Erosion

The Hydrologic (water) Cycle

Water is constantly being evaporated (mostly from the oceans) into the atmosphere by solar energy.  Winds transport some of the water vapor in the air over the land (also driven by solar energy).  When the water vapor produces rain, the water then has to find its way back to the ocean (the ultimate base level).  During this trip back to the ocean, the water can do some wild things to the earth's surface.

Links:

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/home.rxml

http://grads.iges.org/nncc/hydro.html

Stream Erosion

• Dissolved Load - minerals in solution carried seaward.
  • This is why the oceans are salty
    
    
• Suspended Load - silt and clay in suspension carried seaward
  • this is the bulk of a river's load
  • it makes water look muddy
  • it settles out once the water become stagnant (mud after a flood)
    
    
• Bed Load - sand and gravel "dragged" along the river bed
  • This also "weathers" the channel and undercuts bedrock (when the river meanders) so overlying rock layers are no longer supported.  That's when gravity takes over and the walls come a-tumble'n down.
     
  A river keeps moving until it reaches the base level (a lake or the sea).  The ultimate base level is the ocean.  Once reached, suspended particles settle out and are deposited.

  The amount of material a stream carries depends on:

  • the river's discharge (volume of water it carries per second) which depends on the:
    • drainage basin (collection area)
    • amounts of rainfall / snowfall in the area
    • the Amazon River delivers 7,500,000 cubic feet of water to the Atlantic Ocean every second ... by far the greatest river discharge!
  • type of bedrock it flows over
  • shape and size of the river bed
  • slope of the landscape (gradient)
  • speed of the water
     

  Youthful Phase

  Consider a landscape which has just been uplifted - this is a "youthful" landscape.  It is marked by mountains with steep slopes, waterfalls, and rapids.
   
   

  Yellowstone Falls

  Alps (Austria)

  Water will weather steep "V" shaped valleys in this phase.  Water weathering is vertical ... rapid down cutting is evident.  The Grand Canyon is a good example of this "youthful" phase (even through the Grand Canyon is quite old chronologically).

  

  Arial view of the Colorado River

  Mature Phase
   

  

  In time, running water will effect its toll on the landscape.  It eventually carves out  broad valleys.  The weathering power of the river is changed from vertical to horizontal as the river starts to meander (moves in curves).  This is because water (and the material it carries) always "cut" at the outside bank of a river (where the motion is fastest) ... and deposits sediments at the calmer inside bank of the river.  This action tends to extend the meander and widen the valley.

  

  

  There are some incredible meanders in the San Juan River in Utah.  There are many of these "Goosenecks" along the river.  To go 1 mile as the crow flies ... you have to travel along 5 miles of river.  This river is mostly down-cutting at present ... but it should be clear that, in time, the goose's neck will be cut off as the river erodes the sides of the canyon.

  Old Age

  This landscape occurs when a stream has cut out a wide floodplain.  It often is found at the mouth of the river (where it meets the sea) and there is a very gentle gradient (slope).  The lower sections of the Mississippi River exhibit this profile.  This is where a river can be quite dangerous during times of excessive rainfall/snow melt because it can easily overflow its banks.  In fact, one should not consider flooding as a freak occurrence but rather, an inevitable consequence in the region.

  Features of the region:
   

  • Levees - mounds at either side of a river's banks.  These may be formed naturally during times of flood as sediments settle out once the water overflows its banks.  Once water floods the area, it becomes difficult for it to drain back in the river after water levels drop.  This results in backswanps.  Levees may be built up artificially to prevent flooding but this can backfire because sediments then fill the river channel ...making it easier to overflow during the next flood.
     
  • Oxbow Lakes
     

    (animation)

    As the river meanders it will naturally create an extreme "bow" which will eventually be "cut off", leaving an oxbow lake.  The lower Mississippi has hundreds of these features along its banks.  Since the 1930's, the Army Corp. of Engineers has created several artificial cutoffs in an attempt to speed up the river (to reduce flooding).

    Click here to see a Yahoo map of the lower Mississippi (zoom in once and follow the river to see the many places the river has changed direction)
     
     

  • Deltas - When the river meets the sea, water stops moving and sediments are deposited.  This "clogs" the channel forcing the river to find a new route to the sea.  In time, land is built up in a "delta" pattern at the mouth of the river.  The mouth of the Mississippi River was originally closer to southern Illinois ... but filled in the entire region to where it presently meets the sea in New Orleans.  Some day, the Gulf of Mexico may be completely filled in!
     

    Click here to see the Nile delta
    Click here to see New Orleans
    Click here to see the Atchafalaya River Delta, Louisiana

    
    permission from NASA

    In some instances, a river in its "old age" stages can be rejuvenated due to uplifting.  Thus the river (working with a new steeper gradient) enters a "youthful" phase again and reverts to cutting sharp "V" shaped valley walls.  Instances of this have occurred on the upper Mississippi due to a gradual uplift caused by the "recent" loss of thick ice sheets that are no longer present (the ground moves up just like a pillow does when you move off of it).  This is called a rejuvenated stream.

    Slow vs. Fast

    River erosion can affect the landscape in a "slow but steady" time scale (the Grand Canyon) or in an "all at once" flooding event.  The "slow and steady" erosion at the surface may take several lifetimes to make any noticeable change in the landscape, but in the long run, produces the greatest overall effect.  We tend to notice the less frequent but sudden events which make an impact on the surface but not to the same degree.  A good analogy occurs when you talk about human deaths.  One tragic event can make headlines in the newspapers for weeks (we will all remember 9-11 for the rest of our lives) ... but if you look at the obituaries in the paper, you will see that far more people die every day as a result of old age or illness.  Slow and steady far outnumbers the few and infrequent "events".

    Some cataclysmic "events" in the history of the earth are worth note.  Probably the greatest flooding event occurred when Lake Missoula (a former glacier-fed lake in S.W. Canada 12,000-15,000 years ago) broke its ice dam and suddenly released a wall of water (estimated at 60 times the current discharge of the Amazon) into present Montana, Washington, Idaho, and Oregon.  This event repeated itself perhaps up to 100 times during the last ice age.  These events sculpted the terrain in the blink of an eye.

    Also, sections of the Grand Canyon were plugged by lava flows.  These flows created natural dams which eventually broke .... sending enormous volumes of water through the canyon.  These blasts of water sculpted sections of the canyon in an instant.

   

  

  Photo by Barb Wallner
   

  Sometimes when "All Hell Breaks Loose" you get a sudden sculpting of the earth's surface.  Geologists give names to these devastating events - such as Debris Flows, Mudflows, Jökulhlaups (pronounced "yo-kul-h-loip"), and Lahars.  They can be initiated by any of the following:

  • large landslides resulting from water-saturated debris
  • heavy rainfall mixing with volcanic ash deposits
  • sudden melting of glacial snow and ice near the summit of an active volcano
  • rivers or lakes dammed by volcanic eruptions or glaciers

  ©Jim Mihal 2004, 2006 - all rights reserved