Running Water - The Changing Earth: Exploring Geology and Evolution - page 324

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Figure 12.28 Origin of a Superposed Stream

Water gap

a As a stream erodes down and removes the surface layers of
rock, it is lowered onto ridges that form when resistant rocks in the
underlying structure are exposed.

b The narrow valleys through the ridges are water gaps.

c View of a water gap cut by the Jefferson River in Montana.

Geo-Recap

Chapter Summary

Water continuously evaporates from the oceans, rises as
water vapor, condenses, and falls as precipitation. About
20% of this precipitation falls on land and eventually
returns to the oceans, mostly by surface runoff.

Flow velocity and discharge are related, so that if either
changes, the other changes as well.
Erosion by running water takes place by hydraulic action,
abrasion, and solution.
The bed load in channels is made up of sand and gravel,
whereas suspended load consists of silt- and clay-sized
particles. Running water also transports a dissolved
load.
Braided waterways have a complex of dividing and
rejoining channels, and their deposits are mostly sheets of
sand and gravel.
A single sinuous channel is typical of meandering
streams that deposit mostly mud, with subordinate point-
bar deposits of sand or, more rarely, gravel.

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Running water moves by laminar fl ow, in which stream-
lines parallel one another, and by turbulent fl ow, in which
streamlines complexly intertwine. Almost all fl ow in
channels is turbulent.
Runoff takes place by sheet fl ow, a thin, more or less con-
tinuous sheet of water, and by channel fl ow, confi ned to
long, troughlike stream and river channels.
The vertical drop in a given distance, or the gradient, for
a channel varies from steep in its upper reaches to more
gentle in its lower reaches.

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