Week 7 PowerPoint Lecture
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Objectives:
4.01 Evaluate erosion and depositional processes:
Formation of stream channels with respect to the work
being done by the stream (i.e. down-cutting, lateral
erosion, and transportation).
Nature and characteristics of sediments.
Effects on water quality
Effect of human choices on the rate of erosion.
Groundwater and Chemical
Weathering
 As groundwater passes through permeable rock, minerals in the
rock dissolve. The warmer the rock is and the longer it is in
contact with water, the greater the amount of dissolved minerals
in the water.
 Water that contains relatively high concentrations of
dissolved minerals, especially minerals rich in calcium,
magnesium, and iron, is called hard water.
 Water that contains relatively low concentrations of
dissolved minerals is called soft water.
Results of Weathering by
Groundwater
 Watch this video to view how caves
form. Click on “How Caves Form”
on the left side of your screen once
you go to the website. Use the
information to help with your
worksheet.
http://www.pbs.org/wgbh/nova/caves
/form.html
Caverns - a natural cavity that forms in
rocks as a result of the dissolution
of minerals; also a large cave that
commonly contains many smaller,
connecting chambers
Results of Weathering by
Groundwater, continued
Stalactites and Stalagmites
 When water containing dissolved
calcite drips from the ceiling of a
limestone cavern, some of the calcite is
deposited on the ceiling.
 As this calcite builds up, it forms a
suspended, cone-shaped deposit
called a stalactite.
 When drops of water fall on the
cavern floor, calcite builds up to
form an upward-pointing cone
called a stalagmite.
Results of Weathering by
Groundwater, continued
Sinkholes - a circular
depression that
forms when rock
dissolves, when
overlying sediment
fills an existing
cavity, or when the
roof of an
underground cavern
or mine collapses
Results of Weathering by
Groundwater, continued
Natural Bridges
 When the roof of a cavern collapses in several places, a relatively
straight line of sinkholes forms.
 The uncollapsed rock between each pair of sinkholes forms an
arch of rock called a natural bridge.
 When a natural bridge first forms, it is thick, but erosion causes
the bridge to become thinner.
Results of Weathering by
Groundwater, continued
Karst topography -a type of irregular
topography that is characterized by caverns,
sinkholes, and underground drainage and
that forms on limestone or other soluble
rock
 Common features of karst topography
include many closely spaced sinkholes and
caverns.
 Generally, karst topography forms in regions
where the climate is humid and where
limestone formations exist at or near the
surface.
Parts of a River System
 A river system is made up of a main
stream and tributaries.
 Tributary - a stream that flows into
a lake or into a larger stream
Watershed - the area of land that is
drained by a river system
 The ridges or elevated regions that
separate watersheds are called divides.
 Can you find the tributaries that are part of
this watershed?
 Can you identify the divide in the diagram?
Parts of a River System, continued
 The relatively narrow depression that a stream follows as it flows downhill is
called its channel.
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A stream channel gradually becomes wider and deeper as it erodes its banks and
bed.
River systems change continuously because of erosion.
 The edges of a stream channel that are above water level are called the
stream’s banks.
 The part of the stream channel that is below the water level is called the
stream’s bed.
Stages of a River System
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Youthful
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Mature
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V-shaped valley with steep sides
Erodes beds faster than banks
Most rapid water flow
Less steep gradient
Erodes banks more than beds
Meanders often form
Old
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Deposits sediment, does not erose
Broad, flat plain forms in river valley
Slowest water flow
Click to view formation and
evolution of a river:
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it
=swf::640::480::/sites/dl/free/00724
02466/30425/10_22_10_23.swf::Fig
s.%2010.22,%2010.23%20%20Meander%20CutOffs%20and%20Oxbow%20Lakes
Channel Erosion
Stream Load - materials other than the water that are carried
by a stream
1.
The suspended load consists of particles of fine sand
and silt. The velocity, or rate of downstream travel, of
the water keeps these particles suspended, so they do
not sink to the stream bed.
2.
The bed load is made up of larger, coarser materials,
such as coarse sand, gravel, and pebbles. This material
moves by sliding and jumping along the bed.
3.
The dissolved load is mineral matter transported in liquid
solution.
Channel Erosion, continued
Stream Discharge - volume of water that flows
within a given time
The faster a stream flows, the higher its
discharge and the greater the load that the
stream can carry.
