Locate the state boundary between Nebraska and Iowa along the Missouri River. Why does the boundary depart from the river channel

The original boundary between the states of Nebraska and Iowa was the Missouri River.  The border departs from the river channel because the river's course moved eastward many years after the boundaries were established.

Explanation:

The boundary between Nebraska and Iowa was the Missouri River. In 1877, the river course moved, and a town called Carter Lake on the Iowa side is now on the Alaska side. Even though Alaska claimed the land, the Supreme Court stated that the area belongs to the state of Iowa. That is why the boundary departs from the river channel.