Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 43

2.6 Sediment-Transport Processes
Mass wasting is the general term used by geologists to describe the movement of material
from bedrock hillslopes to valleys under the influence of gravity and facilitated by wind
( eolian ) and water ( fluvial ) processes. Gradual movement of material from highlands to
lowlands in deserts is influenced by slope, the presence of surface water and groundwater,
vegetation cover, disturbance by animals, and the characteristics of the sediments
themselves. Large volumes of earth materials also can be moved catastrophically via
landslides, debris flows, and rockfalls (Chapter 5). These events can be somewhat common
in desert regions especially following high precipitation events, earthquakes, removal of
vegetation cover, and/or failure along planes of weakness in the rock mass. The danger of
catastrophic mass-wasting events can be mitigated using a variety of engineering controls
such as subsurface water drainage, rock nets, rock bolts, and grouting.
Other sediment-transport processes are continually active in deserts at smaller scales
than the processes mentioned earlier. Episodic rainfall in deserts dislodges sediments on
hillslopes by rainsplash, essentially the raindrop impacts the soil surface and frees small
particles. The sediment is then free to be transported by water flowing over the soil surface
as overland flow or sheetwash. As more water flows over the soil surface, small channel
networks (rills) began to form, which concentrates and speeds up the flow, allowing larger
sedimentary particles to be transported.
As the rill networks combine and grow larger, they begin to form washes, which then
deliver sediment from the hillslope into the mountain front region or piedmont described in
the following section. At each stage, the ability of water to carry larger and larger sediment
loads and particles increases. Sediments derived from bedrock are transported off the
hillslopes and into valleys by these fluvial processes (Figure 2.4). In deserts, this form of
sediment transport tends to occur in short amounts of time whenever high precipitation
events occur, and as a result, most washes in deserts are dry most of the time yet represent
the major transportation route of sediments (see Chapter 4). During large floods, sediments
in the valleys also can be remobilized and deposited again further downstream from their
original source areas.

(a) (b)

FIGURE 2.4
A typical desert hillslope in the McDowell Mountains located in Scottsdale, Arizona, has relatively sparse veg-
etation cover and gravelly surficial soils (a). Infrequent but powerful precipitation events lead to the formation
of rills and washes on desert hillslopes (light-colored linear features in (b), also in the McDowell Mountains);
these are the major sediment transport pathways from hillslopes to the piedmont. (Courtesy of W.L. Stefanov.)