Transcript 13.2 Stream Erosion and Deposition

13.2 Stream Erosion and
Deposition
Key Idea:
As streams flow, they erode rocks and soil,
transport these sediments over long
distances, and deposit them in areas of
quieter water.
Learning Objectives
1. Describe how streams weather and erode
Earth’s surface.
2. Describe how streams transport and
deposit sediments.
3. Explain how deltas and alluvial fans form.
How Streams Weather and Erode
Material
 Running water in streams and rivers wears
down Earth’s surface by breaking up
bedrock and by removing eroded rock and
soil materials.
 Running water breaks up the bedrock over
which it flows primarily by mechanical
means. Rapidly flowing water has a lifting
effect that can split off and remove rock
fragments.
How Streams Weather and Erode
Material
 Most erosion occurs when running water
abrades and hammers away at its bed by
using sand, pebbles, and boulders as
“cutting tools”.
 Abrasion also wears down the cutting tools
themselves, producing the rounded
boulders, pebbles, and sand grains that are
commonly found in te beds of streams and
rivers.
River Rounded Boulders
Potholes
 Cutting tools (moving pebbles and boulders)
are involved in the formation of potholes in
the bedrock of rivers.
 Potholes are oval or circular basins which
form when water in a river develops small
whirlpools. As sand and pebbles, or even
small boulders swirl around in the whirlpools
they grind potholes in the bedrock.
Potholes
Along James River in Virginia
potholes formed in granite
Along Mohawk River in New York
the potholes formed in limestone
What Is a Plunge Pool?
 A plunge pool is a basin that has been worn
away at the base of a waterfall by the action
of falling water. Abrasion by churning
particles also plays a role in forming a
plunge pool. Many waterfalls have plunge
pools.
How a Plunge Pool Forms
How a Plunge Pool Looks
Chemical Weathering
The rivers also exert a chemical stress on the
bedrock. The water is a powerful solvent,
and many minerals in rocks are easily
dissolved. Such minerals are the ones which
contain calcium: calcite and dolomite are the
most common.
Rocks containing these minerals, like
limestone and marble are easily weathered
away by streams and rivers.
How Streams Transport Material
 All the materials transported downstream by
a river is called load. A river carries its load
in three ways:
1. in solution
2. in suspension
3. in its bed load
Solution
 If the river flows over rocks that dissolve in
water then the river’s load is transported in
solution.
 Common minerals transported in solution
are: calcium, magnesium, and bicarbonate.
Most from rivers’ solution load comes from
groundwater seeping into the river.
Materials in Suspension
 When the river looks muddy, it carries
materials in suspension.
 Suspended material consists in fine particles
of clay, silt , and fine sand.
 Although these materials are heavier than
water, the turbulence of the stream flow stirs
them up and keeps them from sinking.
River carrying materials in
suspension
Transport of Materials on Bed Load
 A river may also transport materials in its bed load. The
bed load materials are too heavy to be carried in
suspension, and are slowly dragged or tumbled on the bed
of the river, especially at high velocity.
 The relative amounts of river’s load depend on the
-nature of the river
-climate
-type of bedrock
-season of the year.
 As a general rule, most of the rivers’ load is transported in
suspension. The load increases with human land use.
Bed Load Materials
Competence and Capacity
The ability of a stream or a river to transport
materials is described by its competence and
capacity.
 Competence is a measure that describes the
maximum size of the materials carried by the river.
 Capacity is the measure of the total amount of
sediment a river or stream can carry.
 The competence and capacity increase during
floods, therefore much of the erosion created by
rivers occurs during floods.
Stream Deposition
 All rivers and streams will eventually deposit all
the materials they carry.
 While some of the materials may be deposited
along the channel, most of the load is deposited
where the river empties into a sea, or lake.
 A delta is a common deposit, that forms when a
river flows into a quiet or large body of water, such
as a lake, an ocean, or an inland sea.
How Deltas Form
 River water comes to a standstill at a delta where
most of the river’s sediment is dropped.
 As long as the amount of sediment is larger than
the amount that can be taken away by the
currents, the deposit grows.
 A river flowing into its delta, it splits into branches
called distributaries.
 A delta is a delicate balance between deposition
and erosion.
Mississippi Delta
Alluvial Fan
 A fan shaped deposit is called an alluvial fan.
Alluvial fans form at the base of a mountain, where
the slope changes drastically.
 An alluvial fan differs from a delta in several ways:
-first, the deposit is formed on land, not in water.
-Second, the sediments are coarse sand and
gravels rather than silt and clay.
-Third, its surface is sloping; it is not flat like that of
a delta.
Alluvial Fans