Geology and El Niño in the desert:
Landslides, debris-flows, and flash floods during El Niño events
Although desert lands appear nearly lifeless and the landscapes appear
static, the geologic processes that created this landscape are among the
most active of any place in the world. This landscape is changing continuously
by running water, blowing sand, and landslides, and many times when changes
occur the results are catastrophic for desert residents.
Geologic processes that act on the land's surface are described below. Each
of these processes is likely to cause large changes, perhaps catastrophically,
as a result of fierce storms forecast for El Niño.
Surface geologic processes.
Most young surface deposits of the deserts were created by water carrying
sediment by alluvial and colluvial processes. Alluvial processes
range from debris flows to sheetwash to stream flow in channels, and make
landforms such as alluvial fans, dry washes (arroyos), and dry lakes
(playas). Colluvial processes are driven by gravity as tumbling,
flowing, or creeping of material down steep slopes (mass wasting),
and make landforms such as talus cones and landslides.
Landslides.
What are they? The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Landslides are the wholesale movement of rock and soil down-slope by gravity; they can occur quickly or slowly. Slow movement of landslides and gradual creep of rock and soil downslope can be as destructive as rapid landslides and debris flows but are not catastrophic, and cause less loss of life.
El Niño effects. Landslides are much more likely to occur when the slopes are saturated by water. Prolonged rainfall and rapid melt of thick snowpack can trigger landslides on steep slopes underlain by weak materials.
More info. Although gravity acting on materials on an over-steepened slope is the primary reason for a landslide, there are other contributing factors:
Examples .
Gros Ventre landslide, Wyoming. Photograph
taken in 1931 of landslide triggered by rainfall and snowmelt in 1925. Landslide
dammed the Gros Ventre River, creating a lake that later drained catastrophically
when the landslide dam failed, killing people downstream.
Madison slide, with Earthquake Lake
behind it. This slide was triggered by an earthquake. 8/59
Landslide in loose rock, Echo Canyon,
Utah. 5/12/68. Note headwall scarp and minor scarps within the landslide.
Slumgullion mudflow, Colorado. Modern
photograph. The active flow extends from the active toe to the prominent
main scarp at the top of the photograph.| 9/22/05
photograph
Rockfall on Interstate 70, Colorado.
5/8/73.
Slump on Boulder County Road. 5/8/73.
Crown of slide is about 20 feet high.
Slump in loose sand down slipface
of dune, White Sands National Monument. 1962.
Debris flows. 
What are they? Debris-flows move so rapidly down steep slopes (to speeds of 35 miles per hour or more) that they are sudden and unexpected, destroying property and taking lives. They are among the most numerous and dangerous types of landslides in the desert. Debris-flows (also referred to as mudslides, mudflows, earth flows, or debris avalanches) are debris-laden flows of water, often the consistency of wet concrete.
El Niño effects. Material on steep slopes that becomes saturated with water after prolonged, intense rain or rapid snow melt may develop a debris flow or mud flow. The resulting slurry of rock and mud may destroy homes, wash out roads and bridges, sweep away cars, knock down trees, and obstruct streams and roadways with thick deposits of mud and rocks. Alluvial fans close to mountain fronts commonly are mostly made up of debris-flow deposits, indicating that this is an area of potential debris-flow hazard, especially during an El Niño event.
More info. Debris-laden flows are sometimes distinguished from mudflows that are soupier in consistency and carry less rock. Deposits left by debris flows are distinctive: they consist of poorly sorted debris, and are easily recognized in parts of alluvial fans distant from the mountains where boulders lie on and within a largely sand-size deposit.
Examples.
Debris flow on trail, Grand Canyon.
1966
Debris flow of mud and trees
across Hogback Road, Jefferson County, CO. 5/7/73
House destroyed by volcanic mudflow,
Mt. St. Helens eruption. Toutle River, WA. 5/80
Huge debris flow in Idaho
Sheetwash
What is it? Sheetwash is a thin sheet of water flowing across the surface as a result of an intense rainstorm. Although the water flows as a thin sheet, it is capable of transporting pebbles, sand, and mud.
El Niño effects. Sheetwash takes place during intense storms and after sustained hard rains have saturated the soil. El Niño events can bring sustained rains and cause widespread sheetwash, and homes that don't normally have flood problems may experience some flooding.
More info. Vegetation and cryptobiotic crusts have a very important role in retarding sheetflow by binding together surface materials, inhibiting erosion and improving absorption of water. As a result, removal of vegetation by fire, grazing, or mechanical stripping can increase the severity of sheetwash flooding. All colluvial and alluvial surfaces are subject to sheetwash.
Flash floods
What are they? Flash floods move so rapidly it can be hard to outrun them. Flash floods are all the more alarming because they can destroy property and wash away cars miles from the place where the rain fell. They occur in channels cut into into alluvial fans and in 'dry' washes (arroyos) in valleys during brief periods of intense rainfall.
El Niño effects. Flash floods take place as a result of intense cloudbursts that cause sheetwash across the surface, which then collects in rills, rivulets, and washes. By the time the water reaches main washes, walls of water several feet deep proceed rapidly downstream.
More info. Flash floods typically have a debris-laden front composed of rock, trees, and mud in water that surges down the channel. If channels are filled by the streams, the water spills over the stream banks to form floodplains. Deposits left by flash floods range from coarse gravel to muds in the channels and generally sand and mud on floodplains; bouldery debris are also left behind.
Examples .
Photograph of flash flood, Twentyfive
Mile Wash, Kaiparowits Plateau, UT. Flood was 8 ft deep and covers the road.
Rain fell 6 to 8 miles away.
Dust storms
What are they? Dust storms can sandblast surfaces, stripping paint and frosting windshields. They.are caused by powerful winds and can carry fine materials away from one place and deposit them in another. Windblown materials are mostly silt and fine sand.
El Niño effects. Prolonged rain in the desert can create many new sand deposits after floods receed. These sand deposits are unstable becuase no plants grow on them yet. After they sand dries it can be blown by strong winds to form dunes that can migrate across roads and into houses.
More info. As with sheetflow, wind erosion of materials is greatly reduced by thick vegetation and cryptobiotic crusts. Dry lakes (playas) and disturbed areas in the desert are the main source of windblown dust.
USGS
web site on dust studies.
USGS
climate program web site for El Nino
For further information, contact David Miller
http://geology.wr.usgs.gov/wgmt/elnino/deserten/processes.html, 30 November 1999, Contact: El Niño Web Team
