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| The Panamint Mountains draped by alluvial fans. |
The work of water: Alluvial fans
From the mouth of Golden Canyon, the top of Artist's Drive, or Dante's
View, you have a sweeping view across Death Valley toward the Panamint
Mountains. Rising almost halfway up the steep mountain front of the
Panamints are great aprons of rocky debris that spread out toward the
deep valley floor, partially burying this majestic range in its own
sediment. If you're observant, you'll notice that the sediment apron
is actually made up of many individual fan-shaped deposits, each radiating
out from a deep canyon cut into the mountain front. Death Valley is
world-famous for the incredible size, shape, and exposure of these alluvial
fans.
 Although rainfall is scarce in the valley, water is the creative
force that builds Death Valley's alluvial fans. At higher elevations,
precipitation is higher, and the water is quickly channeled into the
canyons that drain the mountain front. Death Valley’s occasional, but
intense, storms can quickly flood these canyons, sending surges of water
downslope. Where this deluge passes through narrow canyon passageways,
the water flow rapidly gains speed and strength (just like placing your
thumb over a garden hose creates a spray). The raging water picks up
any loose material in its path. The faster the water moves, the larger
the pieces of rock it picks up.
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| Badwater alluvial fan. Photo from NPS archives. |
Dumping debris
Eventually the rock-laden torrent reaches the narrow canyon mouth. Water
rushes out of the canyon into the open valley. Without the constricting
force of the canyon walls, the racing flood water slows down dramatically,
loses energy, begins to drop its sediment load, and breaks into several
small streams. The largest rocks drop out first, and the smallest sediment
is carried farthest from the canyon mouth. Only the finest silts and clays
are carried all the way to the bottom of Death Valley.
Each flood deposits
an new batch of sediment. In areas with higher rainfall, running streams
would remove this dumped debris, but in dry Death Valley, much of it stays
put. When the next flood washes down the canyon, it may take a new path
to the valley bottom. Over time, a system of braided stream channels radiates
out from the canyon mouth, and gradually builds up an alluvial fan.
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| Fan spreads out from canyon cut into the Black Mountains. |
Old and new
From overlooks like Dante's View, you can easily see braided networks
of light grey channels emanating from canyon mouths and extending to the
valley floor. These light grey washes are the active channels of the alluvial
fan. Older, inactive channels have been darkened by desert varnish. As
Death Valley deepens and tilts, older alluvial fan channels are permanently
abandoned, while new channels cut deep trenches on their way to the base
of the fan. Even the casual observer can often identify several generations
of fan deposits, the darkest, most deeply desert varnished are the oldest,
while the lightest are youngest.
Radiocarbon dates indicate that the highest surviving fan surfaces
in Death Valley are about
(xxx) years old. No doubt countless fans have formed since the opening
of Death Valley about 3 million years ago, but those earlier fan deposits
are now buried thousands of feet beneath younger fans and the valley floor. |
