Geoscience Reference
In-Depth Information
The Graded Stream
Now that the graded stream concept has been discussed, you
can review it by viewing an animation. Go to the
Geo Media
Library
and access
The
Graded Stream
. This animation nicely
illustrates the idealized process of stream evolution and gradi-
ent adjustment. As you watch it, you will first see a longitudinal
profile similar to the uppermost one in Figure 16.20a. Notice how
this profile smoothes as time progresses through the combined
process of knickpoint reduction and filling of lakes and ponds
with sediment. Ultimately, the stream will obtain a slope that en-
ables it just to carry the sediment that is provided through hill-
slope erosion. Once you complete the animation, answer the
questions at the end to test your understanding of this concept.
in the side of the illustration. Each one of these profile lines
represents the gradient of the channel bed at a distinct period
of time in the evolution of the system. For reference, let's say
that the entire sequence of events takes about 1 million years to
complete.
The top profile line represents the stream gradient follow-
ing the initial uplift of the landscape above sea level. Soon after
the landscape was raised, a stream would begin to develop to
handle the runoff associated with heavy rains and groundwater
flow. Given the nature of streams, runoff on this landscape
would naturally flow downhill toward the sea and would fol-
low the path of least resistance. The gradient would naturally
be very steep in some places and relatively shallow in others.
The steepest gradients would most likely be associated with
resistant rock layers because stream erosion would be slow in
these locations compared to the areas of less resistant rock. As a
result, the stream gradient is higher where resistant rocks occur
and lower in areas of softer rock. A place where the stream gra-
dient is significantly steeper than other places within the stream
is called a
knickpoint
. Knickpoints are visible within streams as
rapids and, where gradients are especially steep, as waterfalls.
Where the gradient is low, overland low would collect in lakes.
Assuming that climate and vegetation cover remained essen-
tially the same, the stream would spend perhaps its first 300,000
to 400,000 years adjusting to the variations in gradient that oc-
cur across the landscape. Note, however, that the overall stream
gradient becomes smoother between the top two profiles.
One reason longitudinal profiles gradually become smooth
is that knickpoints slowly retreat upstream as they wear down
by stream erosion. A good example of how this process occurs is
at a great waterfall like Niagara Falls at the Ontario-New York
border (Figure 16.21). Niagara Falls is on the Niagara River,
which flows from Lake Erie into Lake Ontario over the Niagara
Dolomite, which is a resistant layer of limestone that overlies a
layer of easily eroded shale. This limestone was scraped clean
and covered by a continental glacier until about 12,000 years
ago, at which time the ice receded into Canada and an ungraded
landscape remained. Since the ice melted, the falls have re-
treated more than 11 km (6.8 mi) at a rate of about 1.3 m/year
of the water striking the plunge pool erodes the shale at the
base of the falls. As the shale erodes headward over time, the
overlying limestone collapses under its own weight and the falls
retreat. This cycle has been repeated countless times in the past
12,000 years and continues to the present time. Today about
5522 m
3
(195,000 ft
3
) of water pours over the falls every sec-
ond. Most of the water flows over the Canadian (or Horseshoe)
Falls, which measure about 790 m (2592 ft) in length and are
about 49 m (160 ft) high. Although the American Falls are the
same height, they are only approximately 305 m (1000 ft) long
and thus have less water flowing over them.
Returning to the concept of a smoothing stream profile,
let's examine our hypothetical stream about 500,000 years into
its history when it becomes graded. In other words, it can carry
the average sediment load in the basin. At this time, the stream
gradient has the ideal longitudinal profile (see Figure 16.20a),
with a steeper slope in the upper reaches of the basin and a shal-
lower slope downstream. From this point forward, this graded
profile is maintained and gradually lowered as the landscape is
further eroded.
Now let's turn our attention to the place where the stream
enters the ocean (or a large lake). This place is called the
stream's
base level
because it represents the lowest elevation at
which a stream can erode its channel bed. This concept is easy
to visualize because the stream cannot erode its bed below the
level of the water body in which it flows (Figure 16.20b). If it
did, it would be flowing uphill to meet the ocean or lake, which
is physically impossible. The concept of base level also applies
on a more local level, for example, where one stream flows into
another at a confluence. As at the ocean, the elevation of a tribu-
tary's stream bed must be the same as the channel bed of the
stream in which it flows.
Return to Figure 16.20a to see how base level relates to
stream behavior. In this particular example, the elevation
of base level did not change throughout the evolution of the
The lowest level at which a stream can no longer
lower its bed, because it flows into the ocean, a lake, or another
Base level
