Grand Lake St. Marys is a man-made impound located in northwestern Ohio primarily in
Mercer County with about 20% of its 13,500 acres in
Auglaize County. It inspired
Grand Lake St. Marys State Park located on the lake's northeastern shores.
The reservoir was created from 1837 to 1845 to aid in keeping the economically vital Miami-Erie Canal at a constant depth of five feet. At its completion, the lake was the largest of its kind in the world.
Fishing in the lake is good for bass, bluegill and crappie in the spring as well as yellow perch in the winter.
