Geoscience Reference
In-Depth Information
Figure 18.8 Playa in Death Valley, California. Lakes form in
basins and on plains in the American West because they collect
storm runoff. They subsequently dry out during drought periods,
leaving this distinctive landscape behind.
Figure 18.9 Arroyos in the Southwest. Arroyos are steep-
sided gullies cut into alluvium. Flowing water within this arroyo
suggests that a heavy storm recently passed through the area.
In this discussion of southwestern geomorphology, you
may wonder how streams can play such an important geo-
morphic role in such an arid place. Although it might seem
that the influence of water would be minimal in dry lands
where most streams are ephemeral, streams are actually very
important geomorphic agents in these landscapes. One rea-
son for this impact is that when it does rain in arid regions,
it often rains very hard for a short period of time, resulting
in flash flooding. This high discharge has a disproportionate
impact in desert regions because vegetation cover is minimal
and hillslopes are not protected by plants. As a result, the
landscape can be shaped dramatically when streams do flow.
Also remember that these processes have been ongoing for
many millions of years and you are seeing the net effect of
long-term change.
Along with the overall dissected terrain of the Southwest,
ephemeral streams also produce some distinctive landforms.
One such feature is an arroyo , which is a steep-sided gully cut
into alluvial sediments (Figure 18.9). Arroyo formation appears
to be linked to wet/dry climate cycles related to El Niño and
La Niña conditions, respectively, in the Pacific Ocean. Research
shows that episodic cutting of arroyos has been especially
intense during the past 4000 years.
Perhaps the most interesting landform created by streams
in the Southwest and many other arid regions is an alluvial
fan. An alluvial fan is a depositional landform created where
bedload-dominated streams flow out of mountainous or hilly
areas onto an adjacent plain. Very simply, as the stream travels
down the steep gradient, it can carry the coarse sediment de-
rived from the mountain or hill. When the stream reaches the
plain at the base of the hill or mountain, however, it loses en-
ergy because the gradient there is much lower. As a result of this
reduced gradient, stream power is lost and aggradation occurs.
Sediment deposition begins at the fan apex, which is the point
where the stream leaves the mountain. Below the apex, the
stream sweeps back and forth through time and in so doing cre-
ates a semicircular landform that looks much like a hand-held
fan from above (Figure 18.10a).
Alluvial fans can occur virtually anywhere that streams
flow abruptly from areas of high relief to surfaces that have
lower slopes, even in humid areas. However, some of the
most impressive fans in the world are those in the south-
western United States. Here, alluvial fans occur along many
of the mountain fronts within the region and are sometimes
many kilometers wide and have steep gradients. These fans
are so large that they often overlap with one another along
the mountain front to form a bajada . Many of these fans are
complex in that they contain interbedded deposits of mud,
sand, gravel, cobbles, and even boulders. The sand and gravel
layers indicate deposition by a bedload stream, whereas the
very coarse rocks indicate prior debris flows. The fine sedi-
ments accumulate during major mudflows that periodically
occur. Often, the surface of fans consists of a mix of gravels
and cobbles (Figure 18.10b).
A deep, steep-sided gully that is cut into alluvium.
Bajada A landscape consisting of overlapping alluvial fans
along a mountain front.
Arroyo
Alluvial fan A fan-shaped landform of low relief that forms
where a stream flows out of an area of high relief into a broad,
open plane where the gradient is less and deposition thus occurs.
 
 
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