Durango Bill’s
Grand Canyon 3-D Tour
Near River Mile 286.2
Lower Superimposition Rapid
alternately
Driftwood Rapid
(Also please see Superimposition
Rapid)
The topographic map above (Courtesy of TopoZone http://www.topozone.com/ ) shows
the pre-Lake Mead contours in the vicinity of “Driftwood
Island”. (Driftwood Island is just below the center of map.) The
blue shading shows what Lake Mead would cover if it were full. Contour
elevations are in meters - thus multiply by ~3.28 to convert to feet.
If you want to find Driftwood Island via Google Earth, go to about
36.1926 N, 114.0269 W. (Note, the mile markers in the above map do not
correspond with current Grand Canyon river guide maps.)
There are two items of interest in the map. First, the
pre-Lake Mead course of the Colorado River was from right to left in
the deepest part of the old canyon. (The contour lines in this deepest
part of the old canyon average about 270 meters which is equivalent to
about 886 feet above sea level.) In particular, note that this old
route was south of Driftwood Island. The second item of interested is
the red place-marker. It is located on the crest of an old ridge at an
approximate elevation of about 343 meters which is about 1125 feet
above sea level. Before Lake Mead existed, this old ridge connected
what would become Driftwood Island with higher elevations to the north.
We will come back to this “red place-marker”.
After Hoover (Boulder) Dam was built, the upstream
reservoir (Lake Mead) backed up until only a small speck remained of
the “Driftwood Island Ridge”. In the computer generated
picture above, Driftwood Island is the small unlabeled island in the
center. Meanwhile, the Colorado River continued doing what it had been
doing for millions of years. It was transporting silt downstream - only
now the silt was being trapped in Lake Mead instead of continuing down
to the Colorado’s delta. As measured near Phantom Ranch in the
Grand Canyon, this silt transport rate averaged 3 to 5 tons per second.
(Different sources give different amounts.) This went on for 60 seconds
per minute. 60 minutes per hour. 24 hours per day. 365 days per year.
For decades. The subsequent construction of Glen
Canyon Dam has significantly slowed this sediment transport rate (now
Lake Powell is catching most of the sediment), but for nearly 30 years,
Lake Mead got “dumped on”.
The incoming sediment buried the old river channel some
300 feet deep. It also spread sideways to cover the old ridge that
connected Driftwood Island with higher terrain to the north.
In the last few years, the southwestern part of the United
States has been experiencing a drought. Inflow to Lake Mead has been
less than what downstream demands wanted. Thus, water has been released
from Lake Mead to make up the difference. The excess of outflow over
inflow has lowered the lake level.
As the water level in Lake Mead has dropped, the huge
amounts of silt that were deposited are being exposed. There is one
very important difference from the topographic map view (first
picture). In the topographic map view, the original course of the
Colorado River was to flow from right to left to the south of Driftwood
Island. In the recent Google Earth view above, the Colorado River is
still flowing from right to left, but now it is north of Driftwood
Island.
As discussed on the Superimposition
Rapid Page, the process of burying an old landscape with sediment
and then establishing a new drainage system on top of it has a
descriptive name. What we are witnessing here is another example of
“superimposition”. While it only took a few decades instead
of many million years, a new drainage system has been established on
the surface of the recently deposited sediments.
Once again, the new drainage system had no information
about the old river channel. Here, by random chance, the new path for
the river ended up over the old Driftwood Island Ridge. The high saddle
point on the old ridge is about 1125 (+/-) feet above sea level, as
shown by the “red place-marker” on the topographic map.
The photograph above is part of the Bureau of
Reclamation’s Photo Database. The original photograph can be seen
here: http://www.usbr.gov/lc/region/g5000/photolab/gallery_detail.cfm?PICIDTYPE=13210
The view looks northeastward with Driftwood Island (now a sandy
hill) a little to the right of center and Grand Wash Bay in the
distance. The Colorado River enters from the upper right and flows
toward the lower left. Lower Superimposition Rapid is a “potential
rapid”. As of early 2008, it doesn’t exist. However, if the water level
in Lake Mead should drop below the 1100-foot level, then the rapid
would develop where the river crosses a submerged ridge that extends
from Driftwood Island to higher ground above and to the left of the
former island.
In mid 2007, the outlook for Lake Mead called for the Lake Mead water
level to drop to 1100 feet (or lower) over subsequent years. If
subsequent events had followed this scenario, the river would have had
no problem excavating a lot of the old silt downstream from the
Driftwood Island Ridge. The material in the ridge itself is probably
poorly cemented, but it has been in place for several million years. It
would resist erosion whereas as downstream silt could be eroded.
Potentially, another “superimposition type” rapid could develop on the
downstream side of this ridge. If Lake Mead water levels should
drop below the 1100-foot level sometime in the future, a rapid would
form at this location, and it would seem appropriate to name this rapid:
Lower Superimposition Rapid
The winter of 2007/2008 has seen well above normal snowfall in the
Rockies. Runoff from this snowfall will keep water levels in Lake
Powell and Lake Mead well above earlier expectations. Thus “Lower
Superimposition Rapid” will only remain as a potential rapid that is
subject to possible lower water levels in Lake Mead at some
indeterminate time in the future.
The Calm Before the
Storm
The photograph above was taken by Tom Martin on Oct. 9,
2007. The photographic location is part way up a small bluff about 1200
feet northwest of the top of the former Driftwood Island and looks
east-northeastward toward Wheeler Ridge. Grand Wash Canyon is to the
right of center.
If the water level in Lake Mead should ever drop to the
1100-foot level, Lower Superimposition Rapid (Driftwood Rapid) would
begin to form about 1100 feet upstream - approximately to the
right of the white bluff that can be seen on the left edge of the
picture. The location would be near the area where there is a slight
change in the river color from “light muddy” in the foreground to “dark
muddy” in the background.
Up until Jan, 2008, the USBR had been forecasting Lake
Mead to drop below the 1090 foot level in 2009. If this had worked out,
a significant rapid would begin to form on the north side of Driftwood
Island. However, snowfall amounts in the Rockies have been
substantially above normal during the winter of 2007/2008. This will
produce a larger water runoff and keep Lake Mead at higher levels in
2008/2009 than expected earlier. It now looks nothing will develop on
the north side of Driftwood Island for the next couple of years; but if
Lake Mead should ever drop to below the 1100-foot level at some future
date, the potential remains that “Lower Superimposition Rapid” could
still form.
Return to the Superimposition Rapid page
Return to the
Index Page for the Grand Canyon Tour
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