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

Discover Canada records this in one direct sentence. The guide writes: There are five Great Lakes located between Ontario and the United States: Lake Ontario, Lake Erie, Lake Huron, Lake Michigan (in the U.S.A.) and Lake Superior, the largest freshwater lake in the world. The natural feature the test wants is therefore the Great Lakes.

Five lakes, one chain. Discover Canada commits to FIVE specific Great Lakes by name: Lake Ontario, Lake Erie, Lake Huron, Lake Michigan, and Lake Superior. Of the five, four form part of the boundary between Ontario and the United States — Lake Michigan is entirely inside the U.S.A. and is the only Great Lake not shared with Canada. The other four lakes form a continuous shared water border with several American states.

One of the lakes is a world record-holder. Discover Canada writes that Lake Superior is "the largest freshwater lake in the world." So the Great Lakes are not just a regional border feature — they include a world-record body of water. Together, the five Great Lakes hold roughly one-fifth of the world's surface fresh water.

The Great Lakes anchor a population corridor. Discover Canada writes that "more than half the people in Canada live in cities and towns near the Great Lakes and the St. Lawrence" river system. So the Great Lakes are not just a border — they are the heart of Canada's most densely populated region. Toronto, the country's largest city and main financial centre, sits on Lake Ontario. Founded by United Empire Loyalists, Ontario also has the largest French-speaking population outside Quebec — and Niagara, where the lakes drop from one level to the next, is famous for its vineyards, wines, and fruit crops. So the Great Lakes are at once a border, a freshwater wonder, and the geographic backbone of Canadian central population, industry, and agriculture.

The question is testing whether new citizens know which natural feature forms part of the Ontario–United States border. Discover Canada commits to one feature: the Great Lakes — explicitly "located between Ontario and the United States." The right test answer matches that.

The wrong answer choices each substitute a different natural feature. The Rocky Mountains are in the West (Alberta and British Columbia), not on the Ontario border. The St. Lawrence River runs from the Great Lakes through Quebec to the Atlantic and forms part of the Quebec/U.S. boundary, not Ontario's main border. The Mackenzie River is in the Northwest Territories, far from the U.S. border. Only the Great Lakes match the source.