Durango Bill’s
Grand Canyon 3-D Tour
About 1.25 miles
North-Northeast of Pearce Ferry
Near River Mile 280
Superimposition Rapid
alternately
Pearce Ferry Rapid
A chronology of transitions from:
1) Lake Mead
2) To a new superimposed path for the Colorado River
3) Which became a “riffle” near the old Pearce Ferry
take-out.
4) This “riffle” became a moderately severe rapid in late 2007
5)
And this could become an unnavigable waterfall if Lake Mead should drop
to the 1080 to 1090 foot level in future years. Alternately the rapid
would disappear if /when Lake Mead water levels rise above 1125 feet.
Construction of Hoover Dam (Boulder Dam) began in 1931
and was completed in 1935 as part of a project to “reclaim the
desert for productive human usage”. (Hence the name “Bureau
of Reclamation”.) By early 1941, the Colorado River had filled
the
reservoir to capacity to form Lake Mead.
The historic photograph below was taken July 12, 1950, and
shows what the upper end of Lake Mead looked like before large amounts
of sediment had accumulated. (Estimated water level is 1176 feet.) This
and other historic photographs of the area can be found at the USGS
Earth Explorer web site at http://edcsns17.cr.usgs.gov/EarthExplorer/
The silt-laden Colorado River exits from the Grand Canyon
(lower right quadrant) into Lake Mead. Pearce Bay is in the lower left
portion of the lake, and what will become the location for
Superimposition Rapid is just to the right of the two points of land
that stick out into the lake just to the north of Pearce Bay. If you
look just to the right of the northernmost of the two points, you can
see a small island. There is a submerged ridge that connects this
island with the higher ground to the left, and after another 50+ years
of
changes to Lake Mead, the river will hang up on this old ridge to
produce Superimposition Rapid.
Before Glen Canyon Dam was built, the silt load of the
Colorado River (as measured near Phantom Ranch in the Grand Canyon)
averaged 3 to 5 tons per second. (Different sources give different
amounts) Before Hoover Dam was built, this
sediment ended up in the Colorado River’s delta, but after it was
built, it was all deposited in Lake Mead.
The topographic map above from http://www.topozone.com shows the
old ridge that the river is now crossing. (The old ridge is the narrow
peninsula that sticks out into the lake just below the BM 1250 label.)
The lowest lake shoreline shows that the top of the old ridge was
exposed above the 1157-foot level. (The estimate date for the low-lake
contours is about mid 1972.) Also the 1160-foot contour line is shown
on the ridge which increases the pre-erosion level of the old ridge to
above the 1160-foot level.
The small island that could be seen in the previous 1950
photo shows up as the small hill (closed 1200-foot contour) at the end
of the peninsula. The Pearce Ferry take-out is off the bottom edge of
the map to the left of center.
An April 15, 1962 photograph at http://www6.nau.edu/library/scadb/imagedisplay.cfm?item_num=9383&type=Image
also shows this old ridge exposed above an estimated Lake Mead water
level of 1157 feet. (View looks south with the peninsula just above the
lower edge.)
The picture above shows the location for the incipient
rapid as it appeared in September 1992. (Estimated Lake Mead elevation
is 1174 feet.) The upper end of Lake Mead has filled with silt. Pearce
Bay is in the lower left hand corner. The two points of land in the
upper left corner are the same as shown in the earlier pictures. What
was an island in the first picture and the end of the peninsula in the
topo map, is now the small triangular light area just to the right of
the river. This area will be submerged again during the high water
levels seen in the late 1990s (see graph below), but the final path of
the river will be near what can be seen in this photograph.
Vertical view of what used to be the northeast end of Lake
Mead. (Estimated date for the photograph is March 9, 2004. Estimated
Lake Mead elevation is 1140 feet.) The
(historic) take-out at Pearce Ferry (at the lower end of
Pearce Wash) is in the lower left quadrant - near River Mile 280 in the
standard Grand Canyon river guide books. “Superimposition
Rapid” has subsequently formed where the two ridges closely
approach the river - midway between the pin and the left edge. The pin
marks the estimated location of the 1923 photograph shown at the bottom
of page.
Before Hoover (Boulder) Dam was built, the Colorado River
exited from the Grand Canyon (off the right edge of the picture) and
flowed left and up across what are now the mudflats in the picture, to
exit off the top edge (left of center). After Hoover Dam was completed,
the reservoir backed up to cover the dark brown area in the picture
with water. What was part of Lake Mead as seen in the first photograph
has now been transformed into the dark brown mud flats.
