Durango Bill’s
Ancestral Rivers of the World
Ancestral Rivers of
Mexico, Western Canada, and Alaska
by
Bill Butler
Antecedence and superimposition are geologic processes
that explain how and why rivers can cut through mountain systems
instead of going around them. Examples (with pictures) are from Mexico,
western
Canada, and Alaska.
Featured Areas
Bustamante Canyon (Canon Bustamante),
Mexico
Peace River / Williston Reservoir,
British Columbia
Stikine River, British Columbia / Alaska
Alsek River, Yukon Territory / Alaska
Copper River, Chugach Mountains, Alaska
Nenana River, Alaska Range, Alaska
Delta River, Alaska Range, Alaska
Bustamante Canyon
(Canon Bustamante), Mexico
There are several rivers that cut through ridges near
Monterrey, Mexico, but one of the most impressive is Bustamante Canyon
which is some 62 miles north-northwest of Monterrey. In the view above
(which looks north-northwestward), the Rio Sabinas cuts from left to
right (from Ranchito to Bustamante) through a high uplifted ridge of
sedimentary rocks. (The layers are primarily limestone and include
caves.) The resulting canyon is over 3,000 feet deep.
Bustamante Canyon is an example of antecedence. The Rio
Sabinas (Locally, other names are also used.) isn’t much of a
river, but it (or its ancestor) was in place first, and as the ridges
were folded/tilted/uplifted thousands of feet, the river was able to
maintain its original course and cut down through the rising block.
As in most cases of antecedence, the river played the part
of a stationary band saw. If you lift a block of wood up into the teeth
of a horizontal band saw, the saw will cut a groove into the block of
wood. Here, the Rio Sabinas played the part of the band saw. As the
mountains rose, the river maintained its original course and cut a
groove into the rising mountain block. The “groove” is now
a 3,000+ foot deep canyon that cuts across the mountain range.
Peace River /
Williston Reservoir, British Columbia
The picture above looks slightly west of due north at the
headwaters of the Peace River in northeastern British Columbia. The
large lake is Williston Reservoir / Lake which is backed up behind the
W. A. C. Bennett Dam. The dam itself is some 15 miles off the right
edge of the picture. Water flow through the river system originates as
multiple tributaries that feed into the lake on the left side of the
picture. The confluence produces the Peace River which flows to the
right through the Rocky Mountains. Where the river cuts through the
Rockies, the gorge is over 4,000 feet deep.
Williston Lake itself averages about 2,160 feet above sea
level. If you follow the valley southward off the bottom left edge of
the picture, you can reach the Fraser River valley and follow it down
to the Pacific Ocean. If the Peace River followed this route, it would
never have to climb more than 2,400 feet above sea level. (Note: While
the Fraser River itself stays below 2,400 feet, recent uplifts along
its route have forced it to cut through plateaus and mountain ranges,
but this story is not covered here.)
Instead the river cuts through the heart of the Rockies.
The ridge line of the Rockies is consistently 5,500 to 6,000 feet above
sea level in the vicinity of the Peace River Gorge with some local
summits over 7,000 feet. Why did the river cut to the right through the
mountains instead of taking an easy route to the south?
The uplift history of this part of the Rockies is not
known. What is obvious is that the Peace River had established its
route before the mountains rose. If the mountains had been in place
first, the river would have taken the easier route to the south. The
only way the Peace River could cut through the mountains requires that
the river was in place first. As the Rockies rose, the river was able
to abrade down fast enough to maintain its original course.
Stikine River, British
Columbia and Alaska
Several rivers in western Canada cut through the Coast
Ranges to reach the Pacific Ocean. The Fraser River (mentioned above)
is one of these, but is not described further here. Two major rivers
that we will cover are the Stikine River and the Alsek River.
The view above looks southwestward across northwestern
British Columbia and the Alaskan panhandle. The Stikine River
originates some 100 miles off the lower left corner of the picture and
comes into view through the compass sign at the lower left corner. From
there it flows westward which in this view is toward the right and up a
bit. Then the Stikine turns more to the southwest (up in the picture)
and then due south (left and up) to cut through the Coast Range
mountains.
The horizontal yellow line across the top of the picture
delineates the border between Canada and Alaska. After the Stikine
clears the mountains, it exits into the fiords and islands of the
southeast Alaska panhandle near Wrangell, Alaska.
This is wild and only partially explored country. If you
are into river rafting in the middle of nowhere, you might want to jot
down the Stikine for future research.
The Coast Ranges of Canada and Alaska are very young. For
the most part they have been uplifted in the last 20 million years. The
Stikine River was in place before this uplift began. As the Coast
Ranges were uplifted, the river was able to cut down fast enough to
maintain an approximate stable elevation. As the mountains rose, the
river cut a groove into the rising block.
On its path through the Coast Ranges, the Stikine River is
within one thousand feet of sea level. The mountains on either side are
over 6,000 feet above sea level. Thus the Stikine’s gorge through
the ranges is about a mile deep.
