Page 531 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch20_501-520.indd Page 508 9/3/10 6:23 PM user-f465
tiL12214_ch20_501-520.indd Page 508 9/3/10 6:23 PM user-f465 /Users/user-f465/Desktop

RUNNING WATER river, you know that the river is at work moving mountains, bit
Running water is the most important of all the erosional agents by bit, to the ocean. It has been estimated that rivers remove
of gravity that remove rock materials to lower levels. enough dissolved materials and sediments to make the whole
Erosion by running water begins with rainfall. Each raindrop surface of the United States flat in a little over 20 million years,
impacting the soil moves small rock fragments about, but it also a very short time compared to the 4.6 billion year age of Earth.
begins to dissolve some of the soluble products of weathering. In addition to transporting materials that were weathered
If the rainfall is heavy enough, a shallow layer or sheet of water and eroded by other agents of erosion, streams do their own
forms on the surface, transporting small fragments and dissolved erosive work. Streams can dissolve soluble materials directly
materials across the surface. This sheet erosion picks up fragments from rocks and sediments. They also quarry and pluck fragments
and dissolved material, then transports them to small streams at and pieces of rocks from beds of solid rock by hydraulic action.
lower levels (Figure 20.8). The small streams move to larger chan- Most of the erosion accomplished directly by streams, however,
nels, and the running water transports materials three different is done by the more massive fragments that are rolled, bounced,
ways: (1) as dissolved rock materials carried in solution, (2) as and slid along the streambed and against one another. This
clay minerals and small grains carried in suspension, and (3) as results in a grinding and filing action on the fragments and a
sand and larger rock fragments that are rolled, bounced, and slid wearing away of the streambed.
along the bottom of the streambed. Just how much material is As a stream cuts downward into its bed, other agents of
eroded and transported by the stream depends on the volume of erosion such as mass movement begin to widen the channel as
water, its velocity, and the load that it is already carrying. mate rials slump into the moving water. The load that the stream
Streams and major rivers are at work, for the most part, carries is increased by this slumping, which slows the stream. As
24 hours a day every day of the year moving rock fragments the stream slows, it begins to develop bends, or meanders, along
and dissolved materials from elevated landmasses to the oceans. the channel. Meanders have a dramatic effect on stream erosion
Any time you see mud, clay, and sand being transported by a because the water moves faster around an outside bank than it
does around the inside bank downstream. This difference in
stream velocity means that the stream has a greater erosion abil-
ity on the outside, downstream side and less on the sheltered area
inside of curves. The stream begins to widen the floor of the val-
ley through which it runs by eroding on the outside of the mean-
der, then depositing the eroded material on the inside of another
bend downstream. The stream thus begins to erode laterally,
slowly working its way across the land. Sometimes two bends in
the stream meet, forming a cutoff meander called an oxbow lake.

EXAMPLE 20.3
5
Each year a major river in a humid region transports 1.49 × 10 Mg of
3
2
sediment eroded from a drainage area of 6.54 × 10 km . What is the
thickness ( z) of soil, in meters, that would be eroded from this land-
scape in 1 million years? Assume the average density of the soil being
3
eroded in the drainage basin is 1.3 Mg/m and that all of the sediment
being transported in the river has been eroded from the landscape.
SOLUTION
This type of problem is best solved by subdividing it into steps that
consider the volume of sediment eroded in 1 million years and the
thickness of sediment associated with this volume.
Step 1: Volume Removed
5 Mg 6 m _ m _
_

ρ
m = 1.49 × 1 0 (1 × 1 0 yr) ρ = ∴ V =
yr V
11
11
1.49 × 1 0 Mg
= 1.49 × 10 Mg __
V =
Mg
Mg
_ 1.3 _
ρ = 1.3 3
3
m m
11 Mg
1.49 × 1 0 _
V = ? = _

Mg
1.3 _

m 3
FIGURE 20.8 Moving streams of water carry away dissolved
11
3
materials and sediments as they slowly erode the land. = 1.1 × 1 0 m
508 CHAPTER 20 Shaping Earth’s Surface 20-8