What forms the natural border in the east between the U.S. and Canada?

The St. Lawrence River serves as a significant natural border in the east between the United States and Canada. This river flows from the Great Lakes to the Atlantic Ocean, marking part of the boundary between the two countries. The river has historically been important for trade and transportation and continues to play a crucial role in the economy of both nations. It also represents an important geographical feature that influences the regional climate and ecosystems.

The Great Lakes, while being a crucial body of water in North America, are not the primary natural border in the east; they are located to the north of many U.S. states and the southern parts of Canada. The Rocky Mountains, a major mountain range running through western North America, and the Canadian Shield, which is a large area of exposed Precambrian rock in Eastern and Central Canada, do not define the easterly boundary between the two countries. The St. Lawrence River clearly establishes a recognizable and significant boundary in that eastern region, explaining its selection as the correct answer.