Running Water - The Changing Earth: Exploring Geology and Evolution - page 321

A valley might start to erode where runoff has suffi cient
energy to dislodge surface materials and excavate a small rill.
Once formed, a rill collects more runoff and becomes deeper
and wider and continues to do so until a full-fl edged valley
develops. Processes that contribute to valley formation in-
clude downcutting, lateral erosion, headward erosion, and
sheetwash. Mass wasting processes are also important.
Downcutting takes place when a river or stream has
more energy than it needs to transport sediment, so some
of its excess energy is used to deepen its valley. If down-
cutting were the only process operating, valleys would be
narrow and steep sided. In most cases, though, the valley
walls are undercut, a process called lateral erosion , creating
unstable slopes that may fail by mass wasting . Furthermore,
erosion by sheetwash and erosion by tributary streams
carry materials from the valley walls into the main stream
in the valley.
Valleys not only become deeper and wider, but also be-
come longer by headward erosion , a phenomenon involving
erosion by entering runoff at the upstream end of a valley
(

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Figure 12.23 Gullies and Valleys

Figure 12.24a). Continued headward erosion may result in
stream piracy , the breaching of a drainage divide and diversion
of part of the drainage of another stream (Figure 12.24b).
Once stream piracy takes place, both drainage systems must
adjust to these new conditions; one system now has greater
discharge and the potential to do more erosion and sediment
transport, whereas the other is diminished in its ability to ac-
complish these tasks.
According to one concept, stream erosion of an area
uplifted above sea level yields a distinctive series of land-
scapes. When erosion begins, streams erode downward;
their valleys are deep, narrow, and V-shaped, and their pro-
fi les have a number of irregularities (

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Figure 12.25a). As
streams cease eroding downward, they start eroding later-
ally, thereby establishing a meandering pattern and a broad
fl oodplain (Figure 12.25b). Finally, with continued erosion,
a vast, rather featureless plain develops (Figure 12.25c).
Many streams do indeed show the features typical of
these stages. For instance, the Colorado River fl ows through
the Grand Canyon and closely matches the features in
the initial stage shown in Figure 12.25a. Streams in many
areas approximate the second stage of development, and
certainly the lower Mississippi closely resembles the last
stage. Nevertheless, the idea of the sequential development
of stream-eroded landscapes has been largely abandoned
because there is no reason to think that streams necessarily
follow this idealized progression. Indeed, a stream on a gen-
tly sloping surface near sea level could develop features of
the last stage very early in its history. In addition, as long as
the rate of uplift exceeds the rate of downcutting, a stream
will continue to erode downward and be confi ned to a nar-
row canyon.

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a Gullies are small valleys, but they are narrow and deep.
This gully measures about 15 m across.

b This valley has steep walls that descend to a narrow valley
bottom.

Stream Terraces
Adjacent to many channels are erosional remnants of fl ood-
plains that formed when the streams were fl owing at a higher
level. These stream terraces consist of a fairly flat upper

surface and a steep slope descending to the level of the lower,
present-day fl oodplain ( Figure 12.26). Some streams have

several steplike surfaces above their present-day fl oodplains,
indicating that terraces formed several times.

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