 During the early 19th century , at the height of the fur trade , steamboats and <unk> began traveling nearly the whole length of the Missouri from Montana 's rugged Missouri Breaks to the mouth , carrying beaver and buffalo furs to and from the areas that the trappers frequented . This resulted in the development of the Missouri River <unk> , which specialized in carrying furs . Since these boats could only travel <unk> , they were dismantled and sold for lumber upon their arrival at St. Louis . 
