How Streams Transport Material
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SUSPENSION The Colorado River
carries much of its load in suspension.
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The eroded rock and soil materials that are transported downstream by a
river are called its load. A river transports, or carries, its load in three
different ways: in solution, in suspension, and in its bed load.
Mineral matter that has been dissolved from bedrock is carried in
solution. Common minerals carried in solution by rivers include dissolved
calcium, magnesium, and bicarbonate. Most of a river’s solution load
comes from groundwater seeping into the river. Before it reaches the
stream, the groundwater has traveled through fractures in the bedrock,
chemically eroding rock along the way.
When river water looks muddy, it is carrying rock material in
suspension. Suspended material includes clay, silt, and fine sand.
Although these suspended materials are heavier than water, the turbulence
of the stream flow stirs them up and keeps them from sinking. Turbulence
includes swirls and eddies that form in water as a result of friction between
the stream and its channel. The faster a stream flows, the more turbulent
and muddy it becomes. A rough or irregular channel also increases
turbulence.
A river may also transport rock materials in its bed load. The bed
load consists of sand, pebbles, and boulders that are too heavy to be
carried in suspension. These heavier materials are moved along the
streambed, especially during floods. Boulders and pebbles roll or slide
along the river bed. Large sand grains are pushed along the bottom in a
series of jumps and bounces.
The relative amounts of a river’s load that are carried in solution, in
suspension, and in the bed load depend on the nature of the river, the
climate, the type of bedrock, and the season of the year. As a general rule,
most of the load carried by the world’s streams and rivers is carried in
suspension. The size of a river’s suspended load increases with human
land use. Road and building construction and removal of vegetation make
it easier for rain to wash sediment into streams and rivers.
Stream Load
Observe how sediment is transported
by flowing water.
Keycode: ES1303
Flow
Suspension:
Silt and clay
Bed load:
sand,
gravel,
pebbles and
boulders
A stream transports rock materials in solution,
in suspension, and in its bed load. Materials
carried in solution cannot be seen.
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Unit 4 Earth’s Changing Surface