What major river defines the eastern border of Nebraska?

The Missouri River defines the eastern border of Nebraska and serves as a significant geographic and political boundary between Nebraska and its neighboring state of Iowa. This river stretches over 2,300 miles and is the longest river in North America, playing a crucial role in the history, economy, and ecology of the region.

The Missouri River also has historical importance, as it was a key route for indigenous peoples and later for explorers, traders, and settlers moving westward. In addition to its historical significance, the river also influences modern economic activities, including agriculture, transportation, and recreation in the region surrounding it.

In contrast, other rivers like the Arkansas River, Platte River, and Nebraska River do not define the eastern border of the state. The Arkansas River flows through parts of Colorado and Kansas but does not reach Nebraska's eastern boundary. The Platte River runs across central Nebraska and is well-known for its role in the state’s water system, but it does not serve as a border. The Nebraska River is not a recognized river in the context of Nebraska's geography or its borders. Thus, the Missouri River is the correct answer due to its definitive role in marking Nebraska's eastern edge.