Chapter 6
Running Water &
Groundwater
Section 6.2
The Work of
Streams
The work of streams
• Streams are Earth’s most important agents of erosion.
• Weathering delivers material to streams by sheet flow, mass
movements and groundwater.
• Streams generally erode their channels lifting loose particles
by abrasion, grinding, and by dissolving soluble material.
• The stronger the current is, the more erosional power it has
and the more effectively the water will pick up particles.
• Sand and gravel carried in a stream can erode solid rock
channels like sandpaper.
– Ex: Pebbles caught in swirling stream currents can act like cutting
tools and bore circular “potholes” into the channel floor.
The work of streams
• Streams transport sediment in three ways:
– In solution (dissolved load).
– In suspension (suspended load).
– Scooting or rolling along the bottom (bed
load).
The work of streams
1. Dissolved Load:
• Most of the dissolved load enters streams through
groundwater.
–
•
•
The amount of material in a stream is dependent
upon the climate and geologic setting.
Usually expressed in parts per million (ppm).
–
•
•
Some through dissolving rock along stream’s course.
Ex: Some rivers have a dissolved load of 1000 ppm.
Average for the world’s rivers is estimated at 115
to 120 ppm.
Streams supply almost 4 billion metric tons of
dissolved substances to the oceans each year.
The work of streams
2. Suspended Load:
• Most streams carry the largest part of their load in
suspension.
• The visible cloud of sediment suspended in the
water is the most obvious portion of a stream’s
load.
• They usually carry only sand, silt, and clay this
way.
• Streams also transport other materials during a
flood because of increased water velocity.
The work of streams
The work of streams
3. Bed Load:
• The part of a stream’s load of solid material
that is made up of sediment too large to be
carried in suspension.
• Larger, coarser particles move along the
bottom, or bed, of the stream channel.
• The bed load only moves when the force of
water is great enough to move the larger
particles.
The work of streams
•
The ability of streams to carry a load is determined by two
factors:
–
–
1.
•
•
Competence:
Measures the largest particles it can transport.
Increases with velocity.
–
2.
•
•
The stream’s competence
The stream’s capacity.
When the velocity doubles, the competence of a stream increases
by a factor of 4.
Capacity:
The maximum load it can carry.
Directly related to discharge.
–
The greater the volume of water in a stream, the greater the
capacity is for carrying sediment.
Most streams carry the
largest part of their load
A.
B.
C.
D.
As dissolved material.
Along the stream’s bottom.
As bed load.
In suspension.
The suspended load of a
stream
A. Is deposited before the bed load.
B. Consists primarily of highly soluble
substances.
C. Moves along the bottom of the
channel by rolling or sliding.
D. Usually consists of fine sand, silt, and
clay-sized particles.
The capacity of a stream is
directly related to its
A.
B.
C.
D.
Velocity.
Discharge.
Gradient.
Meandering.
What is the measure of the
largest particles a stream can
carry?
A.
B.
C.
D.
Competence.
Capacity.
Discharge.
Gradient.
The work of streams
• Whenever a stream slows down, the situation reverses.
– Velocity decreases = competence decreases, and sediment begins to
drop out (largest particles first).
• Deposition occurs as streamflow drops below the critical
settling velocity of a certain particle size. The sediment in that
category begins to settle out.
• Stream transport separates solid particles of various sizes,
large to small.
• This process is called sorting.
• The sorted material deposited by a stream is called alluvium.
• Many different depositional features are made of alluvium.
The work of streams
1.
•
Deltas:
When a stream enters the relatively still waters of an ocean
or lake, its velocity drops, resulting in the stream depositing
its sediment which forms a delta.
–
•
Delta: An accumulation of sediment formed where a stream enters
a lake or ocean.
As a delta grows, the stream’s gradient lessens and the
water slows down.
The channel becomes “choked” with sediment resulting in
the river changing direction searching for a shorter route to
base level.
The stream often divides into several smaller channels
(distributaries), which act in the opposite way of tributaries.
•
•
–
Distributaries carry water away.
The work of streams
The work of streams
• After many shifts, a delta may grow into a
triangular shape (Δ = Greek letter delta).
• Not all delta’s have this ideal shape.
• Differences in the shapes of the shorelines
and variations in the strength of waves and
currents result in different shapes of the delta.
The work of streams
2. Natural Levees:
• Some rivers occupy valleys with broad, flat floors.
• Successive floods over many years can build
natural levees along them.
– Natural Levee: A landform that parallels some streams.
• Forms when a stream overflows its banks; its
velocity rapidly decreases and leaves coarse
sediment deposits in strips that border the channel.
• As the water spreads out over the valley, less
sediment is deposited.
• This uneven distribution of material produces the
gentle slope of a natural levee.
The work of streams
A depositional feature that
forms where a stream enters a
lake or ocean is a (an)
A.
B.
C.
D.
Natural Levee.
Delta.
Meander.
Oxbow Lake.
A natural levee is
A. An erosional feature perpendicular to the
stream channel.
B. A depositional feature perpendicular to the
stream channel.
C. An erosional feature parallel to the stream
channel.
D. A depositional feature parallel to the
stream channel.
In a stream channel, which of
the following will be deposited
first?
A.
B.
C.
D.
The dissolved load.
Fine sand and silt.
Gravel-sized particles.
Clay-sized particles.
