Desert Processes and Landforms
5 points (missing 3 questions = -1 pt)
Name_________________
The desert environment is made up of landforms shaped by the wind, running water, and weathering.
Sand dunes migrate with the wind as sand grains saltate up the gentle slope of the dune and slip down the steep
slope of the dune, also known as the slip face. This asymmetrical slope pattern provides a clue to the wind
direction. The wind blows from the gentle slope to the slip face. Deflation removes the finer particles and
leaves the coarser particles behind as a desert pavement.
Infrequent but intense thunderstorms occur in mountain deserts due to orographic lifting. Running
water carves up the landscape quite readily in the absence of the protective cover of vegetation. Less resistant
rock layers are more easily eroded than more resistant rock layers. The more resistant layers are referred to as a
cap rock.
As a river leaves a mountain canyon and spills out onto the valley floor, the flow
spreads out in a fan shape. Sediment is deposited because as the flow spreads out into many channels, there is
less discharge in each individual channel on the whole fan and the water can also infiltrate into the previously
deposited sediment. So the discharge and velocity can also decrease at the end of the fan. This feature is called
an alluvial fan. As alluvial fans grow, they often merge into slopes called a bajada. Farther out in the desert
valley, a temporary shallow lake called a playa may occur during snowmelt or after storms in the mountains.
Under the arid conditions, evaporation rates are high. Mineral salts precipitate as the water evaporates leaving
salt pans.
Weathering occurs even though water may be limited. In areas with high enough elevations, freezethaw action can physically break rock apart. Oxidation, a form of chemical weathering, is seen as desert
varnish, a red-brown-black stain on the surface of the rock. Over time, these weathering processes can break
down a mesa into isolated remnants called buttes.
Section 1: Dunes
Kane Springs, California individual stereopair. The North arrow is in the upper right hand corner of the
photo.
What is the dominant wind direction?
How did you make that decision?
What kind of dunes are these?
The representative fractional scale is 1:20,000.
How far away is the dune (A) west of the airstrip in feet? (Be sure to measure with the wind direction in mind)
in miles?
If the dune is traveling at a rate of 35 ft per year, when will it encroach upon the airstrip? Distance = rate x time
Read this article about an area of inactive dunes in Nebraska.
http://online.wsj.com/news/articles/SB10001424052702303342104579101211691661186
In terms of the hydrologic cycle, what is the advantage to having a golf course made of this sediment?
Ashby, Nebraska topo map. The Sand Hills of Nebraska are often referred to as fossilized or inactive dunes.
They are not moving significantly anymore. Most are now naturally vegetated in short grass prairie and used as
rangeland for cattle. This area lies over the High Plains Aquifer.
What is the representative fraction map scale?
One inch on the map represents how many feet?
how many miles?
What is the contour interval?
How much higher than its surroundings would a sand dune have to be, in order to show up on this map?
What is the PLS (1/4, section, Township and Range) location of Graham Ranch in the northwestern part of the
map?
The ridge immediately north of Graham Ranch is a fossilized sand dune. What was the dominant wind
direction?
So then what kind of dunes are these?
Why couldn’t these dunes be classified as longitudinal dunes?
The areas in the blow-outs between the fossilized dunes now have small lakes in them. How does this affect
deflation?
Section 2: Mountain Deserts
Mt. Whitney, California topo map. Owens Valley used to contain a larger lake. Much of the water has been
drained away to supply water to cities in Southern California. The Alabama Hills have provided the scenery for
many western movies. Look at the US atlas before beginning this section.
Sketch a cross-section showing the topography beginning with the western side of the map around Kings
Canyon and Sequoia National Parks and continuing across the divide and down the eastern side of the
mountains into the valley floor.
West
East
Explain the atmospheric reason for the desert conditions on the eastern side of the map when there are so many
lakes on the western side of the map around Kings Canyon and Sequoia National Parks.
Ennis, Montana in the page 58 Topographic Map Book
What is the drainage pattern on the alluvial fan?
What is the stream gradient (drop in elevation / distance) for the creek from the edge of Sections 14 and 23 on
the eastern edge of map to the road with the Benchmark of 6080 ft?
What is the stream gradient (drop in elevation / distance) for the creek from the Benchmark of 6080 ft to where
the creek meets Highway 287?
Why is sediment deposited as the creek leaves the mountain canyon?
Grand Junction, CO plate 26 in the Stereoatlas book
Find the park road in the lower left where it runs on top of the mesa. Explain what is happening to the mesa and
what will be the result.
http://3dparks.wr.usgs.gov/
You will need the cardboard stereoglasses. Go to Death Valley National Park.
Images 1 and 2, Dante’s View
Based on the name of this mountain, what do you think the temperatures would be like in the summer here?
What evidence is there that running water shapes this landscape?
Section 3: Desertification
http://www.telegraph.co.uk/earth/8359076/US-farmers-fear-the-return-of-the-Dust-Bowl.html
After reading this story about the aquifer, explain how water use could result in desertification of an area that is
not actually a true desert.