If we could look at the Stikine’s path through the
mountains as it existed 3 million years ago, we would see a narrow
canyon comparable to today’s Grand Canyon. However, unlike the
Grand Canyon, massive glaciers have periodically flowed down the
Stikine’s valley over the last 2 million years. Glaciers erode
sideways as well as down since the rocks and debris they carry scrape
away the sides of preexisting canyons. The glaciers have transformed
the previous narrow canyon of the Stikine into a classical glacial
valley. The result is a typical “U” shape cross section
where the valley is relatively flat but the terrain rises steeply on
both sides.
Geology is a “work in progress”. While the
glaciers broadened out the valley, the Stikine has already started to
carve a new canyon within this broad glacial valley. If you look
closely just
above the “Image 2006 TerraMetrics” label, you can see the
result of this recent canyon cutting.
Alsek River, Yukon
Territory to Alaska
The picture above is a view to the southwest, and shows
the Alsek River which starts near Haines Junction, Yukon Territory
(junction of routes 1 & 3) and then flows southwestward (up)
through the Alaskan panhandle to the Pacific Ocean. The
tall peak near the left edge is 15,300-foot Mt. Fairweather while the
even higher Logan / St. Elias massif is near the right edge. Several
large glaciers can be seen winding their way downward from the St.
Elias ice-fields, but if you look closely you can see gray piles of
recessional moraines indicating the glaciers are shrinking due to
global
warming.
In the distance the Tweedsmuir Glacier (see below) flows from right to
left - just upstream (toward the foreground) from where the river
crosses the U. S./Canadian border.
This is an active area of mountain building. The
earthquake icon marks the epicenter of a magnitude 5.1 quake that
occurred in 1995. If you have Google Earth, you can zoom in to see the
locations of many more recent earthquakes.
At Haines Junction, the Alsek River is about 1,900 feet
above sea level. If the Alsek didn’t cut through the Coast
Ranges, it could follow today’s topography eastward (toward the
lower left corner and eventually join the drainage system for the Yukon
River. If it followed this route, its maximum altitude would be about
2,800 feet.
Instead the Alsek River flows southwestward through these
growing mountains. Today the mountains clear 7,000 feet within a few
miles of the river. In the future they will be even higher.
The Alsek is another example of antecedence. The river was
in place first. As the mountain ranges rise, the river has enough
abrasive power to “sandblast” and remove material that
keeps trying to “get in the way”.
The view above looks northwestward over the lower Alsek
River with the Tweedsmuir Glacier starting in the upper left corner and
flowing toward the foreground. The Alsek River enters from the upper
right and flows toward the lower left. Irregular surges by the glacier
have squeezed and currently threaten to block the Alsek River. If the
glacier does block the river, a large backup lake will form (going
toward the upper right corner) behind the glacial dam. If a glacial
backup lake forms, the possibility exists that the eventual overflow
may breakup the lower end of the glacier - with a large portion of the
lake water draining all at once in a flood.
Nov. 2007 Update
In the late summer/fall of 2007 the Tweedsmuir Glacier (At
~59.8 N 138.0 W
and which is the major obstruction to the Alsek’s course) had
begun another of its periodic surges. The Tweedsmuir may block the
Alsek River sometime in the next few months and form a lake upstream
above the blockage point. If a back-up lake forms, the river will
eventually
break through again, with a glacial flood ( a jökulhlaup )
downstream. Further information about the Tweedsmuir can be found at http://www.gi.alaska.edu/ScienceForum/ASF18/1878.html
plus background information on Alsek/St. Elias glaciers at: http://pubs.usgs.gov/prof/p1386j/stelias/stelias-lores.pdf
Chris Larsen (Geophysical Institute, University of Alaska
Fairbanks) has been photographing the Tweedsmuir Glacier, and as of
Nov. 5 the
Tweedsmuir’s surge has at least temporarily stopped
before blocking the Alsek. (Good photos of the Tweedsmuir and other
glaciers at http://www.gps.alaska.edu/chris/
)
March 31, 2008 Update
The Tweedsmuir Glacier appears to be surging again in early 2008, and
once again threatens to block the Alsek River. If you are planning to
raft the Alsek in 2008, check the above web links for updated
information.
Copper River, Chugach
Mountains, Alaska
The picture above is a view looking south over the Copper
River in Alaska. The Copper River enters from the lower right corner
and then flows southeastward to where a major tributary, the Chitina
River, joins it. Then the Copper River turns toward the south-southwest
(up in the picture) and cuts through the Chugach Mountains before
reaching the Gulf of Alaska.
The elevation where the Chitina joins the Copper River is
around 600 feet, and its all downhill from there to the Pacific Ocean.
The surrounding Chugach Mountains average 6,000 to 7,000 feet in
elevation.
The many earthquakes in the area are evidence that the
Chugach Mountains are continuing to be uplifted. However, the Copper
River was here first. The Copper River carries a lot of silt, and this
acts as a sandblasting agent to abrade a groove through the rising
mountain range. Thus, the Copper River will be able to maintain its
path even as the Chugach Ranges continue to rise higher in the future.
