 1978.
Split into simpler sentences: It was created in 1948 when the owners of the Vail Ranch constructed the 132-foot high Vail Lake Dam. It has been owned and operated by the Rancho California Water District since 1978.