The St. Marys River is in Ontario.
Ontario . These are the Rainy River , the Pigeon River , Lake Superior , the St. Marys River , Lake Huron , the St. Clair River , Lake St. Clair , the Detroit River , Lake Erie , the Niagara River , Lake Ontario and along the St. Lawrence River from Kingston , Ontario , to the Quebec boundary just east of Cornwall , Ontario . The St. Marys River ( named Saint Marys River by the USGS , and sometimes misspelled as St. Mary 's River ) is a 126 mi river in the southeastern United States . The St. Marys River ( rivière Sainte-Marie ) , sometimes written as the St. Mary 's River , drains Lake Superior , starting at the end of Whitefish Bay and flowing 74.5 mi southeast into Lake Huron , with a fall of 23 ft. For its entire length it is an international border , separating Michigan in the United States from Ontario , Canada .