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 5.3

Sources of Earth ’ s water
Figure 5.2

Running water
 Drainage basin
• Land area that contributes water to a river system
• A divide separates drainage basins

Figure 5.4
Drainage basins and drainage divides

Running water
 Streamflow
• Factors that determine velocity
• Gradient , or slope
• Channel characteristics
• Shape
• Size
• Roughness
• Discharge – volume of water flowing in the stream
(generally expressed 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 5.7

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 backswamps or yazoo tributaries

Formation of natural levees by repeated flooding
Figure 5.21

Running water
 Base level
• Lowest point to which a stream can erode
• 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 5.15

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 5.17 A

Continued erosion and deposition widens the valley
Figure 5.17 B

A wide stream valley is characterized by meandering on a well-developed floodplain
Figure 5.17 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 5.10

A meander loop on the
Colorado River
Figure 5.18

Formation of a cutoff and oxbow lake
Figure 5.11

Running water
 Drainage patterns
• Networks of streams that form distinctive patterns
• Types of drainage patterns
• Dendritic
• Radial
• Rectangular
• Trellis

Drainage patterns
Figure 5.22

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 5.23 top

Same satellite view during flooding in 1993
Figure 5.23 bottom

Water beneath the surface (groundwater)
 Largest freshwater reservoir for humans
 Geological roles
• As an erosional agent, dissolving by groundwater produces
• Sinkholes
• Caverns
• An equalizer of streamflow

Water beneath the surface (groundwater)
 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

Water beneath the surface (groundwater)
 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 5.26

Water beneath the surface (groundwater)
 Distribution and movement of groundwater
• Distribution of groundwater
• Porosity
• Percentage of pore spaces
• Determines storage of groundwater
• 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 –9 ° C 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
Figure 5.29

Water beneath the surface (groundwater)
 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 5.31

Artesian systems
Figure 5.32

Water beneath the surface (groundwater)
 Environmental problems associated with groundwater
• Treating it as a nonrenewable resource
• Land subsidence caused by its withdrawal
• Contamination

Water beneath the surface (groundwater)
 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

Water beneath the surface (groundwater)
 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 )

Cave Features in Lehman Caves, Great
Basin National Park, and Carlsbad
Caverns National Park
Figure 5.38

Water beneath the surface (groundwater)
 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 5.39 C