Clay Township is a civil township of St. Clair County in the U.S. state of Michigan. The population was 8,446 at the 2020 census. The township is located along the mouth of the St. Clair River at Lake St. Clair. The river delta includes numerous islands, in which Harsens Island is the largest. The township mostly surrounds the city of Algonac, but the two are a…Clay Township is a civil township of St. Clair County in the U.S. state of Michigan. The population was 8,446 at the 2020 census. The township is located along the mouth of the St. Clair River at Lake St. Clair. The river delta includes numerous islands, in which Harsens Island is the largest. The township mostly surrounds the city of Algonac, but the two are administered autonomously since Algonac incorporated as a city in 1967. Most of Algonac State Park is located within Clay Township.