Over a period of several decades, silt brought down by the
Colorado River filled in the old river channel and spread out to cover
an area more than a mile wide to form a broad silt flat. The bottom of
the old river channel is some 300 feet down under the silt. (A graph at
http://www.durangobill.com/GCriverProfile.html
shows the river elevations for both before and after Hoover (Boulder)
Dam.)
The graph above shows the water level in Lake Mead
starting with 1980. The plotted levels through 2010 are forecasts via
the U. S. Bureau of Reclamation
http://www.usbr.gov/uc/water/crsp/studies/index.html
Historical water
levels can be seen at
http://www.usbr.gov/lc/region/g4000/hourly/mead-elv.html
The
chart above shows historical measurements
and future water level forecasts as of the April 2008 USBR report.
The water level in Lake Mead reached its maximum elevation
in 1983. When the lake level subsequently receded from this maximum
elevation, the river had no guide as to the location of the old
channel, and established a new path across the silt flats,
approximately in its current position. Most of this new path area was
periodically exposed and resubmerged over the next 20 years, but the
drought that has set in during the early 2000s has severely lowered
Lake Mead. As the water level lowered, the river became entrenched in
its new path.
Please see “Transition to a more arid
Southwest” (and related technical paper links) at http://www.ldeo.columbia.edu/res/div/ocp/drought/science.shtml
for the reasons why drought conditions are developing. (And be sure to
give the authors an A+ for recognizing the current problem in advance.)
The geological sequence of burying an old landscape under
silt/sediment and then establishing a new river drainage system across
the top of these sediments is a process called superimposition. In a
normal geologic time frame, this requires many millions of years. Here,
at the upper end of Lake Mead, a man-made version of the process has
taken only a few decades.
In a normal geologic sequence, a new superimposed river
system might end up over the top of an ancient mountain range. If
subsequent erosion removes the softer sediments, it’s quite
possible the superimposed river might remain stuck in its path and
erode its way down through the old mountain range to form a canyon.
Here, in the remnant of the upper end of Lake Mead, the
silt deposits buried an old ridge. When the Colorado River established
its new course after the 1983 maximum elevation of Lake Mead, the river
happened to end up over the top of an old ridge. When the water level
in Lake Mead subsequently dropped, sediments on the downstream side of
the ridge eroded away. However, the ridge resisted erosion. The river
thus formed a rapid where it dropped from the relatively higher
elevation of the ridge to the relatively lower elevation downstream. It
seems appropriate to name the rapid “Superimposition Rapid”.
(Alternately, if the rapid were named for its geographical location, it
might be named Pearce Ferry Rapid or Pierce Ferry Rapid.)
The picture above shows what the landscape at
“Superimposition Rapid” looked like before Lake Mead dropped low enough
for the rapid to form. The Colorado River is entrenched in a path that
forces it to cross an old ridge. The area
where the river crosses the ridge can also be seen in the first Google
Earth picture - midway between the pin and the left edge. In both views
the river
enters from the right and then flows toward the top of the picture.
The Google Earth satellite picture (above) was probably taken on March
9, 2004 and covers approximately the same area as the Vishnu Temple
Press photo below. With a little work you can find the exact spot where
the Vishnu picture was taken. The white sand in the foreground of the
Vishnu picture will match the lower right end of the white sand patches
in the Google Earth picture. (On the left side of the river, above and
slightly to the right of the center copyright labels.)
The photograph above is similar to the Google Earth
picture and was taken on March 22, 2003. It was originally posted on
the Vishnu Temple Press website (Good source for rafting guide books.),
and while it is no longer posted there, you can see the original web
page at: http://web.archive.org/web/20031208154023/www.vishnutemplepress.com/pearceferry.html
The photograph above was taken by Tom Martin at nearly the same
location as the Vishnu Temple Press photograph - but in Feb. 2008. It
is inserted here (a jump in the time sequence) to show what has
happened in the 5 year period from 2003 to 2008. Notice how the river
has dug down through the sediments. (For example, the ridge a couple of
hundred yards downstream in the upper left is now part of the skyline
as the photographic location is now 20 to 30 feet lower.)
As the elevation of Lake Mead dropped due to the drought,
the Colorado River had an easy chance to erode the silt deposits that
were deposited before the drought lowered the water level. The
exception is the ridge. When Lake Mead dropped below the 1120-foot
level in 2007, the river could erode the silt sediments downstream from
the ridge, but the ridge acted as a dam and protected the sediments
above the ridge. The ridge itself became the crest of the new rapid.
(This is the upstream portion of the rapid which is a couple of hundred
feet upstream from the large rock that subsequently emerged.)
