Imagine a question and stream-of-consciousness explanation for which this is the answer: along the lower Platte River Valley near Fort Kearny , Nebraska Territory
Question: The Oregon Trail was laid by fur traders and traders from about 1811 to 1840, and was only passable on foot or by horseback. By 1836, when the first migrant wagon train was organized in Independence, Missouri, a wagon trail had been cleared to Fort Hall, Idaho. Wagon trails were cleared increasingly farther west, and eventually reached all the way to the Willamette Valley in Oregon, at which point what came to be called the Oregon Trail was complete, even as almost annual improvements were made in the form of bridges, cutoffs, ferries, and roads, which made the trip faster and safer. From various starting points in Iowa, Missouri, or Nebraska Territory, the routes converged along the lower Platte River Valley near Fort Kearny, Nebraska Territory and led to rich farmlands west of the Rocky Mountains.

Where did immigrants gather to begin their journey westward on the oregon trail?
Stream-of-consciousness: The relevant sentence in the passage is: From various starting points in Iowa, Missouri, or Nebraska Territory, the routes converged along the lower Platte River Valley near Fort Kearny, Nebraska Territory and led to rich farmlands west of the Rocky Mountains.