Above the mouth of the Osage, the immediate valley of the Missouri gradually expands, embracing some wide bottoms in which are many settlements gradually increasing in the number of inhabitants. The Manito Rocks, and some other precipitous cliffs, are the terminations of low ranges of hills, running in quite to the river. These hills sometimes occasion rapids, and opposite the Manito rocks a small group of islands stretches obliquely across the river, separated by narrow channels in which the current is stronger than below. This group is called the Thousand Islands. Some of the channels are obstructed by collections of floating trees, which usually accumulate about the heads of islands, and are here called rafts. After increasing to a certain extent, portions of these rafts become loosened, and float down the river, covering nearly its whole surface, and greatly impeding and endangering the progress of the ascending boats. Council Bluffs, the seat of an important military establishment of the United States, about six hundred miles up the Missouri, is a remarkable bank, rising abruptly from the brink of the river to an elevation of one hundred and fifty feet. From the hill tops, a mile in the rear of the Bluffs, is presented a most extensive and beautiful landscape. On the east side of the river, the Bluffs exhibit a chain of peaks, stretching as far as the eye can reach. The river is here and there seen meandering in serpentine folds along its broad valley, chequered with woodlands and prairies, while, at a nearer view, you look down on an extensive plain, interspersed with a few scattered copses or bushes, and terminated at a distance by the Council Bluffs. Taken in connection with the Mississippi into which it flows, this river is the longest on the globe.[7] Its whole course, from its mouth in the gulf of Mexico to its source in the Rocky Mountains, is four thousand four hundred and twenty-four miles, including its windings; and for four thousand three hundred and ninety-six miles of this course it is navigable. From the point of its confluence with the Mississippi to fort Mandan, it is one thousand six hundred and nine miles; to the foot of the rapids at Great Falls two thousand five hundred and seventy-five miles; two thousand six hundred and sixty-four to where it issues from the mountains; two thousand six hundred and ninety to the Gates of the Mountains; three thousand and ninety-six to the extreme navigable point of Jefferson river; and three thousand one hundred and twenty-four miles to its remotest source. In this immense course it receives upwards of fifty large rivers, and one hundred and fifty smaller streams. Its principal tributaries are the Roche Jaune, or Yellowstone, the Kansas, Platte, Osage, Gasconade, Little Missouri, Running Water, Charaton, White, and Milk rivers.