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Chapter 3
Landscapes Fashioned by Water
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Earth’s External Processes
Weathering, mass wasting, anderosion are all called external
processes because they occur at ornear Earth’s surface
Internal processes, such as
mountain building and volcanicactivity, derive their energy fromEarth’s interior
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Mass Wasting: The Work of Gravity
Mass wasting is the downslopemovement of rock and soil due to
gravity Controls and triggers of mass
wasting
Water —
Reduces the internalresistance of materials and addsweight to a slope
Oversteepening of slopes
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Mass Wasting: The Work of Gravity
Controls and triggers of masswasting
Removal of vegetation
Root systems bind soil and regolithtogether
Earthquakes Earthquakes and aftershocks can
dislodge large volumes of rock andunconsolidated material
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Water Cycle
The water cycle is a summary of the circulation of Earth’s water
supply Processes in the water cycle
Precipitation
Evaporation Infiltration
Runoff
Transpiration
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The Water Cycle
Figure 3.5
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Distribution of Earth’s
Water
Figure 3.4
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Running Water
Streamflow
The ability of a stream to erode and
transport materials is determinedby velocity
Factors that determine velocity
Gradient, or slope Channel characteristics including
shape, size, and roughness
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Running Water
Streamflow Factors that determine velocity
Discharge —The volume of watermoving past a given point in a certainamount of time
Changes along a stream Cross-sectional view of a stream is
called the profile
Viewed from the head (headwatersor source) to the mouth of a stream
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Running Water
Changes from upstream todownstream
Profile Profile is a smooth curve
Gradient decreases downstream
Factors that increase downstream Velocity Discharge
Channel size
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Longitudinal Profile
of a Stream
Figure 3.9
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Base Level
Base level and stream erosion
Base level is the lowest point to
which a stream can erode Two general types of base level
Ultimate (sea level)
Local or temporary
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Base Level
Base level and stream erosion
Changing conditions causes
readjustment of stream activities Raising base level causes deposition
Lowering base level causes erosion
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Adjustment of Base Level to Changing Conditions
Figure 3.16
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A Waterfall Is an Example of a Local Base Level
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The Work of Streams
Stream erosion
Lifting loosely consolidated
particles by Abrasion
Dissolution
Stronger currents lift particles moreeffectively
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The Work of Streams
Transport of sediment by streams Transported material is called the
stream’s load Types of load
Dissolved load
Suspended load
Bed load Capacity — the maximum load a
stream can transport
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The Work of Streams
Competence
Indicates the maximum particle
size a stream can transport Determined by the stream’s
velocity
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The Work of Streams
Deposition of sediment by a stream
Caused by a decrease in velocity
Competence is reduced
Sediment begins to drop out
Stream sediments
Generally well sorted Stream sediments are known as
alluvium
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The Work of Streams
Deposition of sediment by a stream
Delta —Body of sediment where a
stream enters a lake or the ocean Results from a sudden decrease in
velocity
Natural levees — Form parallel to thestream channel by successivefloods over many years
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Formation of Natural Levees
Figure 3.22
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The Work of Streams
Deposition of sediment by a stream
Floodplain deposits
Back swamps
Yazoo tributaries
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Stream Valleys
The most common landforms onEarth’s surface
Two general types of streamvalleys
Narrow valleys V-shaped
Downcutting toward base level
Features often include rapids andwaterfalls
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Stream Valleys
Two general types of streamvalleys
Wide valleys Stream is near base level
Downward erosion is less dominant
Stream energy is directed from side toside forming a floodplain
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Stream Valleys
Features of wide valleys ofteninclude
Floodplains Erosional floodplains
Depositional floodplains
Meanders Cut banks and point bars
Cutoffs and oxbow lakes
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Erosion and Deposition along
a Meandering Stream
Figure 3.11
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A Meander Loop on the Colorado River
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Floods and Flood Control
Floods and flood control
Floods are the most common and
most destructive geologic hazard Causes of flooding
Result from naturally occurring andhuman-induced factors
Causes include heavy rains, rapidsnow melt, dam failure, topography,and surface conditions
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Floods and Flood Control
Floods and flood control
Flood control
Engineering efforts
Artificial levees
Flood-control dams
Channelization
Nonstructural approach through soundfloodplain management
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Drainage Basins and Patterns
Drainage networks
Land area that contributes water to
the stream is the drainage basin Imaginary line separating one basin
from another is called a divide
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Drainage Basin of the
Mississippi River
Figure 3.6
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Drainage Basins and Patterns
Drainage pattern
Pattern of the interconnected
network of streams in an area Common drainage patterns
Dendritic
Radial Rectangular
Trellis
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Drainage Patterns
Figure 3.23
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Water Beneath the Surface
Largest freshwater reservoir forhumans
Geological roles As an erosional agent, dissolving by
groundwater produces
Sinkholes Caverns
An equalizer of stream flow
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Water Beneath the Surface
Distribution and movement of groundwater
Distribution of groundwater Belt of soil moisture
Zone of aeration
Unsaturated zone
Pore spaces in the material are filledmainly with air
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Water Beneath the Surface
Distribution and movement of groundwater
Distribution of groundwater Zone of saturation
All pore spaces in the material arefilled with water
Water within the pores is groundwater
Water table —The upper limit of the zoneof saturation
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Features Associated with
Subsurface Water
Figure 3.28
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Water Beneath the Surface
Movement of groundwater
Porosity
Percentage of pore spaces
Determines how much groundwater
can be stored
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Water Beneath the Surface
Movement of groundwater
Permeability
Ability to transmit water throughconnected pore spaces
Aquitard — An impermeable layer of material
Aquifer — A permeable layer of material
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Water Beneath the Surface
Springs
Hot springs
Water is 6 –9ºC warmer than the meanair temperature of the locality
Heated by cooling of igneous rock
Geysers
Intermittent hot springs
Water turns to steam and erupts
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Water Beneath the Surface
Wells
Pumping can cause a drawdown
(lowering) of the water tablePumping can form a cone of
depression in the water table
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Water Beneath the Surface
Artesian Wells
Water in the well rises higher than
the initial groundwater level Artesian wells act as ―natural
pipelines‖ moving water fromremote areas of recharge greatdistances to the points of discharge
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Formation of a Cone of Depression
Figure 3.32
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An Artesian Well Resulting
from an Inclined Aquifer
Figure 3.33
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Water Beneath the Surface
Environmental problemsassociated with groundwater
Treating it as a nonrenewableresource
Land subsidence caused by its
withdrawalContamination
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Water Beneath the Surface
Geologic work of groundwater
Groundwater is often mildly acidic
Contains weak carbonic acid Dissolves calcite in limestone
Caverns
Formed by dissolving rock beneathEarth's surface
Formed in the zone of saturation
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Water Beneath the Surface
Caverns Features found within caverns
Form in the zone of aeration
Composed of dripstone
Calcite deposited as dripping waterevaporates
Common features include stalactites (hanging from the ceiling) andstalagmites (growing upward from thefloor)
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Water Beneath the Surface
Karst topography Formed by dissolving rock at, or
near, Earth's surfaceCommon features
Sinkholes — Surface depressions
Sinkholes form by dissolving bedrock
and cavern collapse Caves and caverns
Area lacks good surface drainage
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End of Chapter 3