 As of the 2010 census, there were 1,518 people living in St. Clair township, in 634 households (including 446 families).The average household size was 2.39 people.In the census, 98.4 percent of the population identified as white, with 0.5 percent Asian, 0.3 percent African American or black, and 0.2 percent Alaskan or Native American.