This IBA includes the St. Marys River corridor and the neighboring Grand Lake, as such representing elements of both the Great Lakes and the Mississippi River watersheds. It also includes the St. Marys Fish Hatchery on the east side of the lake and the Mercer Wildlife Area on the west. Grand Lake is Ohio’s largest inland lake, functioning as a constructed reservoir with an outlet flowing west to Beaver Creek and eventually to the Wabash River in the Mississippi watershed. The St. Marys River originates just east of Grand Lake, flowing north and west to eventually join the St. Joseph River in Indiana to form the Maumee River, which flows back into Lake Erie. Rare species include state listed swamp cottonwood, catchfly grass (only site in Ohio), lake cress, raven’s-foot sedge, pigeon grape, and pugnose minnow. The St. Marys River corridor is a hydrologically intact river valley with a remaining riparian corridor, many oxbows and much floodplain forest and wetlands, in great contrast to most of surrounding farmlands. This corridor is the single most forested corridor in west-central Ohio. High-quality sites along the corridor include the Barber-Werner Oxbow, Speckman Woods, and The Thoroughfare.

The Barber-Werner Road Oxbow is a floodplain forest of high quality and diversity with large oxbows exhibiting an intact riparian corridor and hydrology. This is one of the better sites along the riparian chain of sites along the St. Marys. 302 species of plants and animals were recorded from the site in 1988; one is state listed – swamp cottonwood.

Speckman Woods is 10 acres of high diversity mixed swamp forest within a beech/maple woods, part of a buffer to an area of deeper swamps on state-controlled land to the Northwest. The deep forest character and high plant diversity exhibit habitats uncommon in Northwest Ohio.

The largest block of local area without roads is known locally as The Thoroughfare due to the abundance of wildlife there. The Thoroughfare contains an intact riparian corridor within an intensive agricultural area; it is the largest forested area. The largest block of local area without roads is known locally as The Thoroughfare due to the abundance of wildlife there. The Thoroughfare contains an intact riparian corridor within an intensive agricultural area; it is the largest forested area in 6-8 county area (including three Indiana counties) with several state-listed plants and animals. It is a high-quality un-dredged river corridor with diverse riparian habitats: upland forest with a few old growth trees, oxbows and other wetlands, swamp forest, floodplain forest, remnant wet prairie, and buttonbush swamp.

The St. Marys River corridor contains unique and rare habitats and species in the most agriculturally developed part of the state. Barber-Werner Road Oxbow is a high-quality site of a rare habitat type-floodplain forest. Forty-two species were recorded from this site alone by the nominator in 1988 with high numbers during migration in spring.

The Thoroughfare has a large amount of good quality wetland, riparian, and forest habitat in an area of the state that has had most of the native habitats removed.

It includes migratory stopover and feeding area for forest and wetland species, including some state-listed species. Large numbers of waterfowl use the area when migrations and seasonal floods coincide.

The 160 acres of the St. Marys Fish Hatchery ponds attract diving and puddle ducks during migration peaks, as well as shorebirds and wading birds during times of peak mudflat exposure in spring and fall.