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Why do many of North America’s longest rivers flow eastward?

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Map of North America showing international boundaries, country names, and capital cities. The map includes Canada, the United States, Mexico, Greenland, Central American countries, and Caribbean nations, with a locator inset at the bottom left for global context. Major bodies of water such as the Pacific Ocean, Atlantic Ocean, Arctic Ocean, and Caribbean Sea are labeled.
North America

Many of North America’s longest rivers flow eastward primarily due to the continent’s east-west drainage asymmetry, which is dictated by the Continental Divide.

This divide, a continuous ridge of mountain summits, runs largely along the crest of the Rocky Mountains in the western portion of the continent. Because this principal drainage divide is positioned well to the west, the vast majority of the continent's landmass slopes toward the east. Consequently, the longest rivers—such as the Missouri-Mississippi-Ohio system—must travel great distances across the Interior Lowlands to reach the sea.12

Furthermore, the major discharge points for these river systems, including the Gulf of Mexico, Hudson Bay, and the Gulf of St. Lawrence, are located on the eastern side of the continent, effectively drawing the water eastward.12