Which Great Lake has the largest watershed?

Lake Huron has the largest watershed of the five Great Lakes.

Its drainage basin covers approximately 134,100 square kilometres, excluding the lake’s own surface area. That watershed includes parts of Ontario and Michigan and receives water from Lake Superior through the St. Marys River and from Lake Michigan through the Straits of Mackinac.

Lake Huron’s basin includes Georgian Bay, North Channel, and thousands of islands. Although Lake Michigan and Lake Huron are often described separately, they are hydraulically one body of water because the broad Straits of Mackinac allow free exchange between them. Their water surfaces also stand at essentially the same elevation.

A watershed is the land area draining into a lake, not the lake’s surface area. This explains why Huron can have the largest watershed even though Lake Superior has the greatest surface area and volume among the Great Lakes.