Algonac is a city in St. Clair County of the U.S. state of Michigan. The population was 4,110 at the 2010 census. Algonac is located at the southern end of the St. Clair River, just before it splits into a large delta region known as the St. Clair Flats. (Coordinates 42°36′N 82°34′W / 42.6°N 82.56°W) The St. Clair River drains Lake Huron into Lake St. Clair and is part of the Great Lakes Waterway. At the center of Algonac is Algonac City Park, a park which contains a half-mile long boardwalk along the St. Clair River. Just to the north of the city is Algonac State Park.