Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Brooke Willsey
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.
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.
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.
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.
Photo taken in a wash between Whetstone and Benson, AZ (van pictured above is circled)
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
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.
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
Tree hanging over side of wash, near Benson, AZ
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.
A view looking into a wash, the sides are approximately 30 ft. high, taken near Benson, AZ
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.
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://www.youtube.com/watch?v=Z0bPYnU6w9s (Video specific to debris flows in southern AZ in 2006, has lots of pictures)