What natural feature forms part of the border between Canada and the United States?

The Great Lakes serve as a significant natural feature that forms part of the border between Canada and the United States. This group of five freshwater lakes—Superior, Michigan, Huron, Erie, and Ontario—creates a border between the two countries, particularly between Canada and various U.S. states such as Michigan, Wisconsin, and New York.

The Great Lakes are not just a border but also play a crucial role in trade, transportation, and ecosystem health in both countries, illustrating the interconnectedness of their geography. The location of these lakes has historically influenced settlement patterns, economic activities, and environmental policies in the region, making them a central feature of the Canada-United States border.

In contrast, the other options do not align correctly with the geographical border. The Rocky Mountains, while a prominent geographic feature in North America, do not mark the border between Canada and the United States in their entirety. The Mississippi River primarily runs through the central United States and does not act as a border between the two countries. The Atlantic Ocean is an ocean and does not create a direct border between Canada and the U.S. in the manner that the Great Lakes do, which highlights their importance in understanding the geography of North America.