The Platte River is an approximately 310 mi (500 km) long river in the Western United States. It is a tributary to the Missouri River, which in turn is a tributary to the Mississippi River. Platte River being one of the most significant river systems in the watershed of the Missouri, it drains a large portion of the central Great Plains in Nebraska and the eastern Rocky Mountains in Colorado and Wyoming. The river was highly significant in the westward expansion of the United States, providing the route for several major westward trails, including the Oregon Trail and the Mormon Trail. In the 18th century, it was also known among French fur trappers who explored it as the Nebraska River.