Which body of water separates Michigan's Upper Peninsula from Wisconsin?

The Menominee River serves as the natural boundary that separates Michigan's Upper Peninsula from Wisconsin. It flows from the north, originating in Michigan, and continues to the south where it meets the waters of Green Bay. This river is significant because it provides both a geographic and historical context for the region, having served as a transportation route and a boundary between different territories.

The other options, while they are significant bodies of water in the Great Lakes region, do not act as the dividing line between the Upper Peninsula and Wisconsin. Lake Michigan borders the eastern side of the state but its waters do not separate the Upper Peninsula from Wisconsin. Lake Huron is situated to the east of Michigan's Lower Peninsula, affecting the geography in a different way. The Straits of Mackinac connect Lake Michigan and Lake Huron, but they are located further to the north and do not define the boundary between the Upper Peninsula and Wisconsin. Thus, the Menominee River is the correct body of water that serves this purpose.