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Lake
(shallow gradient)
Lake disappears
(draining and infilling)
Knickpoint
Waterfall
(steep gradient)
Steep upstream
gradient and waterfall
V-shaped
valley
(a) Initial uplift
(b) Vigorous downcutting
Tributaries
form and extend
headward
Meanders
Deeply incised
valley
Floodplain
(c) Reaching equilibrium
(d) Horizontal expansion
Figure 16.22 Evolution of a graded stream and associated valley. During the first stages of valley evolution, the stream downcuts
vigorously because it has a steep gradient and excess energy. When the stream reaches a graded state, it begins to migrate horizontally,
resulting in a wide valley.
stream profile in Figure 16.20a. Lakes and waterfalls are
present in both illustrations. Recall that lakes occur in areas
where the slope is shallow, whereas waterfalls are associated
with knickpoints where the rocks are resistant and the gradi-
ent is steep. In this early stage of gradation, the stream has a
capacity to carry more sediment than is being delivered to it.
As a result, the stream begins to downcut vigorously in an at-
tempt to reach a state of equilibrium. In so doing, the stream
carves out a deep V-shaped valley (FigureĀ  16.22b) that is
consistent with the Grand Canyon of the Yellowstone River
(Figure 16.23).
As time progresses (Figures 16.22b and c), the lake
disappears through a combination of infilling by eroded up-
stream sediments and draining because the river cut back into
it after eroding through its downstream knickpoint. During
this interval of time, the gradient becomes smoother, as in
the second curve in Figure 16.20a. Turning back to Figures
16.22b and c, the stream begins to extend its valley farther
upstream (or headward ) because the gradient is steepest in
this part of the system. Because of this steep gradient, the
stream naturally flows at a greater speed and thus has more
energy to erode sediments. In Figure 16.22c, you can see
that tributaries begin to develop and also extend their val-
sediment than is provided to it from eroded hillslopes.
This stream will downcut its bed t h rough a process
called degradation .
4.
Stream aggradation occurs when the sediment load is
greater than the stream's capacity to carry it.
5.
Base level is the point where a stream flows into the
ocean and is a major control of a stream's behavior.
When base level rises, streams deposit sediment to
match that elevation; when base level falls, streams
erode their channel beds to cut down to sea level.
Evolution of Stream Valleys
and Floodplains
The next step in this discussion of stream systems is to investi-
gate the landforms that evolve in association with the long-term
process of stream gradation. As you work your way through
this section of the chapter, refer to Figure 16.22. This diagram
shows a simplified four-stage evolution of a stream and its
associated valley.
For simplicity, let's begin by imagining that a landscape
was uplifted or deglaciated at some point in the distant past.
 
 
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