The land surface of the County is a glacial landform, with characteristic slopes, substrata, soils, and drainage conditions, and as a result of these physical factors, certain vegetation evolved and now exist. According to the United States Geological Survey, St. Clair County is part of the landform called Washtenaw-Maumee Lake Plain, consisting primarily of clay soils. There are several one to three mile-wide end moraines, a product of glacial action.
There are six major water bodies affecting St. Clair County. The largest of these water bodies is Lake Huron, which as part of the Great Lakes water system encompassing 12.5 miles of St. Clair County's boundaries. The St. Clair River, a connecting waterbody for the Great Lakes system, is a receptor of a significant portion of the county's surface drainage.
The St. Clair flats area is at the mouth of the St. Clair River. The flats represent one of the world's largest freshwater deltas. Lake St. Clair, towards the southern end of the Great Lakes system, receives 218,000 cubic feet per second of water from the St. Clair River. Lake St. Clair supports a range of recreational activities and wildlife habitat.
The inland waters within the County total only 0.7% of land the area, and include the Black River, the largest drainage basin in southeastern Michigan (encompassing 455,040 acres); the Pine River, the largest drainage basin completely contained within the County (measuring 126,110 acres); and the Belle River, which has the smallest drainage basin (59,810 acres), but has a river bed significantly deeper than the Pine River bed.
Water level fluctuations of two to three feet are common along the St. Clair River shoreline, from the City of St. Clair to the southern County line. Over 218,000 cubic feet per second of water passes through the St. Clair River from Lake Huron into Lake St. Clair, which causes tree mortality, shoreline erosion, and major alteration in species composition of marshes and wet prairies. According to the Great Lakes Information Network (Army Corps of Engineers), there are six locations along the St. Clair River that are monitored for lake levels on a monthly basis.
St. Clair County is located within the path of two flyway corridors for stop-over and migratory birds, thus indicating a rich wildlife habitat, including watersheds and other aquatic places, terrestrial habitats, and a variety of 'edge' lands. Bird species tend to be the leading indicator of the quality of wildlife habitat and are always directly or indirectly related to the supportive plant and microorganism species in a given area.
The majority of the woodlands still existing within the County are along the banks of the Black (including Mill Creek), Pine and Belle rivers, and in smaller qualities in varied locations throughout the County. Woodlands cover only nine percent of the County (approximately six percent is managed by the Michigan Department of Natural Resources), and provide a visual relief from the relatively flat topography of the area, along with providing a prime wildlife habitat.
There is a total of 58 miles of shoreland along the mainland, most of which has been developed or is unable to be developed. The majority of the shoreland has been stabilized by sea walls, but the remaining shoreland is significant because of its influence by and involvement in the geological erosion processes (deposition). However, very little beach erosion can be found along the shorelands due to their limited accessibility and recently consistently low water levels. At times of high water levels, evidence of erosion does exist, primarily along Lake Huron and the St. Clair River.