Channel Erosion, continued
Stream Gradient - the change in elevation
over a given distance
 Near the headwaters, or the beginning of
a stream, the gradient generally is steep.
This area of the stream has a high
velocity, which causes rapid channel
erosion.
 As the stream nears its mouth, where the
stream enters a larger body of water, its
gradient often becomes flatter.
Evolution of River Channels
Meandering Channel - one of the bends, twists, or
curves in a low-gradient stream or river
 When a river rounds a bend, the velocity of the
water on the outside of the curve increase.
However, on the inside of the curve, the velocity
of the water decreases.
 This decrease in velocity leads to the formation of
a bar of deposited sediment, such as sand or
gravel.
Evolution of River Channels, continued
Meandering Channels, continued
 Meanders can become so curved
that they almost form a loop,
separated by only a narrow neck of
land.
 When the river cuts across this
neck, the meander can become
isolated from the river, and an
oxbow lake forms. (See Sims Lake,
Wedgeworth Lake to right)
Evolution of River Channels, continued
Braided stream - a stream or
river that is composed of
multiple channels that divide
and rejoin around sediment
bars
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direct result of large
sediment load, particularly
when a high percentage
of the load is composed
of coarse sand and gravel.
can cause much erosion,
affecting water quality.
Deltas and Alluvial Fans
Delta a fan-shaped mass of rock material deposited at
the mouth of a stream
Alluvial fan a fan-shaped mass of rock material
deposited by a stream when the slope of the land
decreases sharply
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Deltas form where streams flow into the ocean at
the edge of a continent
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Alluvial fans form when streams flow from
mountains to flat land
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A stream may deposit sediment on land or in
water.
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When a stream descends a steep slope and reaches
a flat plain, the speed of the stream suddenly
decreases. As a result, the stream deposits some of
its load on the level plain at the base of the slope.
Alluvial fans differ from deltas in that alluvial fans form on land instead
of being deposited in water.
Floodplains
A floodplain an area along a river that forms from sediments deposited when
the river overflows its banks
 The volume of water in nearly all streams varies depending on the
amount of rainfall and snowmelt in the watershed.
 When a stream overflows its banks and spreads out over the floodplain, the
stream loses velocity and deposits its coarser sediment load along the banks
of the channel.
 The accumulation of these deposits along the banks eventually produces
raised banks, called natural levees.
 Finer sediments are carried farther out into the floodplains by the flood
waters and are deposited there.
 A series of floods produces a thick layer of fine sediments, which becomes a
source of rich floodplain soils.
Human Impacts on Flooding
 Human Impacts on Flooding
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Vegetation, such as trees and grass, protects the ground
surface from erosion by taking in much of the water that
would otherwise run off.
Logging and the clearing of land for agriculture or housing
development can increase the volume and speed of runoff,
which leads to more frequent flooding.
 Flood Control
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Indirect methods of flood control include forest and soil
conservation measures that prevent excess runoff during
periods of heavy rainfall.
More-direct methods include the building of artificial
structures that redirect the flow of water, such as dams or
artificial levees.
Do you remember a recent natural disaster that resulted from failed levees?
Hint: New Orleans, LA
Bibliography
Pictures and information gathered from:
http://www.nps.gov/cave/naturescience/images/entrance_jones_556.jpg
http://www.blm.gov/education/expert/3/small/cave4.jpeg
http://www.blm.gov/education/expert/3/small/cave9.jpeg
http://ga.water.usgs.gov/edu/pictures/sinkhole1.jpg
http://www.dcr.virginia.gov/natural_heritage/images/ccvup56.jpg
www.adoptawaterbody.utah.gov/what_is.htm
http://salvinia.er.usgs.gov/MS_site_Forrest_County_web.JPG
http://3dparks.wr.usgs.gov/3Dbayarea/images2/ArroyoSeco2.jpg
http://www.fws.gov/.../bhg/images/Ebutte_delta2.jpg
http://wrgis.wr.usgs.gov/parks/deva/Badwater_fan08.jpg
http://www.water.ky.gov/NR/rdonlyres/5D74D194-3780-4B2A-8C1A-6A4840B2CE20/0/02goose.JPG
https://www.llnl.gov/str/June07/gifs/dam1.gif
Allison, DeGaetano, Pasachoff, Holt Earth Science. 2008, Holt, Rinehart and Winston, New York