Running water is another effective agent that transports rock debris and
has helped dissect the Teton Range. The damage a broken water main can
wreak on a roadbed is well known, as is the havoc of destructive floods.
The spring floods of streams in the Tetons, swollen by melting snow and
ice (annual precipitation, mostly snow, in the high parts would average
a layer of water 5 feet thick), move some rock debris onto the adjoining
floor of Jackson Hole.
[Illustration: Figure 5. _The Lower Gros Ventre slide, air oblique
view south. The top of the scar is 2,000 feet above the river; the
slide is more than a mile long and one-half mile wide. It dammed the
Gros Ventre River in the foreground, impounding a lake about 200
feet deep and 5 miles long. Gros Ventre Mountains are in the
distance. Photo by P. E. Millward._]
Now and then the range is deluged by summer cloudbursts. Water funnels
down the maze of gullies on the mountainsides, quickly gathering volume
and power, and plunges on to the talus slopes below, as if from gigantic
hoses. The sudden onslaught of these torrents of water on the saturated
unstable talus may trigger enormous rock and mudflows that carry vast
quantities of material down into the canyons. During the summer of 1941
more than 100 of these flows occurred in the park.
Wherever water moves, it carries rock fragments varying in size from
boulders to sand grains and on down to minute clay particles. _Erosion_
(wearing away) by streams is conspicuous wherever the water is muddy, as
it always is each spring in the Snake, Buffalo Fork, and Gros Ventre
Rivers. Clear mountain streams likewise can erode. Although the volume
of material moved and the amount of downcutting of the stream bottom may
not seem great in a single stream, the cumulative effect of many streams
in an area, year after year and century after century, is enormous.
Streams not only transport rocks brought to them by gravitational
movement but also continually widen and deepen their valleys, thereby
increasing the volume of transported debris.
The effectiveness of streams as transporting agents in the Tetons is
enhanced by steep _gradients_ (slopes); these increase water velocity
which in turn expands the capability of the streams to carry larger and
larger rock fragments.