Geology 101 - Lecture 16

I. Hydrologic Cycle

• 97.2% in oceans
• 2.15% in glaciers (largest source of fresh (non-saline) water on Earth)
• 0.62% in groundwater (underground aquifers)
• 0.017% in lakes
• 0.005% in soil moisture
• 0.001% in atmosphere
• 0.0001% in stream channels

• 50% is evaporated and transpired by plants
• 30% goes into runoff, eventually reaching the ocean
• 20% goes into groundwater

III. Streams and Rivers

A. Importance of Rivers

B. Overview of Drainage Networks

1. How do streams flow?
• turbulent flow. Can erode channel and keep sediment suspended

2. Ability of stream to erode depends upon velocity of water which depends on:
a. gradient (slope)

b. discharge (amount of water); measured in m3/sec
=channel width (m) x depth (m) x velocity (m/sec) = m3/sec

c. shape, size of a channel


• When stream velocity decreases, sedimentation occurs

3. Longitudinal profile of stream (see text)

• headwaters - source of stream (erosion) - high gradient. In this region, generally have active downcutting, V-shaped valleys
  
  
• middle region - deposition (channel deposits and flood plain deposits) and erosion (cutbanks). Valleys are wide with flat floor.
  Channel- trough in which stream travels
  Floodplain - overflow region for floods of the main channel
  
  
• mouth - deposition - low gradient
  
  

  Main goal of river is to get channel down to level of mouth (base level) by downcutting

4. Erosion, how does it happen?
• dislodge particles
  
• abrasian
  
• solution activity

5. Sediment transport
• dissolved load
• suspended load
• bedload

6. Evolution of a stream/river
• Oxbow lakes

  - Meanders and oxbow lakes

  Once a meander forms, the bend grows more pronounced with time. Why?
• Braided streams - interwoven because of high sediment accumulation
• Floodplain

Mississippi River - flood plain (picture)
Mississippi River Floods '93 - a virtual field trip


7. Deltas and Alluvial Fans


• Delta - stream enters into another body of water (i.e. lake or ocean)
  
  
• Alluvial fan - stream enters out into dry valley
  
  
• Streams slow down when they reach their mouths. Decreased velocity causes deposition of suspended sediment. Coarsest material is dropped first, finer grained material deposited further away.
  
  
  
  

Groundwater

A. Groundwater as a resource

• Drinking water
• Erosion (subterranean caverns)
• Equalizer of stream flow

B. Water holding capabilities of rock

Consider properties of rock for storage & transport of water:


• Porosity - percentage of rock's volume that is openings
  
  
• Permeability - ability of rock to transmit a fluid
  
  
• Aquifer - Rock that is permeable, able to store and transport water (examples)
  
  
• Aquitard - Rock that is impermeable, acts to hinder or prevent water movement (examples)

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