Three great rivers receive the waters of the Yellowstone Park—the Yellowstone, the Missouri, and the Snake. The first two rivers are on the Atlantic slope; the third is on the Pacific slope. The areas drained by them are approximately:
| By the Yellowstone, | 1,900 square miles. |
| By the Missouri, | 730 square miles. |
| By the Snake, | 682 square miles. |
The Yellowstone River has its source in the snow drifts of Yount Peak, twenty-five miles south-east of the Park. It enters the Reservation six miles west of the south-east corner; crosses it in a direction somewhat west of north, and leaves it at a point about nineteen miles east of the north-west corner. Near the center of the Park it flows through the celebrated lake of the same name, and further north passes through two remarkable cañons before it leaves the Reservation. Its principal tributaries within the Park are the Lamar River (commonly called the East Fork), from the east, and Gardiner River from the west. The Lamar River rises nearly due east of the outlet of Yellowstone Lake and flows north-westerly, joining the main stream near Junction Butte. Its principal tributary is Soda Butte Creek, which rises just outside the north-east corner of the Park and joins the Lamar River near the extinct hot spring cone from which it derives its name.
Gardiner River is the second largest tributary of the Yellowstone, and drains the extensive area between the Washburn and Gallatin Mountains.
The low-water discharge of the Yellowstone River, as measured by the writer, in 1891, a little below the lake outlet, is 1,598 cubic feet per second; as measured by the United States Geological Survey, in 1886, 1,525 cubic feet. The discharge at the north boundary of the Park can not be less than 2,000 cubic feet.
The Missouri River drainage flows into the Gallatin and Madison forks of that stream. The Gallatin drains only a small area in the extreme north-west corner of the Park. The Madison is formed by the junction of the Gibbon and Firehole Rivers, about twelve miles east of the west boundary of the Park. The Gibbon takes its rise a few miles west of the Falls of the Yellowstone, and flows in a south-west direction. The Firehole rises in Madison Lake, and flows north to its junction with the Gibbon. Its principal tributaries are the Little Firehole River and Iron Creek on the west, and Nez Percé Creek on the east.
The Snake River drains the south-west portion of the Park. It rises about fifteen miles south of Yellowstone Lake, just outside the Park. It then takes a northerly circuit into the Park, receiving the waters of Hart and Lewis Rivers, and leaves the Reservation just north of Jackson Lake. Its principal tributary is the Lewis River, which drains Shoshone and Lewis Lakes. Several large streams, Bechler and Falls Rivers among them, cross the south-west boundary of the Park and join the main Snake further south.
The line of separation between this water-shed and those of the Yellowstone and the Missouri, is the Continental Divide, the irregular course of which can be readily understood by consulting the map.
In the entire Park there are about thirty-six named lakes with a total area of nearly 165 square miles. Of these lakes, twenty-one, with an area of 143 square miles, are on the Yellowstone slope; eight, with an area of perhaps two square miles, are on the Missouri slope; and seven, with an area of about twenty square miles, are on the Snake River slope. The four principal lakes—Yellowstone, Shoshone, Lewis, and Hart—are clustered near the Continental Divide at its lowest point, the first being on the Atlantic slope, and the others on the Pacific.
There are upon the various streams of the Park no fewer than twenty-five interesting water-falls, where the streams descend from the Park plateau to the lower surrounding country.