Student Papers in Geology-Cochise College
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Wind and Deserts
Wind and Deserts-Articles         

Roger Weller, geology instructor

wellerr@cochise.edu


Erosion
by Brooke Willsey
Physical Geology
Fall 2012
       

 

                                          Weathering and Erosion of the Arizona Desert

 

     The Arizona landscapes are filled with a vast variety of scenery. Luscious green terrain and powerful rivers can be found in areas of the northern part of the state. And while this can still be seen in the various mountainous regions of the southern parts of Arizona, there is a lot of the opposite scenery as well, an arid desert setting. Where mesquite trees grow like weeds and snakes and coyotes can be common neighbors. 

 

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                                                                        A view of the Whetstone Mountains

 

     The desert weathers in a very different way than a green and moist landscape.  Where lush green wildernesses usually have a dense covering of foliage and soil, the desert does not.  Leaving the surface of this dry environment more exposed to the effects of certain types of mechanical weathering and erosion. Mechanical weathering is the physical disintegration of rocks in the environment.  Erosion is when the weathered material is actually removed and transported from its original location.

 

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                                                Wall of a wash weathered by water flows

 

     Flash floods occur often in southern Arizona. It is not uncommon for washes and rivers to fill with raging water, even when there isnít a storm for miles. Not only can these be lethal to unaware humans and animals, but it can quickly change the environment with the rapid currents, carving deeper washes and new channels throughout the region.  It causes a weathering process that within a few months, especially during the monsoons, can change the landscape drastically. Banks of dry washes, that before were easily climbable and commutable, can become impassable from new carved steepness. Or the opposite can occur where landmarks and obstacles are corroded and washed away.  The following pictures actually demonstrate the impact that floods during the monsoon season can have on materials in the path.

 

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                                                            Photo taken in a wash between Whetstone and Benson, AZ

 

     The above shows an abandoned van that has been left in this wash for only a few years. Granted there has been some damage done by people during this time as well. But in that timeframe, the paint has been completely stripped, the interior has disappeared as with all the glass windows, and the frame has been twisted and dented. Not to mention the tree through it. Unfortunately, no pictures were taken of this vehicle in the past few years to show the change. (It wasnít a planned experiment at that time!) But this was a functional vehicle only a few short years ago.

 

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            Photo taken in a wash between Whetstone and Benson, AZ (van pictured above is circled)

 

     This picture was taken from the perspective of where the van had been as of June 2012, before the wash swept it a good 75 yds. from its original position. Before it also didnít have a tree through the windshield, the entire top wasnít dented, and the rear wasnít twisted. All of that damage had been done in only two months.
 

 

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  A wash near Benson, AZ where part of the wall has collapsed after the past rainy season and also where
 tree roots have been exposed from the continual erosion of the sides of the wash
.

 

     Rough water flows cause many washes to appear like the one in the picture above. The collapsed portion is new to the appearance. This wall was intact through June 2012, but by September there were many places along the wash that had the same erosion from continual flood waters throughout the monsoon season.  After recurrent erosions to washes, the ground beneath trees and vegetation disappears and the plantís root systems may fight to keep in place temporarily. Eventually many of these trees and other foliage plummet to the wash, and/or are swept away in floodwaters. An example of a tree that is hanging on by mere roots is shown below.
 

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                                                Tree hanging over side of wash, near Benson, AZ

 

     Debris flows are water flows that carry materials through their path. These flows can carry anything from mud, to boulders and trees.  Intense debris flows mostly occur in mountain areas with steeper inclines. Arizona has had some extremely damaging debris flows, and often they are linked with wildfires. Wildfires burn any of the vegetation and material that may be keeping the soil, boulders, and other debris in place. Though some washes may not be directly on steep incline, it isnít uncommon for the debris from steep inclines to travel into washes that are less sloped. Even without these slopes, some of the rapid floods can cause washes to end up with much of the same material as could be found in the debris flows found on mountainsides, perhaps only with mass and quantities differing.
 

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A view looking into a wash, the sides are approximately 30 ft. high, taken near Benson, AZ

 

     The southern Arizona desert scenery is an entirely different world when it rains. The water carves channels and everything seems to flow.  Water races down the hillsides with thousands of small streaming crevices combining until they reach raging waterways. The monsoon season changes these washes, roads, and environments year after year, creating an ever-changing canvas of a desert landscape.
 

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                                                                From Dry Canyon in the Whetstone Mountains overlooking the valley

 

All photos used were taken from personal collection.

 

Sources for more information on desert weathering and erosion:

http://pubs.usgs.gov/of/2004/1007/erosion.html

http://www.azgs.az.gov/hazards_floods.shtml

http://www.dem.azdema.gov/operations/docs/mitplan/chapter5.4.7.pdf

http://www.youtube.com/watch?v=Z0bPYnU6w9s     (Video specific to debris flows in southern AZ in 2006, has lots of pictures)

http://azgeology.azgs.az.gov/article/environmental-geology/2012/04/rapid-erosion-emigrant-canyon-chiricahua-mountains-arizona