The graph below shows the
elevation of the Colorado River and its downstream gradient as
it flows from Lees Ferry to Lake Mead through the Grand
Canyon. Dark red and dark blue data for the graph were taken
from USGS Topographic maps combined with the Martin &
Whitis “Guide to the Colorado River in the Grand Canyon”.
(Better detail is available at TopoZone http://www.topozone.com
although the maps are the same.) The USGS maps show 40-foot
contours with interpolations by the author.
Prior to July 21, 2007, an older graph was used
that was based on contours in Belknap’s River Guide. Both
sources are roughly similar although there are some minor
differences over short ranges. Similar data (in orange and
light blue) has been added to show what existed before Lake
Mead backed up into the lower canyon. Light colored lines to
mile 261 are from Larry Stevens’ “The Colorado River in Grand
Canyon”. Data to mile 280 is an interpolation based on a pre
Lake Mead USGS topographic map that shows 870 feet at Wheeler
Ridge (Mile 284).
The table below interprets some
of the observations.
Mile 0: Lees Ferry
Mile 22 to 26: There are several rapids in this area, but they
appear to be a result of the steeper gradient rather than the
cause of the gradient. The rock layers in this area are the
lower members of the Supai Group and the top of the Redwall
Limestone. The steeper gradient in this section appears to be
the result of a small recently-active fault that crosses the
river at mile 30. The downstream side of this fault has been
dropped 50 to 100 feet. The gradient is thus increased as the
river crosses the fault zone. The increased gradient then
works its way back upstream as canyon cutting propagates
upriver. If the solid Redwall Limestone influenced the
gradient, then the gradient should decrease instead of
increasing in this area.
Mile 37 to 50: This area has the least gradient observed in
the Grand Canyon. The Eminence Break Fault crosses the river
at mile 49.8 with the strata uplifted downstream. It appears
this fault has also been active in the recent past. The uplift
downstream from mile 50 acts as a partial barrier to river
erosion with the result the gradient is less as the river
approaches the uplifted barrier.
Mile 71 to 79: The steepest gradient within the canyon
is observed in this section. The gradient increases sharply
after mile 71, but this is not due to local rock conditions,
as the Vishnu Schist isn't seen until mile 78 and even the
Shinumo Quartzite doesn't appear until mile 75. The crest of
the Kaibab Anticline is well to the west of this area. The
pattern is consistent with the river acting as a "stationary
band saw", and cutting a gorge into a still rising block. The
peak gradient occurs at Hance Rapids between miles 76 and 77.
Mile 98 to 99: The local peak in the gradient (to above
15) is entirely due to a "little river riffle" at the mouth of
Crystal Creek. A photo on page 376, Beus and Morales, shows a
large river raft being swallowed by an even larger 5-6 m.
standing wave at Crystal Rapid in June 1983.
Mile 152 to 175: This section of the river has the least
gradient of any area west of the Kaibab Plateau. Intuitively,
this area should show a steep gradient as it is just above the
Toroweap Fault and includes some of the sheerest parts of the
inner canyon. It seems probable a major debris flow in
Prospect Canyon some 3,000 years ago created Lava Falls Rapid,
and the debris fan has been partially blocking the river for
20 miles upstream ever since.
Mile 179 to 180: Everyone's favorite "riffle" otherwise
known as Lava Falls Rapid. This was the greatest obstacle to
the Racing Chris Crafts that ran the river upstream in 1950.
It took several hours before they made it. Earlier
(unsuccessful) motorized attempts to run Lava Falls upstream
were filmed with narration by a then unknown commentator who
has since gone on to making a ski movie or two. (Hint - W. M.)
Mile 225 to 231: The river has one last steep section
before entering the backup from Lake Mead. This is sort of the
“Home Stretch” as the river approaches the Grand Wash Cliffs.
Before Hoover Dam was built, there were several more rapids
(steep gradient) from here to the Grand Wash Cliffs.
Mile 235 to 242: (River elevations and gradient are derived
from The Stevens Guide Book and an old USGS topographic map
that shows the pre Lake Mead elevation at River Mile 284.) The
old riverbed in this area is buried by the backup from Lake
Mead. Before Hoover Dam was built, the gradient in this
section (Lower Granite Gorge) rivaled the current gradient
observed in Upper Granite Gorge.
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