The Hydrologic Cycle

• Water sources:
  • Oceans – 97.2%
  • Ice sheets / glaciers – 2.15%
  • Lakes, rivers, groundwater – 0.65%
• The hydrologic cycle is a summary of the circulation of Earth’s water supply
  • Powered by the sun
  • Links oceans to continents
  • Processes involved:
    • Precipitation
    • Evaporation
    • Infiltration
    • Runoff
    • Transpiration
• The Three Fates of Precipitation
  • Infiltration
    • the movement of water into rocks or soil through cracks and pore spaces
  • Runoff
    • water that flows over the land
  • Transpiration
    • the release of water vapor to the atmosphere by plants
• Infiltration capacity of the soil is controlled by:
  
  • Intensity and duration of rainfall
    
  • Soil saturation
    
  • Soil texture
    
  • Slope of the land
    
  • Nature of the vegetative cover
• Main Sources of Water
  
  • Surface Water
    
    • Lakes, rivers, reservoirs
      
  • Ground Water
    
    • In the Earth, flows through fractures and pores
      

Surface 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 is the single most important agent sculpturing Earth’s land surface
  
  • Begins as sheetflow
    
  • Sheetflow develops into tiny channels called rills
    
  • Eventually flows into streams, rivers & lakes
    

The Hydrologic Cycle | Surface Water | Surface Water – Streams | Surface Water – Stream Valleys | Surface Water – Flooding | Links | top | Classes Home

Surface Water – Streams

• Profile of a Stream
  
• Streams
  
  • Upstream
    
    • Higher velocity
      
    • Smaller discharge
      
    • Smaller channel
  • Downstream
    
    • Lower velocity
      
    • Greater discharge
      
    • Larger channel
      
• 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
    
  • A waterfall is an example of a local base level
    
• Erosion = transportation of sediments
  
  • AKA load
    
  • Dissolved load
    
  • Suspended load
    
  • Bed load
    
• Erosion = transportation of sediments
  
  • Load is related to a stream's
    
    • Competence - maximum particle size
      
    • Capacity - maximum load
      
      • Capacity is related to discharge
        
• Deposition
  
  • Caused by a decrease in velocity
    
    • Competence is reduced
      
    • Sediment begins to drop out
      
• Stream sediments
  
  • Known as alluvium
    
  • Well-sorted deposits
    
• 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
      
  • Structure of a simple delta
    
  • Formation of natural levees by repeated flooding
    

Surface Water – Stream Valleys

• River Erosion
  
  • Vertical erosion
    
    • Produces deep, V-shaped valleys with rapids
      
  • Lateral erosion
    
    • produces broad valleys, flood plains, and meandering streams
• Characteristics of narrow valleys
  
  • V-shaped
    
  • Downcutting toward base level
    
  • 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
    
• Floodplain
  
  • Features often include
    
    • Meanders
      
    • Cutoffs
      
    • Oxbow lakes
      
  • Stream Processes and Floodplain Development
    
• Drainage basins and patterns
  
  • A divide separates drainage basin
    
  • Drainage basins and patterns
    
  • Types of drainage patterns
    
• Flood Plains
  
  • Periodic floods deposit rich soils
    
  • Agricultural production on floods plains is followed by urbanization
    
  • Natural levees
    
    • Forms as floods deposit coarse detritus near the river
      
    • Naturally constraint the river except in the larger floods
      

The Hydrologic Cycle | Surface Water | Surface Water – Streams | Surface Water – Stream Valleys | Surface Water – Flooding | Links | top | Classes Home

Surface Water – Flooding

• Floods and flood control
  
  • Floods are the most common geologic hazard
    
  • Causes of floods
    
    • Weather
      
    • Human interference with the stream system
      
  • 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
    
  • Same satellite view during flooding in 1993
    

The Hydrologic Cycle | Surface Water | Surface Water – Streams | Surface Water – Stream Valleys | Surface Water – Flooding | Links | top | Classes Home