Yellowstone is, among its many attributes, the source of large and
mighty rivers. Located across the Continental Divide, the Park feeds two
of the most extensive drainage systems in the nation—(1) the Missouri
River system (and ultimately the Mississippi River) on the Atlantic
side, via the Yellowstone, Madison, and Gallatin Rivers, and (2) the
Columbia River system on the Pacific side, via the Snake River (fig. 1).
These streams are fed by an annual precipitation which averages about 17
inches at Old Faithful and Mammoth, but which is considerably greater in
the mountain ranges.
Many stretches of the main river valleys in Yellowstone are broad and
flat bottomed. In these, the stream gradients range from about 10 to 30
feet per mile, and there is little erosion going on at present (Hayden
Valley is a good example, fig. 4). But here and there the gradients are
steeper, and the valleys are narrow and rugged. In some places these
streams drop 50 or even 100 feet per mile, and the fast-moving waters
have carved deep V-shaped gorges (fig. 36).
Waterfalls, features for which Yellowstone is also justly famous (fig.
40), generally result from abrupt differences in rock hardness. If a
stream flows over rocks that are more resistant to erosion than the
rocks immediately downstream, a ledge or bench will form across the
streambed at that place because the less resistant rocks are worn away
faster. And, as the ledge becomes higher, the softer downstream rocks
will erode even faster. A true waterfall is one in which there is a
free, vertical fall of water. If the ledge or ledges form only a rough,
steep runway in the streambed, then the term “rapids” or “cascades” is
more appropriate.
The existence of many waterfalls in Yellowstone today is due in large
part to the fact that, because of recent volcanism and glaciation, much
of the region’s topography is very young in terms of geologic time.
Streams, even some of the largest ones, have not had enough time to wear
away all the features that may produce waterfalls, cascades, or rapids
along their channels. This is particularly true along the margins of
lava flows, where there are sharp dropoffs between the tops of the flows
and the lower ground beyond. The Grand Canyon of the Yellowstone and the
Upper and Lower Falls, well illustrate the erosive power of running
water.