The two pictures below are from an aerial U.S. Bureau of
Reclamation photograph that can be seen at http://www.usbr.gov/lc/region/g5000/photolab/gallery_detail.cfm?PICIDTYPE=13209
This first picture looks east across the (historic)
easternmost end of Lake Mead. In the distance, the Colorado River exits
from the Grand Canyon, flows westward (toward the foreground) across
the silt flats, and then turns north to flow off the left edge of the
picture. The old Pearce Ferry takeout is well off the lower right edge
while the location for the new rapid is in the lower left corner.
The picture above is an enlargement of the lower left
corner of the previous picture. Both views show what the incipient
rapid looked like on Feb. 2, 2006 - Lake Mead elevation about 1140 feet.
Standing waves from a submerged pour-over rock (part of
the old
ridge) can be seen.
As of late July 2007, the surface elevation of Lake
Mead had dropped about 29 feet from where it was in Feb 2006. The soft
sediments that were deposited by the river over the last 6 decades can
be easily eroded. This leaves two major possibilities. 1) If the rock
in the old ridge can resist further erosion, then a severe rapid will
form on the downstream (left) side of the old ridge. 2) The river just
might be able to erode down through the old ridge. If it can erode
downward, there will still be a rapid, but the drop-off will be less,
and hence the rapid will be less severe.
The photograph at the right was taken by
“bobkerry44” on Oct. 7, 2006 and is a copy of the original
which is posted in the photos section at the gcpba Yahoo Group. The
photograph was taken from near the right bank of the river and looks
northwestward across the river to the same portion of the ridge
that appears on the left side in the Vishnu picture.
The Vishnu picture was taken in March 2003 with the water level in Lake
Mead at about 1154 feet. By Oct 7, 2006 the lake had dropped to about
1126 feet. The river had already taken advantage of this height
difference to erode downward toward the old ridge. The pour-over rock
that is in the center of the photograph is part of the top of the old
ridge. As the lake continues to drop in the future, the river will
become increasingly hung up on the old ridge and the rapid will
gradually become more severe.
The following 3 pictures were taken from “North
Point” (the left ridge in the Vishnu picture) and look southeast to
south over the rapid. The sequence is a record of what was happening in
the fall and winter of 2007/2008.
The
first picture was taken on Sept. 1, 2007 by Duwain Whitis (“Guide to
the Colorado River in the Grand Canyon” by Tom Martin and Duwain Whitis)
This next photograph was taken on Oct. 8, 2007 by Tom
Martin (River Runners For Wilderness) and coauthor of the above river
guide.
Finally, the third picture (below) was by Tom Martin in Feb. 2008.
Two items of interest are of note. The most obvious is the large rock
in the middle of the river. Also there is a smaller riffle about 200
feet upstream from the rock.
Analysis using old photographs (e.g. http://www6.nau.edu/library/scadb/imagedisplay.cfm?item_num=9383&type=Image
)
indicates the top of the rock is reasonably close to or
possibly slightly lower than 1155 feet above
sea level. It is also apparent that the river has moved laterally since
the satellite photograph that was used for Google Earth. Hence old
pictures and even the USGS topo map shown earlier are only
“squiggly” guides as the topography is rapidly evolving.
First, the old peninsula that can be seen in the topo map
and in the above (linked) picture has eroded downward. What is left of
it is causing the riffle that is some 200 feet upstream from the rock.
The rock on the other hand was still submerged as of the
nau 1962 photo (Lake Mead at 1157 feet), and there is no indication
of its existence in the topo map. As the river has eroded downward, the
rock (and who knows what underwater extensions it might have) has
emerged. While the old peninsula has been eroded downward, the rock is
holding - well - “like a rock”. Thus local variations in
erosional resistance exist - which throws another unknown into the
expectations of what will evolve next.
The situation at the rapid will continue to change.
The ultimate effect on the rapid can not be precisely determined, but
you should expect changes.
The Future
Updated April 14, 2008: In mid 2007 the water level
forecast for Lake Mead had expected the Lake Mead water level to drop
to near the 1080-foot level by summer 2009. If the water level had
subsequently followed this forecast, Superimposition Rapid would have
evolved into an extremely severe rapid (and possibly a waterfall) that
might have been unrunable.
However, snowfall in the
Rockies during the winter of 2007/2008 has been substantially above
normal. This will produce a much above normal spring runoff that will
partially fill Lake Powell more than 30 feet above its low levels of
recent years. In turn this will allow water releases from Lake Powell
during the summer months of 2008 to be well above what we have seen in
recent years. These larger releases from Lake Powell will keep Lake
Mead from dropping as much as was expected in earlier river management
forecasts.
With Lake Mead only dropping a few feet this year, the summer 2008
outlook for the rapid’s severity
will not change much from what was observed in winter 2007/2008.
