YELLOWSTONE NATIONAL PARK, an American national reservation, situated mainly in N.W. Wyoming, U.S.A., dedicated by the United States government as "a public park or pleasure ground for the benefit and enjoyment of the people." It is nearly a rectangle in shape, with a length, from N. to S., of 62 m., a width of 54 m. and an area of approximately 3350 sq. m. It extends into Montana, on the N., about 2¼ m. and into Montana and Idaho, on the W., 2 m. Except at its main entrance, through the valley of the Yellowstone on the N., the park is entirely surrounded by national forests: the Gallatin and Absaroka national forests, on the N.; the Shoshone and the Beartooth, on the E.; the Teton, on the S.; and the Targhee, the Madison and the Gallatin, on the W.
The central portion, comprising an area of about 2000 sq. m., is an undulating volcanic plateau with a mean elevation above the sea of about 8000 ft. Along the entire E. border stretches the Absaroka range, with peaks exceeding 11,000 ft. (Index Peak, 11,740 ft.) in height. On the N. is the Snowy range with its snow-capped peaks. W. of the Snowy the Gallatin range extends S. for 20 m. along the W. border. Electric Peak, in the N.W. corner of the park, rises to a height of 11,155 ft. Near the S. end of the park are the Red Mountains, which culminate in Mt. Sheridan (10,385 ft.) and afford a magnificent view of the whole region; and farther S. the N. spur of the lofty Tetons juts across the S. border.
In the production of these mountains and plateau there was first, at the close of the Cretaceous period, an upheaval of the earth's substance to form a mountain rim and a depressed basin. Subsequently, in the Tertiary period, there were two enormous outpourings of volcanic material—first andesitic lava, and later, after a long interval of quiet, rhyolitic—which nearly half filled the basin, converted it into a plateau and broke up the mountain rim. Two centres of volcanic activity were Mt. Sheridan, in the S., and Mt. Washburn, in the N. The volcanoes have long been extinct, but the diminished energy now causes hot springs and geysers in all parts of the plateau, about 100 in number. More than half, including the largest and finest, are in the upper and the lower Geyser basins, near the bead of the Madison, here known as the Firehole, river. Several others are farther N. in the Norris basin upon Gibbon river, a branch of the Madison, and others are farther S. in the Shoshone basin.
Excelsior, the largest geyser, with a crater about 300 ft. long and 200 ft. wide, has not been active since 1890, but for several years after its discovery it threw up at intervals a huge mass of water to a height of 200–250 ft. Old Faithful, at regular intervals of 65–70 minutes, throws up a column of hot water 2 ft. in diameter to a height of 125–150 ft., and the eruption lasts 4–4½ minutes. The Giant, at intervals of 2 to 4 days or more, throws up a column to a height of 250 ft. for 90 minutes. The Beehive (so called from the shape of its cone), the Grand and the Lone Star throw up columns to a height of 200 ft. but at irregular intervals. In the Norris basin are the Black Growler and the Hurricane, which consist of small apertures through which steam rushes with such tremendous force that it may be heard for miles. The hot springs are widely distributed over the plateau and number from 3000 to 4000. The water of most of the springs and geysers holds silica in solution in considerable quantities, so that as it cools and evaporates it deposits a dazzling white sinter which has covered many square miles of the valleys and contrasts strongly with the dark green of the surround mg forests. The springs, geysers and steam vents are scattered over it in the most irregular fashion. The silicious matter has also built up around the springs and geysers cones or mounds of considerable size and great beauty of form. The water of many of the springs contains sulphur, iron, alum and other materials in solution, which in places stain the pure white sinter with bright bands of colour. The tints and hues of some of the pools are of matchless beauty. Near the N. boundary of the park there is a group of about 70 active springs, known as the Mammoth Hot Springs, which hold carbonate of lime in solution. Their deposits have built across a small valley or ravine a series of broad, flat, concentric terraces beautiful in form and 300 ft. in height. The water which trickles over the rims of the pools and basins on the upper terraces is a transparent blue, while the formation itself contains a network of fibrous algae which gives it a wonderful variety of colours. In the lower Geyser basin are the Mammoth Paint Pots, a group of mud springs with colours varying according to the mineral ingredients in the steam, which not only colours the mud but also forms it into imitative figures. Near the centre of the park is Mud Caldron, a circular crater about 40 ft. deep with the boiling mud at the bottom. Although there have been some changes in the thermal energy in the park since 1871, there has been no appreciable diminution. Certain springs and geysers lose some of their energy at intervals, while others gain; certain geysers have become quiescent, but some new ones have been formed.
