Geoscience Reference
In-Depth Information
Suspended load
(clay and silt particles)
Dissolved load
(mineral ions
in solution)
Direction of
stream flow
Turbulence
Rolling
Saltation
Sliding
Bedload
Figure 16.18 The sediment load of streams.
Streams carry their sediment load as
dissolved, suspended, and bedload.
other way to look at this change is that streams ideally evolve
in a way that allows them ultimately to balance the amount of
sediment they carry with the capacity of the stream. This sedi-
ment load is derived from hillslopes and tributary channels in
the drainage basin and can be carried in the following ways
(Figure 16.18):
1.
Dissolved load
—Mineral ions that are carried in solution
and are invisible during transport.
2.
Suspended load
—Sediment that floats along in the
stream. This load usually consists of clays and silts that
are held up by turbulent flow within the water.
3.
Bedload
—Larger particles such as sand and gravel that
roll, slide, or bounce along the channel bed in a pro-
cess called
saltation
. This form of transport dominates
in mountain streams where slopes are steep and stream
velocity is high.
KEY CONCEPTS TO REMEMBER ABOUT
FLUVIAL EROSION AND DEPOSITION
Erosional landforms are created where flowing water
carries sediment away, as on a steep hillslope.
Depositional landforms are constructed through accu-
mulation of sediment when the power to carry sediment
lessens, as on a level floodplain.
Hillslope erosion is most active on unvegetated slopes
and begins with a process called
splash erosion
, caused
by raindrops.
Flowing water first begins to erode hillslopes in rills. As
rills coalesce and enlarge, they form trench-like features
called gullies, which eventually broaden into canyons.
It is usually possible to determine the dominant kind of
sediment a stream is carrying by noting its fundamental channel
characteristics. If a stream is transporting mostly suspended sedi-
ment, it will typically have a single, deep channel that gradually
curves from side to side across the landscape (Figure 16.19a).
Such a stream is called a
meandering stream
and often contains
muddy-looking water due to the high concentration of suspended
sediment that was eroded from deposits containing abundant silt
and clay. In the Mississippi River, for example, approximately
90% of the sediment carried within the river is suspended load.
No wonder this river is frequently called
Big Muddy
! If a stream's
dominant load is bedload, then it will have a
braided stream
pat-
tern (Figure 16.19b), which consists of a maze of interconnected,
wide, shallow channels that look like the braids in someone's hair.
These streams have this form because the channel banks consist
of easily eroded deposits of sand, gravel, and even cobbles that
allow the stream to develop a wide, shallow form. In many cases,
these coarse-textured deposits are derived from melting glaciers
somewhere upstream. This relationship will be examined more
thoroughly in Chapter 17.
Stream Gradation
Let's now examine how landscapes evolve along larger, high-
order streams. Although these streams do not vigorously erode
like rills and gullies, they nevertheless shape the landscape in
subtle ways that have profound effects over a long period of
time. In most higher-order streams the combined processes
of runoff, erosion, and deposition have been occurring for
thousands of years.
Think of a stream in your neighborhood or city. Although
its fundamental appearance (e.g., its width, depth, or valley
width) probably has not changed much in the past few thousand
years, it was likely a very different system tens of thousands (or
even millions) of years ago in the early stages of its develop-
ment. Perhaps this ancestral stream had a much steeper gradient
and narrower valley than it currently does. Through time, the
system slowly adjusted internally to the environmental setting
in which it occurs—specifically, the amount of precipitation that
falls, the size of the drainage basin, the density of vegetation on
the landscape, and the rock and sediment types found there. An-
Braided stream
A network of converging and diverging
stream channels within an individual stream system that are
A river or small stream that curves back
and forth across its valley.
Meandering stream
