Positioned on Ohio’s western border with Indiana, Mercer County is in the Till Plains ecoregion. The St. Lawrence Continental Divide runs through the center of the county. The Wabash River flows west into the Mississippi watershed, and the St. Marys River flows northwest into the Maumee, making it part of the Great Lakes Basin. It was dammed early in the 19th century to support the canal system, creating the 13,500-acre Grand Lake, which is the largest inland lake in Ohio. See pp. 11-12 in Ohio Coastal Atlas: Chapter 2 for a detailed watershed map of the northern half of the county.

Places to Visit

• Grand Lake St Marys State Park

  Grand Lake is the largest inland lake in Ohio. There are many access points around the lake, including parks and boat launches.

• Coldwater Creek Treatment Train

  CCTT is a created wetland to filter runoff nutrients. Water is pumped from Coldwater Creek into a connected series of shallow wetlands. The wetland areas support a good Odonata population. 31 species recorded here. There is parking, no restrooms.

• Prairie Creek Treatment Train

  PCTT is a wetland complex to filter water pumped from Prairie Creek before it enter Grand Lake. A diverse wetland that can support a variety of Odonata species. 21 species recorded here. Parking available, no restrooms.

• Gilliland Nature Sanctuary

  GNS is a developing wildlife area. It has several wetland and vernal pool areas. 20 species have been recorded for GNS. There is a small parking lot and mowed trails. No restrooms.

County Observation Activity

This graph shows the total number of odonata observations for the county in blue, with the average for all Ohio counties in orange. The season is divided into 10 or 11 day segments by Month – basically early-month, mid-month, and late-month.