Where does Grand Lake St. Marys’ water come from? And where does it go?
Grand Lake St. Marys occupies a fascinating and unusual place in Ohio’s water system. Because of outflows created during the canal era, water from the lake travels in two directions: west through Beaver Creek toward the Wabash, Ohio and Mississippi rivers, and east through the Miami & Erie Canal toward tributaries that eventually reach Lake Erie.
A new research paper, Great and grand connection: Grand Lake St. Marys is the hydrologic link between the Wabash River and Lake Erie watersheds, documents and quantifies this connection. The study was authored by Stephen J. Jacquemin, Morgan C. Grunden, Haley N. Hoehn, Jason C. Doll and Silvia E. Newell, with researchers representing Wright State University-Lake Campus, Francis Marion University and the University of Michigan.
Using data collected during the 2024 and 2025 water years, the researchers developed a comprehensive hydrologic budget examining how water enters, moves through and leaves Grand Lake St. Marys.
The paper covers:
- The lake’s canal-era history and how the construction of Grand Lake St. Marys and the Miami & Erie Canal created a connection between two major drainage basins.
- The lake’s two primary outflows: Beaver Creek, which flows toward the Wabash River, and the Miami & Erie Canal, which carries water toward the St. Marys and Auglaize river systems and ultimately Lake Erie.
- The complete water budget of Grand Lake St. Marys, including tributary inflows, precipitation, evaporation, groundwater loss, water use and outflows.
- The volume of water moving in each direction. The study estimates average annual outflows of approximately 32.6 million cubic meters toward the Wabash River and 15.5 million cubic meters through the canal toward Lake Erie.
- The movement of phosphorus and other water-quality implications associated with water leaving the lake and entering downstream waterways.
- The lake’s water residence time, estimated at an annual average of approximately 293 days, and what that may mean for nutrient management and harmful algal blooms.
- Potential aquatic invasive species pathways created by the connection between the Great Lakes and Mississippi River drainage basins.
- Considerations for future lake and canal management, including flood control, habitat, nutrient loading, biodiversity, cultural history and recreational uses.
The findings provide valuable new information for researchers, conservation organizations and public agencies working to understand Grand Lake St. Marys and make informed decisions about its future management.