The Continental Divide crosses the park in a S.E. direction from the meeting-point of the states of Wyoming, Idaho and Montana. The small section S. of the Divide is drained by the Snake river into the Columbia river and the Pacific Ocean; the large section N. of the Divide is drained by the Yellowstone and Madison rivers into the Missouri, the Mississippi and the Gulf of Mexico. The Lewis river, a fork of the Snake, has its origin in the beautiful Shoshone Lake, and the Heart river, another fork of the Snake, rises in Heart Lake, under Mt. Sheridan.
The Yellowstone drains the entire E. section. Rising just beyond its S. limits, it flows into and through Yellowstone Lake, a magnificent sheet of water, very irregular in shape, dotted with forested islands, having an area of about 140 sq. m., lying 7741 ft. above the sea and nearly surrounded by lofty mountains. A few miles below the lake, the river, after a succession of rapids, leaps over a cliff, making the Upper Fall, 109 ft. in height. Half a mile lower down it rolls over the Lower Fall, which has a clear descent of 308 ft. The river at this point carries, at the average stage of water, about 1200 cub. ft. per second. With this fall the river enters the "Grand Canyon," which in many scenic effects is unequalled. Its depth is not great, at least as compared with the canyons upon the Colorado river system; it ranges from 600 ft. at its head to 1200 near the middle, where it passes the Washburn Mountains. Its length to the mouth of Lamar river is 24 m. It is cut in the volcanic plateau, and its ragged broken walls, which are inclined at very steep angles, are of a richness of colouring that almost defies description, a colouring that is produced by the action of the thermal springs, at the base of the canyon, upon the mineral pigments in the lava. Bright orange, yellow, red and purple hues predominate and are set off very effectively against the dark green pines with which the margins of the canyon are fringed, and the white foam of the river at the bottom of the chasm. Near the foot of the Grand Canyon, Tower creek, which drains the concavity of the horseshoe formed by the Washburn Mountains, enters the Yellowstone. Just above its mouth this stream makes a beautiful fall of 132 ft. into the gorge in which it joins the river. A few miles farther down, the Yellowstone is joined by an E. branch, Lamar river, which drains a large part of the Absaroka Range. Then it enters the Third Canyon, from which it emerges at the mouth of Gardiner river. The latter stream drains an area of elevated land by means of its three forks, and upon each of them occurs a fine fall in its descent toward the Yellowstone. The Madison rises in the W. of the park and flows in a generally N. and then W. course out of the park. Its waters are mainly collected from the rainfall upon the plateaux, and from the hot springs and geysers, most of which are within its drainage area. Upon this river and its affluents are several fine falls. Indeed, all the streams in this region show evidence, in the character of their courses, of a recent change of level in the surface of the country.
The climate, influenced by the high elevation, is characterized by long and severe winters and short summers with great diurnal extremes of temperature. But the low temperature causes the moisture-laden winds to deposit here greater quantities of rain and snow than in the semi-arid regions below, which not only promote the growth of vegetation, but cause the activity of the springs, geysers and waterfalls. The mean annual temperature at the station of the United States Weather Bureau, near the N. boundary, is 39° F. The summer (June, July and August) mean is 59°; the winter (December, January and February) mean, 20°.
Extremes have ranged from 96° in July to −35° in February. The temperature has fallen to 30° in July, and a warm summer day may at any time be followed by frost at night. The mean annual precipitation is 19·6 in. Much of this is in the form of snow, and nearly half of it is during the four months from December to March; in the four months, from July to October, it is only 4·4 in. Some snow falls in every month except July and August, and the average annual snowfall amounts to 94·7 in. The prevailing winds are S.
About four-fifths of the park is covered with dense forests of black pine (Pinus Murrayana), balsam, fir, spruce, cedar and poplar. These trees do not attain a large size. A low blueberry (Vaccinium myrtilis) forms a thick underbrush in much of the forest. Choke-cherries, gooseberries, buffalo berries, red currants and black currants grow along the streams and in moist places of the lower altitudes. In the glades are
