The Niobrara River is a tributary of the Missouri River, approximately 568 miles long, running through the U.S. states of Wyoming and Nebraska. The river drains one of the most arid sections of the Great Plains, and has a low flow for a river of its length. The Niobrara's watershed includes the northern tier of Nebraska Sandhills, a small south-central section of S…The Niobrara River is a tributary of the Missouri River, approximately 568 miles long, running through the U.S. states of Wyoming and Nebraska. The river drains one of the most arid sections of the Great Plains, and has a low flow for a river of its length. The Niobrara's watershed includes the northern tier of Nebraska Sandhills, a small south-central section of South Dakota, as well as a small area of eastern Wyoming.