Yellowstone, the largest affluent of the Missouri, rises high up in the Rocky Mountains in Wyoming, flows 25 miles NW. to the mountain-girt Yellowstone Lake (22 miles long, 7788 feet above sea-level), thence N. through the National Park into Montana, partly through stupendous canons, and then ENE. and NE. to the Missouri, on the western border of North Dakota. It is some 1300 miles long, and is navigable for steamboats 300 miles, to the mouth of the Big Horn, its largest affluent.

The Yellowstone National Park occupies the extreme north-western corner of Wyoming, and forms a square about 75 miles in diameter, almost all of it more than 6000 feet above sea-level, and rising in the snow-covered mountains to 10,000 and 14,000 feet. Situated on the 'Great Divide,' 'its pine-clad mountains form the gathering-ground for the head-waters of large rivers flowing away to the Atlantic and Pacific oceans.' The region is remarkable as well for its scenery as for its famous hot springs and geysers. The river has two falls about 15 miles below the lake, the lower one a magnificent cataract 330 feet in height; then it passes through the Grand Canon (20 miles), and receives Tower Creek, which itself has leapt out of a deep and gloomy canon known as Devil's Den over a beautiful fall of 156 feet. Near the river are many of the hot springs, those of White Mountain, near the northern boundary of the Park, extending for 1000 feet up the sloping side, and their snow-white calcareous deposits standing like a series of great frozen cascades. A few miles from Sulphur Mountain, with its vapours rising from fissures and craters, is the active Mud Volcano, with a crater 25 feet in diameter. All the hot springs of the Park number nearly 10,000. But the most singular feature of the region is its geysers, with columns of hot water 50 to 200 feet high, the most magnificent in the world. These are found principally on the Firehole River, a fork of the Madison, at the western end of Shoshone Lake, and in the Norris basin, to the north of that on the Firehole. The region was visited and described by surveyors "in 1869, and explored and mapped in 1871. In 1872 Congress dedicated and set it apart ' as a public park or pleasuring ground for the benefit and enjoyment of the people;' and increased the area in 1891. Two troops of cavalry are quartered in the Park to preserve the forests and wild animals (bisons, elks, antelopes, &c), and to act as police. A branch of the Northern Pacific Railway extends to the northern boundary of the Park. See works by G. M. Synge (1892) and Wiley (1893).