Occasionally, deposition
causes the main channel of a
stream to divide into several
smaller channels called
A.
B.
C.
D.
Oxbow lakes.
Distributaries.
Meanders.
Deltas.
The work of streams
•
There are two different types
of stream valleys:
1. Narrow Valley:
• A narrow V-shaped valley
shows that the stream’s
primary work has been
downcutting toward base level.
• Prominent features are rapids
and waterfalls, which occur
where the stream profile drops
rapidly.
The work of streams
2. Wide Valleys:
• Once a stream has cut its channel closer to base
level, it starts widening.
• The side-to-side cutting of a stream eventually
produces a flat valley floor, or floodplain.
• Streams that flow on floodplains move in meanders.
• Most of the erosion occurs on the outside of the
meander – often called the cut bank – where the
velocity and turbulence are greatest.
• The debris that is removed is then deposited
downstream as point bars.
• Point bars form in zones of decreased velocity on
the insides of meanders.
The work of streams
• Erosion is more effective on the downstream side of
a meander because of the slope of the channel.
• The bends gradually travel down the valley.
• Sometimes the movement of a meander slows when
it reaches a more resistant portion of the floodplain,
which results in the next meander overtaking it.
• The meander is narrowed, and eventually the
meander is cutoff, and because of its shape, the
abandoned bend is called an oxbow lake.
The work of streams
The flat portion of a valley floor
adjacent to a stream is called a
A.
B.
C.
D.
Floodplain
Meander
Divide
Tributary
The most prominent features of
a narrow, V-shaped valley,
where the stream profile drops
rapidly, are
A.
B.
C.
D.
Meanders and Floodplains
Rapids and Waterfalls
Lakes and Ponds
Deltas and Natural Levees
A floodplain forms where a
stream
A.
B.
C.
D.
Cuts downward rapidly.
Is far above its base level.
Carries no bed load.
Cuts mainly side to side.
Which of the following would
least likely be found in a wide
valley?
A.
B.
C.
D.
Oxbow Lake
Meanders
Rapids
A Cutoff
What type of stream valley
would form in a mountainous
region?
A. A wide, flat valley.
B. No valley would form.
C. The type of valley would depend on the
stream discharge.
D. A narrow V-shaped valley.
The work of streams
• A flood occurs when the discharge of a
stream becomes so great that it exceeds the
capacity of its channel and overtakes its
banks.
• Floods are the most common and most
destructive of all natural geologic hazards.
• Most floods are caused by rapid spring snow
melt or storms that bring heavy rains over a
large region.
– Ex: Mississippi River Valley, Summer of 1993.
The work of streams
The work of streams
• Flash floods occur with little warning, and they can be deadly
as walls of water sweep through river valleys.
• Several factors influence flash floods:
– Rainfall intensity/duration.
– Surface conditions.
– Topography.
• Human interference can worsen or even cause floods.
– Ex: Failure of a dam or an artificial levee.
• There are several flood control strategies:
– Artificial levees.
– Flood control dams
– Placing limits on floodplain development.
The work of streams
1. Artificial Levees:
• Artificial levees are earthen mounds built on the
banks of a river.
• They increase the volume of water a channel can
hold.
• Because the stream cannot deposit material
outside of its channel the bottom gradually builds
up, thus it takes less water to overflow the levee.
• They are not built to withstand periods of extreme
flooding.
The work of streams
2. Flood-Control Dam:
• Store floodwater and then let it out slowly.
• Since the 1920’s, thousands of dams have
been built on nearly every major river in the
U.S.
• Dams are not built to last forever, as sediment
builds up behind the dam, and stored water
will gradually diminish.
• Large dams also cause ecological damage to
river environments.
The work of streams
3. Limiting Development:
• Many scientists and engineers advocate
sound floodplain management instead of
building structures.
• Minimizing development on floodplains allows
them to absorb floodwaters with little harm to
homes and businesses.
The work of streams
• Every stream has a drainage basin.
• A drainage basin is the land area that contributes
water to a stream.
• An imaginary line called a divide separates the
drainage basins of one stream from another.
• Divides range in scale from a ridge separating two
small gullies on a hillside to a continental divide,
which splits continents into enormous drainage
basins.
• The Mississippi River has the largest drainage basin
in North America.
– The river and its tributaries collect water from more than
3.2 million square kilometers of the continent.
The work of streams
One major cause of floods
is
A.
B.
C.
D.
Rapid spring snow melt.
A decrease in stream discharge.
Light rain over a large area.
Increased capacity of stream channels.
One traditional flood control
method has been to attempt to
keep the stream’s flow within its
channel by creating
A.
B.
C.
D.
Meanders.
Artificial Cutoffs.
Artificial Levees.
Flood Control Dams.
Limiting development on
floodplains is effective
because it
A. Allows floodplains to absorb floodwaters
with little harm to structures.
B. Eliminates wide stream meanders.
C. Is more expensive to build structures on
flat land.
D. Helps keep floodwaters within stream
channels.
A drainage basin is
A. The channel of a stream.
B. The land covered by floodwaters.
C. The land area that contributes water to a
stream.
D. All streams that flow directly into an
ocean.
What are the boundaries
called that separate streams
in adjacent drainage
basins?
A.
B.
C.
D.
Mountain ranges.
Divides.
Valleys.
Levees.