Geoscience Reference
In-Depth Information
Base flow is the average stream discharge in a given
stream at any particular time of the year. Bankfull dis-
charge occurs when the channel is full. Floods occur
when discharge is sufficiently high that water spills out
of the river channel.
erosion occurs when sediment is removed from one place by a
geomorphic process such as slumping, stream flow, or glacia-
tion. Erosion is usually more intense in areas of high relief
because the force of gravity enhances the ability of geomor-
phic processes to do work by increasing their power. Other
factors that influence the process of erosion are vegetation and
climate. Erosion tends to be less in areas of dense vegetation
because the plants protect hillslopes from the effects of run-
ning water. Stream erosion is also limited in the driest des-
erts because very little running water occurs there. In fact, the
most intense erosion tends to occur in semi-arid to subhumid
zones where plant cover is relatively sparse but heavy rains
that can move sediment sometimes occur.
In contrast to erosion, the process of deposition occurs
when sediments that are being transported (after being eroded
from someplace else) stop moving and are dropped. Deposition
occurs when the transporting agent (such as wind or running
water) simply loses the power to carry the sediment, which can
happen for a variety of reasons. Although erosion and deposi-
tion often work in tandem to shape a landscape, one process or
the other tends to dominate in any particular place at specific
periods of time. Thus, it is possible to classify landforms gen-
erally as being either erosional or depositional in their nature.
Erosional landforms are created when sediment, soil, or rock is
stripped away by some geomorphic process. Depositional land-
forms, in contrast, form when sediment accumulates after being
dropped.
Figure 16.15 shows a simplified example of these catego-
ries. Here, the mountain slopes have largely been shaped by
erosion due to the high energy created by the steep relief. This
Prediction of flood discharge is based on statistical
probability. The return period refers to the frequency
with which a particular discharge is expected to occur.
High-magnitude floods occur less frequently than less
intense discharges.
Fluvial Processes
and Landforms
Now that the nature of stream channel flow has been discussed,
let's next examine the landforms that streams produce. Land-
forms created by fluvial processes are called fluvial landforms .
Although processes such as glaciation, wind, and ocean waves
also act to shape the Earth's surface (and will be covered in sub-
sequent chapters), running water is the single most important
geomorphic agent on Earth. This process is driven by gravity,
which is the most important geomorphic force on Earth.
Erosion and Deposition
Discussion of stream landforms requires an understand-
ing of erosion and deposition as geomorphic processes be-
cause they act in concert to shape the landscape. Simply put,
Landfor m s due to erosion
Peak
Canyon
(deeper than
ravine)
Saddle
Landfor m s due
to deposition
Spur
Alluvial
fan
River floodplain
Gully
Ravine
(deeper than
gully)
Figure 16.15 Fundamental erosional and depositional landforms.
Although erosion or deposition can occur anywhere,
 
 
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