Some claim that actually the Mississippi River is a tributary to the Missouri River as it is much longer than the Upper Mississippi River and also contains about double the amount of water than the Mississippi River at their confluence.
From Montana to the confluence with the Mississippi River just north of St. Louis the Missouri River is 2,341 miles long.
The Missouri River aways had been a nightmare from a navigational standpoint, seldomly holding enough wof its muddy water for safe navigation. Nevertheless it was an important route for settlers on the westward expansion and of course the Missouri River also played an important role in the Lewis and Clark expedition.
The first steamboat up the Missouri River, the INDEPENDENCE, ran from St. Louis to the vicinity of the Chariton River as early as 1819. The second boat up the river was the WESTERN ENGINEER, which made it almost up to the Yellowstone River - a journey of almost 3 months, oneway.