Grand Lake St. Marys is the largest inland lake in Ohio in terms of land area, but is extremely shallow, with an average depth of only 5 to 7 feet (1.5 to 2 m) with a soft bottom of silt. Some isolated areas of sandy or clay bottoms exist where wave action keeps the bottom cleaner. Lots of rock rip rap has been added to reduce shoreline erosion; these rocky shorelines provide cover for several species of fish. Numerous boat docks, shoreline brush, and fallen trees provide cover for fish. It was constructed 1837-1845 as a reservoir for the Miami and Erie Canals. Two earthen dams were built on the headwaters of two major drainage systems flooding the area between them. The spillway on the west end discharges water into the Wabash and Ohio River drainage systems; water from the east end spillway flows into the St. Marys River, and from there into the Maumee River and Lake Erie. In the 1890's oil was discovered in the area, and oil derricks were set up in the lake. These were the first off-shore oil rigs ever constructed.
In 1949, it was made one of Ohio's first state parks. The lake and the surrounding state park are a popular site for recreational fishing and camping and there are four swimming beaches. At one point it was the largest human-made body of water in the world.
Officially, according to the U.S. Geological Survey, the name of the lake is simply "Grand Lake". But the lake is nearly always printed on maps as "Grand Lake St. Marys", possibly because the address of the park office is listed as "St. Marys, OH".