There is an interesting story regarding the origin of the
name “Copper River”. The word “copper” is of
course named for the chemical element copper. In August 1900 two
prospectors, Jack Smith and Clarence Warner, found a “large green
spot” on the south side of the Wrangell Mountains. (Upstream on
the Chitina River and off the left edge of the picture.) The
“large green spot” turned out to be one of the richest
outcrops of copper ore that has ever been discovered. Mining operations
were eventually financed by the Guggenheim family and J.P. Morgan. The
name of the corporation was a slight misspelling of a glacier that
terminates in the valley just below the mine. The glacier is the
“Kennicott Glacier” which was named after explorer Robert
Kennicott. The original mines along the headwaters of the Copper River
have long since shut down, but the Kennecott Utah Copper Corporation
still runs the world’s largest open pit mining operation. (The
original Kennecott Copper is now a subsidiary of Rio Tinto.)
Nenana River, Alaska
Range, Alaska
There are two river systems that cut from south to north
through Alaska’s Alaska Range and thus indicate a south to north
drainage system existed before the mountain range rose. We will first
take a look at the Nenana River.
The view above looks north where the Nenana River cuts
through the Alaska Range. The Nenana River enters the field of view
from just below the center of the right edge. It then flows westward
behind the “Reindeer Hills” (Remember this is Alaska).
The Jack River is a major tributary to the Nenana. It
enters from the lower edge (left of center) and flows around the west
(left) end of the Reindeer Hills. The two rivers join before they
continue northward through the Alaska Range.
As indicated by the many earthquake icons, the Alaska
Range is actively being uplifted today. Uplift of the range is mostly
in back of a fault that extends left to right across the center of the
picture. The Nenana Canyon is already 3,000 feet deep, but it is
destined to become deeper with time.
The small town of McKinley Village is in the valley behind
the first range of mountains. It is the jumping-off spot for tours of
Denali National Park. If you are taking a trip to the area, be sure to
allow enough time to take one of the guided tours. Also, if the weather
is good, take a “flight-seeing trip” that originates from
the McKinley Park “Airport”.
(Landing instructions from the FAA at
http://www.alaska.faa.gov/fai/images/TANVLY/INR-c.jpg
)
“Activate landing alert siren system for people and moose on
runway”
Bring your camera, camcorder, etc. for “whatever”.
Delta River, Alaska
Range, Alaska
Alaska’s Delta River does a copycat story of the Nenana
River
(see above), but it is located some 90 miles further east. The Delta
River enters from the lower edge (near the compass icon), and
heads north to cut through the Alaska Range. This end of the Alaska
Range is also
undergoing uplift with multiple earthquakes in the area although only
one shows on this more distant view.
The Delta River on the near side of the range is about
2,800 feet above sea level and drops to about 1,800 feet on the far
side. Mountains immediately adjacent to the canyon are over 7,000 feet
high while the high peaks near the edges are in the 8,000 to 12,000
foot
range.
This portion of the Alaska Range is the site of the famous
Black Rapids Glacier. While the Black Rapids Glacier is hidden in this
view, it originates near the left edge of the picture and flows to the
right in a valley that is between the highest visible peaks and the
lower range just in front of it.
The Black Rapids Glacier “galloped” to fame in
1937 when the following excerpt appeared in Time Magazine.
"Out of Central Alaska last week came
an exciting story. The Black
Rapids Glacier, long dying in its valley 125 miles south of Fairbanks,
had come to life. Its mile-and-a-quarter face was shoving toward the
Delta River and the Richardson Highway (sole motor road from Fairbanks
to the coast), rearing ice crests to 500 feet, breaking off great land
icebergs which tumbled thunderously ahead onto the mossy valley floor.
Geologist Ernest N. Patty at Fairbanks declared this week that if the
Black Rapids Glacier is moving as reported, it is traveling 220 feet
per day, a world record."
Please see http://www.gi.alaska.edu/ScienceForum/ASF13/1342.html
for
more information.
Glacial surges are a result of water build-up within the
glacier. If ice movement within a glacier obstructs normal drainage
water, then a pool of water will accumulate. The water will tend to
float the glacier as well as act as a lubricant between the
glacier’s ice mass and the underlying rock. Both actions reduce
friction between the glacier and the underlying rock. The reduced
friction can result in a temporary rapid flow of ice down the valley.
The 1937 surge in the Black Rapids Glacier stopped before
it blocked the highway. If you use Google Earth, you can see that most
of the 3-mile surge has completely melted away. However the glacial
advance did obliterate all the vegetation in its path. If you view the
area today, there is a visible “trimline” that marks the
greatest extent of the glacial surge. Areas that were not buried by the
ice still have their original vegetation and are a relatively dark
color. Areas that were run over by the advancing ice, are still mostly
a lighter color.
Actually, if you look closely at the north side of Black
Rapids Creek, you can see traces of older trimlines and terminal
moraines. The hillside on the east side of the Delta River has at least
one trimline indicating that at least one of these surges dammed the
Delta River and produced a lake several hundred feet deep. It would
have produced a “jokulhlaup” when the ice dam broke.
There’s a research project waiting here for somebody’s
thesis.
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