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  • The Northern Boundary of the Louisiana Purchase
  • William E. Lass (bio)
Key Words

Canada, Convention of 1818, forty-ninth parallel, Thomas Jefferson, Lake of the Woods

Ever since the United States purchased Louisiana from France in 1803, historians have universally and rightfully concluded that neither country was aware of its northern boundary. But the historiography and cartographic portrayals of the Louisiana acquisition fundamentally disagree about how the northern boundary was finally established. One version is that the forty-ninth parallel is the northern boundary. Other sources report it was the continental divide separating the Gulf of Mexico and Hudson Bay drainage systems. This acclaimed “natural boundary” extends eastward through southern Alberta and Saskatchewan before entering the United States in northwestern North Dakota. From that point it follows a southeastern route across North Dakota and nicks the northeastern corner of South Dakota before reaching into Minnesota, where it extends northeastward to the Hudson Bay–Gulf of St. Lawrence divide between North and South Lakes, only about sixty miles west of Minnesota’s northeastern tip. However, the differences between the advocates of the forty-ninth parallel and those of the continental divide boundaries were limited to the region west of the Mississippi River and a direct line from the river’s northernmost reach to the northwesternmost point of Lake of the Woods. That is, both sets of boundary advocates recognized that the northern boundary of the original United States extended to the northwesternmost point of Lake of the Woods.

The contrasting versions are presented by the “Louisiana Purchase” articles in contemporary encyclopedias such as The Louisiana Purchase: A Historical and Geographic Encyclopedia, Encyclopedia of the American West, Encyclopedia of the Great Plains, and the popular [End Page 27] online encyclopedia Wikipedia. With an apparent disregard for editorial consistency, the Louisiana Purchase encyclopedia endorses both the forty-ninth parallel and the natural boundary contentions. The sketch titled “Convention of 1818” notes that the treaty between the United States and Great Britain “also established the northern boundary of the Louisiana Purchase Territory when it fixed the boundary between the United States and Canada at 49 degrees north latitude, from the Lake of the Woods, in present-day Minnesota, to the crest of the Rocky Mountains.” But the article “North Dakota” specifies that “the western two-thirds of the state is situated within the Missouri-Mississippi drainage system and therefore was acquired by the United States when it purchased the Louisiana Territory from France.” Thus, the author concluded that the Hudson Bay drainage portion of the state was not included in the purchase but was acquired from Great Britain by the Convention of 1818.1

The article in the Encyclopedia of the American West is likewise contradictory. It specifies that all of North Dakota was added to the nation by the Louisiana Purchase, which amounts to a tacit endorsement of the contention that the forty-ninth parallel was the territory’s northern boundary. But the accompanying map shows the continental divide as the northern boundary (Map 1).

The “Louisiana Purchase” sketch in the Encyclopedia of the Great Plains states “the northern boundary was never clearly defined (although it roughly followed the forty-ninth parallel).” The author does not explain how the boundary “roughly” follows the forty-ninth, but he may have been aware that this boundary, as marked on the ground, is actually a series of short-line courses, with most of the turning points a few inches north or south of the latitudinal line.

Without specifically addressing Louisiana’s northern boundary, the Wikipedia article states “the United States claimed the Louisiana Purchase included the entire western portion of the Mississippi River drainage system to the crest of the Rocky Mountains.” The contributors assumed the purchase must have included the complete Missouri River watershed, and thus it must have excluded the Hudson Bay drainage area between the continental divide and the forty-ninth parallel. This assumption led them to claim that “the territory north of the 49th (including the Milk River and Poplar River watersheds) was ceded to the UK in exchange for parts of the Red River Basin south of the 49th parallel in the Anglo-American Convention of 1818.” On an accompanying map...


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