Earth as a system: the
hydrologic cycle
Illustrates the circulation of Earth's water
supply
Processes involved in the cycle
•
•
•
•
•
Precipitation
Evaporation
Infiltration
Runoff
Transpiration
The hydrologic cycle
Figure 5.2
Sources of Earth’s water
Figure 5.1
Groundwater and Soils
Groundwater
Water beneath the surface
Largest freshwater reservoir for humans
Geological roles
• As an erosional agent, dissolving by groundwater produces
• Sinkholes
• Caverns
• An equalizer of stream flow
Groundwater
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 filled mainly with
air
Groundwater
Water beneath the surface
Distribution and movement of groundwater
• Distribution of groundwater
• Zone of saturation
• All pore spaces in the material are filled with
water
• Water within the pores is groundwater
• Water table – the upper limit of the zone of
saturation
Features associated with
subsurface water
Figure 5.25
Groundwater
Water beneath the surface
Distribution and movement of groundwater
• Distribution of groundwater
• Porosity
• Percentage of pore spaces
• Determines how much groundwater can be
stored
• Permeability
• Ability to transmit water through connected pore
spaces
• Aquitard – an impermeable layer of material
• Aquifer – a permeable layer of material
Water beneath the
surface (groundwater)
Features associated with groundwater
• Springs
• Hot springs
• Water is 6-9ºC warmer than the mean air
temperature of the locality
• Heated by cooling of igneous rock
• Geysers
• Intermittent hot springs
• Water turns to steam and erupts
Old Faithful geyser in
Yellowstone National Park
Groundwater
Water beneath the surface
Features associated with groundwater
• Wells
• Pumping can cause a drawdown (lowering) of the
water table
• Pumping can form a cone of depression in the water
table
• Artesian wells
• Water in the well rises higher than the initial
groundwater level
Formation of a cone of
depression in the water table
Figure 5.30
An artesian well resulting
from an inclined aquifer
Figure 5.31
Groundwater
Water beneath the surface
Environmental problems associated with
groundwater
• Treating it as a nonrenewable resource
• Land subsidence caused by its withdrawal
• Contamination
Groundwater
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 beneath Earth's surface
• Formed in the zone of saturation
Groundwater
Water beneath the surface
Geologic work of groundwater
• Caverns
• Features found within caverns
• Form in the zone of aeration
• Composed of dripstone
• Calcite deposited as dripping water evaporates
• Common features include stalactites (hanging
from the ceiling) and stalagmites (growing
upward from the floor)
Dripstone formations in
Carlsbad Caverns National Park
Groundwater
Water beneath the surface
Geologic work of groundwater
• Karst topography
• Formed by dissolving rock at, or near, Earth's
surface
• Common features
• Sinkholes – surface depressions
• Sinkholes form by dissolving bedrock and
cavern collapse
• Caves and caverns
• Area lacks good surface drainage
Features of karst topography
Figure 5.37
Infrared image of karst
topography in central Florida
Figure 5.38 A
Running water
Drainage basins
• Land area that contributes water to a river
system
• A divide separates drainage basins
The drainage basin of the
Mississippi River
Figure 5.3
Running water
Streamflow
• Factors that determine velocity
• Gradient, or slope
• Channel characteristics
• Shape
• Size
• Roughness
• Discharge – volume of water flowing in the stream
(generally expresses as cubic feet per second)
Running water
Upstream-downstream changes
• Profile
• Cross-sectional view of a stream
• From head (source) to mouth
• Profile is a smooth curve
• Gradient decreases from the head to the mouth
• Factors that increase downstream
• Velocity
• Discharge
Running water
Upstream-downstream changes
• Profile
• Factors that increase downstream
• Channel size
• Factors that decrease downstream
• Gradient, or slope
• Channel roughness
Longitudinal profile
of a stream
Figure 5.6
Running water
The work of streams
• Erosion
• Transportation
• Transported material is called the stream's load
• Dissolved load
• Suspended load
• Bed load
Running water
The work of streams
• Transportation
• Load is related to a stream's
• Competence - maximum particle size
• Capacity - maximum load
• Capacity is related to discharge
Running water
The work of streams
• Transportation
• Deposition
• Caused by a decrease in velocity
• Competence is reduced
• Sediment begins to drop out
• Stream sediments
• Known as alluvium
• Well-sorted deposits
Running water
The work of streams
• Transportation
• Features produced by deposition
• Deltas – exist in ocean or lakes
• Natural levees - Form parallel to the stream
channel
• Area behind the levees may contain back
swamps or yazoo tributaries
Formation of natural levees
by repeated flooding
Figure 5.19
Running water
Base level
• Lowest point a stream can erode to
• Two general types
• Ultimate – sea level
• Temporary, or local
• Changing causes readjustment of the stream –
deposition or erosion
Adjustment of base level
to changing conditions
Figure 5.13
A waterfall is an example
of a local base level
Running water
Stream valleys
• Valley sides are shaped by
• Weathering
• Overland flow
• Mass Wasting
• Characteristics of narrow valleys
• V-shaped
• Downcutting toward base level
Running water
Stream valleys
• Characteristics of narrow valleys
• Features often include
• Rapids
• Waterfalls
• Characteristics of wide valleys
• Stream is near base level
• Downward erosion is less dominant
• Stream energy is directed from side to side
A narrow, V-shaped valley
Figure 5.15 A
Continued erosion and
deposition widens the valley
Figure 5.15 B
The resulting wide stream valley
is characterized by meandering
on a well-developed floodplain
Figure 5.15 C
Running water
Stream valleys
• Characteristics of wide valleys
• Floodplain
• Features often include
• Meanders
• Cutoffs
• Oxbow lakes
Erosion and deposition along
a meandering stream
Figure 5.8
A meander loop on the
Colorado River
Formation of a
cutoff and
oxbow lake
Figure 5.9
Running water
Drainage patterns
• Networks of streams that from distinctive
patterns
• Types of drainage patterns
•
•
•
•
Dendritic
Radial
Rectangular
Trellis
Drainage patterns
Figure 5.20
Running water
Floods and flood control
• Floods are the most common geologic hazard
• Causes of floods
• Weather
• Human interference with the stream system
Running water
Floods and flood control
• Engineering efforts
• Artificial levees
• Flood-control dams
• Channelization
• Nonstructural approach through sound
floodplain management
Satellite view of the Missouri
River flowing into the
Mississippi River near St. Louis
Figure 5.21
Same satellite view during
flooding in 1993
Figure 5.21