A small percentage of the northwest area of the reservoir is in Mariposa County.

The lake was named in honor of Judge H. V. Eastman (1891–1972) who had served as Secretary Manager of the Chowchilla Water District.

The lake formed in 1975 (50 years ago) (1975) from the construction of Buchanan Dam across the Chowchilla River as a flood control and irrigation project of the United States Army Corps of Engineers.

The earthen dam, 218 feet (66 m) high with a length of 1,746 feet (532 m) at the crest, impounding a maximum capacity of 150,000 acre-feet of Chowchilla River water in the reservoir, is owned and operated by the Corps.

The proposal was abandoned, and the dam was constructed by conventional means.