The Missouri River is the longest river in the USA draining over one sixth of the nation's area. It starts with the confluence of the Jefferson, the Madison and the Gallatin Rivers at Three Forks, Montana. It is also a tributary of the Mississippi River contributing about 45% of the Mississippi's water. The claim is that the Missouri River contains 4,000 trout (wild trout that is) per mile. Meriwether Lewis and William Clark set off on an expedition up the Missouri from Wood River, Ill. on May 14, 1804.
Do not be intimidated with its size and reputation; it can be an angler's dream come true. The spring runoffs can dramatically change the character of the water flow, but mostly it lopes along at a leisurely pace. It's four dams help to control the pace, and the temperature making it an ideal habitat for large wild brown and wild rainbow trout. It has some excellent hatches of Tricos as well as other aquatic insects. The key to fishing the Missouri them becomes the presentation. You must conceal your presence, make your first cast accurate, and maintain a drag free drift.
You can learn more about this mighty river at TroutU.