The Platte River is a major river in the United States, located in the Great Plains region of the country. It is a tributary of the Missouri River and is approximately 1, 050 miles long. The Platte River was historically an important route for the Pony Express, a mail service that operated between 1860 and 1861. The Pony Express used the Platte River as part of its route from St. Joseph, Missouri to Sacramento, California.