How Idaho's Borders Were Formed

Idaho's distinctive shape—including its northern panhandle—resulted from a series of treaties, territorial acts, and geographic compromises:

1. 1846 Oregon Treaty: Set the 49th parallel as the U.S.–British boundary, forming Idaho's northern border.
2. 1803 Louisiana Purchase: The 42nd parallel (southern border of the Oregon Country) became Idaho's southern boundary with Utah and Nevada.
3. 1804–06 Lewis & Clark Expedition: Mapped the Continental Divide separating the Columbia and Snake River drainages, which later defined the western border.
4. 1863 Idaho Territory Created: Originally included present-day Montana and most of Wyoming. Subsequent acts (1864 Montana Territory, 1868 Wyoming Territory) carved away the eastern sections.
5. 1890 Statehood: Admitted as the 43rd state with today's borders finalized.

The panhandle reflects the 49th parallel's eastern extension; the jagged western border follows the Continental Divide's sinuous course.