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One of the Earliest Detailed Maps of the Upper Missouri River and Yellowstone River

Eastern Part of Oregon Territory

This fine large format map of the northeastern part of what was then Oregon Territory extends from the Yellowstone River and Missouri River in the northeast to the Yankton, South Dakota and Sioux City, covering portions of Eastern Montana and Wyoming and most of North and South Dakota. The map shows region during a period when the Columbia Fur Company, American Fur Company, Missouri Fur Company and Rocky Mountain Fur Company were trapping and trading in the region.

The map provides a tremendous and highly detailed region of the lands traversed by Lewis and Clark only a decade after the official map of the expedition was first published in Philadelphia. The river and mountain systems in the region are covered with tremendous detail, as is the illustration of the Indian villages in the region. Extensive annotations in French appear throughout the map, describing geographical features primarily related to the courses of rivers. The estimated populations of several Indian tribes are also given.

The region known as the Oregon Territory emerged from a complex history of imperial rivalry, exploration, and negotiation in the Pacific Northwest. In the 1820s, Oregon Territory originally included all of present-day North Dakota, South Dakota, Iowa, Nebraska, Kansas, as well as Oregon, Washington, Idaho, Montana, Wyoming and Western Minnesota. These were lands which in whole or part had long been claimed by Spain, Great Britain, Russia, and the United States. The basis for U.S. claims rested on the explorations of Lewis and Clark (1804–06), the establishment of the American fur trade by the Pacific Fur Company at Astoria in 1811, and the terms of the Adams-Onís Treaty of 1819, which fixed the northern boundary of Spanish claims at the 42nd parallel.

The period before 1830 was characterized by loose control and overlapping claims. Following the War of 1812, the Treaty of 1818 established a joint occupation agreement between the U.S. and Great Britain, allowing citizens of both nations to settle and trade in the Oregon Country without prejudice to either side’s claims. During this time, the Hudson’s Bay Company (HBC), through Fort Vancouver on the Columbia River (established 1825), dominated the region economically, while American missionaries and fur traders began to assert an embryonic American presence. Despite the area’s strategic value and abundant resources, there was no formal territorial government, and sovereignty remained unresolved.

The early decades of the 19th century laid the foundation for U.S. political control. These years saw the expansion of the overland fur trade, the mapping of key river systems such as the Upper Missouri, Yellowstone, Columbia and Snake, and increasing American interest in westward migration and Manifest Destiny. 

The eastern portions of what later became Oregon Territory were also part of a broader continental vision, which had its own distinct histories of exploration and exploitation. While these areas were formally part of the Louisiana Purchase (1803), they came under the broader administrative rubric of Oregon Territory. However, long before this legal incorporation, U.S. government-sponsored and private expeditions began to chart and traverse the lands now comprising the Dakotas, eastern Montana, and eastern Wyoming.

The Lewis and Clark Expedition (1804–1806) was the first major American venture into this region, following the Missouri River through the Dakotas and into Montana before turning west along the Jefferson and Columbia River systems. Their detailed maps and ethnographic notes formed a foundation for future exploration. In the years following, fur trading companies—particularly the American Fur Company—established trading posts and supply lines throughout the Upper Missouri basin, including Fort Pierre (founded by the Columbia Fur Company in 1817 as Fort Tecumseh, then absorbed by the American Fur Company in 1827 and renamed as Fort Pierre in 1832) and Fort Union (founded in 1828 by the American Fur Company).

The northern plains and mountain frontiers, including present-day eastern Wyoming and Montana, were further explored by figures such as Jedediah Smith and William Ashley in the 1820s. Smith’s 1823–24 expedition followed the Cheyenne and Platte Rivers and crossed the Continental Divide, opening vital knowledge of mountain passes and overland routes. These routes, particularly South Pass in Wyoming, would become critical arteries for future migration.

Though sparsely settled by Euro-Americans before 1830, this eastern expanse of Oregon Territory was of immense interest to the federal government, traders, and Native nations. It was home to numerous Indigenous groups, the Lakota, Arikara, Mandan, Crow, Shoshone, whose trade and resistance shaped the dynamics of U.S. exploration and encroachment.