High Quality Content by WIKIPEDIA articles! Raystown Lake is a reservoir in Huntingdon County, Pennsylvania. It is largest lake that is entirely within Pennsylvania. The original lake was built by the Simpson family of Huntingdon as a hydroelectric project. The current 8,300-acre (34 km2) Raystown Lake was completed in 1973 by the Army Corps of Engineers. The lake was created primarily to control floods, provide electricity, and support recreational activities.