Yellowstone, the largest affluent of the Missouri River, rises high up in the Rocky Mountains in Wyoming, about N. lat. and W. long., flows 25 miles north-west to the mountain-girt Yellowstone Lake (22 miles long, 7788 feet above sea-level), thence northward through the National Park into Montana, partly through stupendous cañons, and then east-north-east and north-east 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. Its area was originally 3575 sq. m., to which congress in 1891 added a tract of nearly 2000 sq. m. to the south and east—nearly all more than 6000 feet above sea level, and rising in the snow-covered mountains to 10,000 to 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; and for the sake of the rainfall and the rivers its forests are carefully preserved. 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 Cañon (20 miles), and receives Tower Creek, which itself has leapt out of a deep and gloomy cañon 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. The semicircular basins, in which the water gathers in pools, and from one to another of which it flows over with gradually lessening temperature, are bright with bead-like tracery of scarlet, yellow, orange, and green on the white groundwork—the colours being nearly always due to the presence of certain coloured algae that live in waters of high temperature, and by whose instrumentality the great siliceous and calcareous sinter deposits have been built up. 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 (see GEYSER, and illustration there), 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 pleasing ground for the benefit and enjoyment of the people.' At the same time it provided against the wanton destruction of fish and game, or their capture or destruction for merchandise or profit; and, as a happy result of this enactment, several hundred bisons and some thousands of elk, antelopes, Rocky Mountain sheep, &c. have found a refuge within the Park. A branch of the Northern Pacific Railway extends to the northern boundary of the Park. According to the report of 1898, everything was going admirably; though the bisons do not increase, the wapiti and many other species have greatly increased. The cavalry police is very efficient.
See the Official Guide, and books by Hayden (1877), Synge (1892), Wiley (1893), and Chittenden (1896).