Ancestral Rivers of the World
of Mexico, Western Canada, and Alaska
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.
Bustamante Canyon (Canon
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
(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
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
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
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
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
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.
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.
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
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
Fish populations in the upper Alsek River are a
mixture of what is normally found in both Pacific drainage rivers
and the Yukon River system. Today, the Alsek River drains to the
Pacific Ocean, and if this drainage had been continuous in the
past, then the fish population would be similar to other Pacific
drainage rivers. The Yukon drainage fish component shows that in
the past, glaciers have blocked the lower Alsek River; and the
river was forced to find an alternate escape route - which
connected to the Yukon River. The current threat of the Tweedsmuir
Glacier to block the river (see below) is a minor example of a
much larger blockage at some point in the past.
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.
Sept. 2, 2008 Update
The Tweedsmuir Glacier has continued to surge and has
nearly blocked the Alsek River. The National Park Service has a
photo and update information at: http://www.nps.gov/glba/parknews/tweedsmuir.htm
June 2009 Update
It appears that the Tweedsmuir Glacier has stopped
surging before it blocked the Alsek River. The icefront is up
against the river, but it now looks like it will not block it.
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.)
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
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
Bring your camera, camcorder, etc. for “whatever”.
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
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
main Ancestral Rivers Page
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