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

(Also please see Lower Superimposition 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/

A 1950 aerial view of the upper end of Lake Mead. 

   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.

Topographic map of the location for the new rapid

   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.)


A 1992 close-up picture of the future location for Superimposition Rapid.

   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.

Overview of (historic) Lake Mead and the location of the new rapid.

   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 shows the historic and expected water level in Lake Mead.

   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.)


A new major rapid is expected to form on the far side of the low ridge.

   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.)


A 2003 photograph of the location for Superimposition Rapid.

   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


Approximately the same view but 5 years later

   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

Aerial view of the (historic) easternmost end of Lake Mead

   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.

Close-up view of the new rapid.

   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.


Superimposition Rapid as of Oct. 7, 2006   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)

Superimposition Rapid as of Sept. 1, 2007

   This next photograph was taken on Oct. 8, 2007 by Tom Martin (River Runners For Wilderness) and coauthor of the above river guide.

Superimposition Rapid as of Oct. 8, 2007


   Finally, the third picture (below) was by Tom Martin in Feb. 2008.

Superimposition Rapid in mid 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”

An early view of Pearce Ferry from USGS photographic archives.

“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.

Same view as the 1923 picture, but via Google Earth as it looked in 2004.

   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.



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