These network of trails meander along the lake at Grand Lakes St. Marys State Park into wooded areas and traverse covered bridges. Local Attractions: Behm's Lighthouse, Bella's Italian Grille, Grand Lake St. Marys State Park (pictured), St. Marys Memorial Park. Type: Asphalt Originally constructed as a feeder reservoir for the Miami-Erie Canal, Grand Lake St. Marys was for many years recognized as the largest man-made reservoir in the world. The 13,500-acre lake is the gateway to swimming, boating, camping and fishing in one of Ohio's oldest state parks.