Writers Wallace Stegner and Irene D. Paden have each compared the Oregon Trail to a rope that is raveled at both ends because, although the trail basically followed a single path across most of the Great Plains, it parted into several distinct strands at both the eastern and the western ends.

It is a vivid metaphor, but not exactly an accurate one. Over the two decades or so when the trail saw its heaviest use, the long center section became divided into separate strands, too. That meant travelers who headed west in the later years were faced with a succession of sometimes difficult decisions to make about which route to follow.

In its infancy, the trail was reasonably simple and straightforward. The route it followed was dictated mainly by geography. In their westward movement, American pioneers had always followed rivers, and the...