Blue Marsh Lake is an artificial lake located northwest of the city of Reading, Pennsylvania, USA and managed by the U.S. Army Corps of Engineers Philadelphia District. It is in western Berks County, fed into by the Tulpehocken Creek. The main span of the lake is along the border between Bern and Lower Heidelberg Townships. However, the northwesternmost portions lie in the more sparsely populated North Heidelberg and Penn Townships. In the middle of the lake is a large, uninhabited island.