The state park is open for year-round recreation, including boating, fishing, swimming and hunting.

[3] It is west of St. Marys, and southeast of Celina, 23 miles (37 km) southwest of Lima in the northwestern part of the state.

[citation needed] The body of water has also been referred to as "Grand Reservoir" and "Lake Celina" among other names.

[1] The land on which Grand Lake St. Marys is located was once part of a forest that stretched from the Appalachian Mountains in Pennsylvania to the prairie in Illinois.

Much of the forest was cleared by pioneer farmers as the area was settled in the years following the Northwest Indian War.

Now the shore of the lake includes woodlands, wetlands and prairie ecosystems that are surrounded by residential development and farms.

Historically, these birds bred in the western United States and Canada, but appear to be expanding their range east.

The most commonly found mammals in the park are cottontail rabbits, mink, fox squirrels, white-tailed deer, eastern coyote, beaver, muskrat, and raccoons.

[4] Due to the increasingly high levels of lake pollution, E. coli bacteria,[10] and related algae levels, Grand Lake could be dying off as a destination lake and is considered by the Ohio Environmental Protection Agency to be "impaired" due to "stream channelization, drainage tiles, loss of floodplains and streamside vegetation, manure runoff and untreated sewage flowing from failing home septic systems and small communities without any wastewater collection or treatment.

Microcystin can harm the liver and cause other health problems including mild rashes and sneezing and even severe gastrointestinal ailments.

[15] Grand Lake St. Marys State Park is open for year-round recreation, including boating, hunting, fishing, swimming and picnicking.

The park also has a miniature golf course, along with basketball and volleyball courts, horseshoe pits, playground equipment, and a recently added swimming pool.

[4] St. Marys Fish Hatchery, located on the lake's eastern shore is operated by the ODNR Division of Wildlife.

The hatchery raises saugeye, walleye, channel catfish and bass for stocking in the public fishing waters of the state.