Previous forecasts for the water level in Lake Mead would have
intensified Superimposition Rapid to the 6 to 8 range in 2008. It now
appears that the rapid will remain near a category 4 to 5 rating (Grand
Canyon rating scale) for the next year and may weaken in 2009. The
correspondence between Lake Mead water levels and the potential
intensity of the rapid is given below.
A rough calculation of
the potential drop at the rapid can be made as follows. The elevation
of the downstream side of the rapid will gravitate toward the lake
level plus a river gradient of about 2 feet per mile for the 13 miles
to the lake. (For example, a lake level of 1115 feet yields 1115 + 2 x
13 = 1141 feet). The upstream side of the rapid will be anchored by the
ridge. Originally the ridge elevation was about 1160 feet, but this is
being eroded downward some unknown amount. The drop across the rapid
will be equal to the difference between the upstream number and the
downstream number.
If we look still further out
into the future over the next couple of decades, Superimposition Rapid
will just be a temporary phenomenon. Even with Glen Canyon Dam blocking
the silt flow that used to come down the Colorado River, there is still
an inflow of silt from the Paria, the Little Colorado, and smaller
tributaries. This silt will continue to accumulate in what used to be
the upper end of Lake Mead. The exact details of where and when this
new silt will settle out will vary with the water level in Lake Mead,
but the new silt will eventually bury everything that is visible today.
This will include Superimposition Rapid, which will eventually be
buried - never to reappear again.
Notes on the origin of
the name “Pearce Ferry”
“Grand Canyon National Park,
Arizona. Upstream toward the west portal of the Grand Canyon. Note the
ruins of the old stone house in the foreground. This was once one of
the buildings at Pierce (Pearce) Ferry, where a wagon trail from
Kingman to Moapa crossed the river. Colorado River Survey of 1923
(Birdseye).”
The historic photograph above was originally taken on the
“Colorado River Survey of 1923” and was subsequently
published in the May 1924 issue of National Geographic Magazine. You
can see the original photo at http://libraryphoto.cr.usgs.gov/cgi-bin/show_picture.cgi?ID=ID.%20LaRue,%20E.C.%20%20771
If you click on the photo at the above web link, you can get greater
magnification.
The estimated location of the photographer is about 36.130
N, 113.977 W or about 0.63 miles east and a little south of the
developing Superimposition Rapid. (Please see the pin location in the
first Google Earth picture.) The view in the 1923 photograph looks
slightly north of
due east. Based on the estimated location of the photographer and the
location of the river in the photograph, it is estimated that the river
originally turned toward the north just off the left edge of the
photograph, and passed about one-half mile to the east-northeast of the
new rapid.
After Hoover Dam was built, the old river channel was
buried under 300 feet of silt and sediments brought down by the
Colorado River. You can get some idea of the magnitude of 300 feet of
sediment by following the ridge that starts in the dark shadow next to
the river and climbs to where it disappears off the right edge of the
photograph. The uppermost portion of this ridge (just off the right
edge) remains barely above the silt flats. If you look at the first
Google Earth picture, you can see a small light-colored “sand
island” to
the right and a little below the pin location. The “sand
island” is the top of the ridge that is just off the right edge
of the
1923 photograph. Also, the “sand island” is visible just to
the right of the river in the middle distance in the first 2006 Bureau
of Reclamation photograph.
The picture above was generated by Google Earth and shows
approximately the same view (with slightly wider sides) as the old 1923
photograph, except that as of 2004, the viewpoint is some 300 feet
higher due to the influx of silt. In the Google Earth picture, the low
flat area on the skyline to the left of center appears relatively
higher due to the ~300-foot increase in viewpoint elevation. Most of
the skyline is formed by the inner rim (Grand Wash Cliffs), but the low
flat area is part of the outer rim and is some 5 - 6 miles more distant
(and 1,900 feet higher) than the inner rim. Thus the increase in
viewpoint elevation makes it
appear somewhat higher in the Google Earth view vs. the 1923 photograph.
The name “Pearce Ferry” goes back to 1876 when
Harrison Pearce established a ferry service here to cross the river.
There is good original documentation at:
http://search.ldslibrary.com/article/view/2579935
and
http://www.familytreelegends.com/records/45546?c=read&page=118
The spelling of “Pearce Ferry” was
subsequently corrupted to “Pierce Ferry”. (The
“spelling checker” in Microsoft Word didn’t exist
yet.) While
“Pearce Ferry” (also “Pearce’s Ferry”) is
the original
name, “Pierce Ferry” is widely used.
Return to river
miles 272 to 280
Continue
to the Lower Superimposition Rapid page
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Index Page for the Grand Canyon Tour
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