The river that both the oregon trail and the mormon trail followed on the great plains was the .

 snake
 missouri
 columbia
 north platte

The river that both the Oregon Trail and the Mormon Trail followed on the Great Plains was the River.

Snake   Missouri  Columbia   North Platte

Explanation:

The route of Oregon was one of the main routes of migration by earth of North America, that left from several places in the Missouri river and arrived until the Territory of Oregon.

The Mormon route, after crossing the Missouri River on ferries and organizing the wagon caravans near what became Omaha, the Mormons followed the north bank of the Platte River in Nebraska to Fort Laramie, in present-day Wyoming.

The answer is: The Missouri River.