Geoscience Reference
In-Depth Information
3.
As streams approach a graded state, they begin to
erode horizontally through the process of meandering.
This process produces a wide valley because bordering
bluffs are eroded.
4.
In the process of meandering, erosion occurs on the
outside of the meander at the cutbank, whereas depo-
sition is focused at the point bar on the inside of the
meander.
5.
You can tell that a stream is actively meandering (in
present time) if it has well developed oxbow lakes and
meander scars.
For example, when a graded, meandering stream responds
to a new period of landscape uplift associated with a major tec-
tonic event or base-level drop, the gradient of the stream is dra-
matically increased. Such a stream once again has a capacity to
carry more sediment than is delivered to it from the hillslopes.
As a result, the rejuvenated stream will begin a new period of
downcutting and channel-bed erosion.
A great place to see the effects of stream rejuvenation
is the Colorado Plateau region in the southwestern United
States. This part of the country has been affected by broad-
scale regional uplift for the past 10 million years. This uplift
raised the plateau about 1.5 km (0.9 mi), which caused the
stream gradients with the Colorado River system to steepen
dramatically. As a result, the Colorado River has downcut ex-
tensively, resulting in the famous Grand Canyon discussed in
Chapter 12 (Figure 12.20). This wave of erosion also spread
to tributaries of the Colorado such as the Green River and San
Juan River, creating the widespread canyons that are found
throughout the region.
The San Juan River is interesting because it contains well-
developed
entrenched meanders
. Prior to uplift of the Colorado
Plateau, the San Juan River was clearly a meandering stream
and the channel bed was somewhere near what is now the lip of
the gorge. In other words, the elevation of the stream channel
was much higher at that point in time, and the gorge had not
yet formed. As uplift of the plateau progressed and the stream
began to downcut, it preserved its meandering form because it
was no long eroding horizontally. This process produced the
stunning landscape of the famous
Goosenecks of the San Juan
River
pictured in Figure 16.27.
Figure 16.27 Goosenecks of the San Juan River in Utah.
These entrenched meanders formed as a result of extensive uplift
of the Colorado Plateau in the past 10 million years, which in-
creased the stream gradient of the San Juan River. The resulting
gorge is about 370 m (1400 ft) deep.
Although these adjustments do not result in dramatic canyons
and gorges, such as the Goosenecks of the San Juan River, they
nevertheless create distinctive landforms.
The most prominent landform created by smaller-scale flu-
vial adjustments is an alluvial terrace. An
alluvial terrace
is
a broad, flat surface that occurs within the stream valley and
is elevated with respect to the river so that it floods much less
frequently than the topographically lower point bars. Although
an alluvial terrace is usually considered to be part of the generic
floodplain, it is also useful to think of the surface as being an
abandoned floodplain because it is no longer frequently flooded.
Floodplain abandonment can occur in one of two ways.
One way is that, through repeated flooding, thick deposits of
alluvial sediment (or
alluvium
) accumulate on the active flood-
plain in a process called
alluviation
. As these sediments gradu-
ally thicken with repeated flooding, the active floodplain slowly
becomes elevated with respect to the stream so that progres-
sively higher discharges are required to flood the surface.
Another way in which floodplain abandonment and terrace
formation occur is when a stream downcuts following a period
of meandering (Figure 16.28a). This downcutting may be asso-
ciated with either a subtle period of regional uplift, base-level
drop, or regional climate change. When such an event occurs, the
stream responds to the new ungraded conditions by downcutting
through its own alluvium. This downcutting elevates the former
Alluvial Terraces
Entrenched meanders illustrate the kind
of landscape evolution that can occur when extensive steepen-
ing of a stream's slope happens due to large-scale regional up-
lift. Most of the time, however, the environmental adjustments
within a drainage basin are more subtle, consisting of relatively
minor fluctuations in base level or in the regional climate.
Alluvial terrace
A level, step-like landform that forms when
a stream erodes its bed so that a horizontal surface is raised
Sediment deposited by a stream.
Alluvium
