Geoscience Reference
In-Depth Information
Overlying hard
limestone collapses
Retreating
escarpments
Plunge pool
Figure 16.21 Niagara Falls.
(a) Niagara Falls is a 51-m (167- ft)
waterfall on the Niagara River. It actually consists of two water-
falls, the Horseshoe Falls on the Canadian side (to the right) and
the American Falls (to the left). The eastern side of Lake Erie lies
in the background. (b) The Niagara River flows over a resistant
layer of limestone that overlies relatively soft shale. This shale is
eroded headward in the plunge pool, which results in collapse of
the overlying limestone and retreat of the falls. Niagara Falls has
retreated more than 11 km (6.8 mi) in the past 12,000 years.
Soft shale
is eroded
by water
stream profile. In reality, however, sea level—in other words,
the stream's base-level elevation—would probably change fre-
quently over such a period of time and would thus be a major
control in the behavior of the stream. If sea level fell, then a
wave of erosion would proceed upstream as the stream eroded
down to the new base-level elevation. On the other hand, if sea
level rose, then the stream would begin to deposit sediment and
aggrade its bed as it rose to the new base-level elevation. The
stream would also begin to erode if uplift occurred in the source
area of the stream, which would cause the channel slope to in-
crease and result in more energy. If such a scenario occurred,
the stream system would be said to be
rejuvenated
; that is, it
KEY CONCEPTS TO REMEMBER ABOUT
STREAM GRADATION
1.
Streams carry sediment as dissolved load, suspended
load, or bedload.
2.
A graded stream is a stream that has evolved to the point
where the amount of sediment it carries is balanced
with the capacity of the stream.
3.
An example of an ungraded landscape is one that
