Which river forms part of Nebraska's eastern border?

The Missouri River is significant as it forms a substantial part of Nebraska's eastern border. This river serves not only as a natural boundary between Nebraska and Iowa but also plays an essential role in the geography and history of the region. It is one of the major rivers in North America and has been crucial for trade, transportation, and settlement since early American history.

The other rivers listed do not form part of Nebraska's eastern border. The Kansas River runs through Kansas, the Platte River flows west to east but does not serve as a border, and the Nebraska River is not a recognized river by that name; rather, the state is named after the Platte River, which is an entirely different entity. Understanding these geographical distinctions helps provide clarity on Nebraska's borders and the significant waterways that define them.