St. Mary’s



The St. Marys River watershed is one of eight subwatersheds within the Maumee River Watershed and comprises 13% of that watershed. It is delineated by the United States Geological Survey as 8-digit hydrologic unit number 04100004. The 524, 800- acre (820 square mile) watershed is located in Allan, Adams and Wells counties in northeastern Indiana, and Van Wert, Mercer, Shelby and Auglaize counties in northwestern Ohio. The watershed contains approximately 434 miles of tributary perennial streams. The St. Marys River originates near Minster, Ohio flowing to the northwest through St. Marys in Auglaize County, then through, Mercer and Van Wert counties in Ohio. The St. Marys River flows into Indiana southwest of Pleasant Mills, near the Indiana State Line and Highway 33. The river continues to the northwest, flowing through Decatur, Indiana into Allen County. It joins the St. Joseph River in Fort Wayne to form the Maumee River, which flows northeast and empties into Lake Erie.


Quick Facts


  • Watershed Area – 848 square miles
  • Empties To – Maumee River in Fort Wayne, Indiana.
  • Land Use – 84% cropland, 7% woodland, 7% impervious
  • Largest Cities – Fort Wayne & Decatur, Indiana. Celina & St. Mary’s Ohio.
  • Watershed Population – 768,810
  • Watershed Counties – Ohio – Shelby, Auglaize, Mercer, Allen, and Van Wert. Indiana – Adams, Allen, and Wells.


Getting Local


Several local organizations play a role in watershed stewardship in the St. Mary’s watershed. Among them is the Maumee River Basin Partnership of Local Governments.






Studies & Reports


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