Yellowstone River, a tributary of the Missouri, rising in Yellowstone lake, in the Yellowstone national park, N. W. Wyoming, near the sources of the Madison (the main constituent of the Missouri) and the Snake. It flows first N. through a series of cafions, about 100 m., into and through a portion of Montana, when, issuing from the mountains, it pursues an E. N. E. course of about 500 m. to its mouth on the border of Montana and Dakota. The lake is 22 by 15 m. in extent, with an elevation of 7,788 ft. ab6ve the sea and a maximum depth of 300 ft. Its shores are rugged but picturesque. It contains immense numbers of salmon trout. The Upper Yellowstone, about 25 m. long, the ultimate source of the river, flows into it. The elevation of the mouth of the Yellowstone is 2,010 ft. About 15 m. below the lake are the upper falls, where the river, after passing through a series of rapids, makes an abrupt descent of 140 ft. The lower falls, ¼ m. further down, are 360 ft. high. The stream then flows for 20 m. through the Grand caiion, whose perpendicular sides, from 200 to 500 yards apart, rise to the height of 1,000 ft.

Just below the Grand canlon the river receives Tower creek, which flows through a gloomy canon, 10 m. long, known as the "Devil's Den." About 200 yards above its mouth the creek has an abrupt fall of 156 ft., surrounded by columns of breccia resembling towers. Below the mountains the course of the Yellowstone lies through a wide open valley bounded by high rolling hills. Lignite is abundant along its banks. Its chief tributaries, proceeding down the stream, are Shields river and Great and Little Porcupine rivers on the left, and Beaver river, Big Rosebud creek, Clarke's fork, Pryor's, Big Horn, Rosebud, Tongue, and Powder rivers on the right. The Big Horn and Powder are much the largest of these tributaries. The Yellowstone is navigable to near the mouth of the Big Horn, about 300 m. above the Missouri.