The Cheyenne River, also written Chyone, referring to the Cheyenne people who once lived there, is a tributary of the Missouri River in the U.S. states of Wyoming and South Dakota. It is approximately 295 miles (475 km) long and drains an area of 24,240 square miles (62,800 km2). About 60% of the drainage basin is in South Dakota and almost all of the remainder is in Wyoming.
Cheyenne River at the I-90 bridge crossing, near Wasta in Pennington County.
The Missouri River is a river in the Central and Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Mountains of southwestern Montana, then flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river drains semi-arid watershed of more than 500,000 square miles (1,300,000 km2), which includes parts of ten U.S. states and two Canadian provinces. Although a tributary of the Mississippi, the Missouri River is slightly longer and carries a comparable volume of water. When combined with the lower Mississippi River, it forms the world's fourth-longest river system.
The Missouri River in Montana
Holter Lake, a reservoir on the upper Missouri River
The Missouri in North Dakota, which was the furthest upstream that French explorers traveled on the river
The Yellowstone River, the fifth longest tributary of the Missouri, which it joins in North Dakota