Durango Bill’s
Ancestral Rivers of the World
Ancestral Rivers in
Africa
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. The examples here (including pictures)
are from Africa, but other examples can be found throughout the
world.
Featured areas
Arroyos (“intermittent rivers”), Algeria
Blue Nile (Abay River), Ethiopia
Congo River, Democratic Republic of the Congo and the
Republic of the Congo
Lufira River, Upemba Natl. Park, Democratic Republic of
the Congo
Lukuga River, Lake Tanganyika, Democratic Republic of the
Congo
Luvua River, Lake Mweru, Democratic Republic of the Congo
& Zambia
Zambezi River, Kariba Dam/Reservoir, Zambia & Zimbabwe
Arroyos
(“intermittent rivers”), Algeria
We think of a river as something that has
water all the time. In some places in the world, a “river”
only has water on the rare occasions when it rains. The rest of the
time the “river” is a dry arroyo.
The picture above is a vertical view of typical arroyos in
the Sahara Desert, some 30 miles west-northwest of Laghouat, Algeria.
The arroyos are cutting across folded sedimentary layers. The harder
sedimentary layers are more resistant to erosion, and where they are
tilted upward, they form hogback ridges. When it does rain, the muddy
run-off water is quite capable of ordinary river erosion and has cut
short canyons through these ridges. The deepest of these canyons are
found in the dark ridge in the center of the picture, and are up to 800
feet deep.
The drainage pattern for the arroyos was probably
established some 10 million years ago across a beveled surface and
before more recent uplifts. (This is a “very fuzzy”
estimate.) More recently, the general area has been uplifted and
erosion has set in again. When it does rain, softer surface material is
washed away, and the old more resistant ridges are left intact.
The sedimentary layers themselves are of interest. 100 to
200 million years ago, this area was the bottom of a shallow sea. Silt,
mud, algae, the remains of other organic material, etc. settled out on
the bottom of this ancestral sea. These accumulated into multiple
layers totaling several thousands of feet thick. These sediment
accumulation areas extended well to the southeast of the area in the
picture. With time, these sediments were buried deep enough to cook the
organic remains into oil.
Beginning about 50 million years ago (again “very
fuzzy”) the area in the picture was crumpled into anticlines and
synclines. Erosion beveled these earlier mountains to an essentially
flat surface. The current arroyo drainage systems were established
across this beveled surface. The drainage patterns are thus an example
of superimposition.
Finally, renewed recent uplift has allowed erosion to set
in again. The entire surface has been eroded down from the earlier flat
surface. The greatest surface erosion has taken place in the
“softer” layers while the more resistant layers have been
exposed as “hogback” ridges. The arroyos were able to erode
down through these more resistant ridges, and thus they have been able
to maintain their paths.
Blue Nile River (Abay
River), Ethiopia
The picture above shows the origin of the Blue Nile (Abay)
River in Ethiopia. The river originates in Lake Tana (Tana Hayk) which
can be seen in the upper right corner of the picture. The elevation of
the lake is about 5,865 above sea level.
After the river leaves the lake it flows southeastward to
about the center of the picture and then turns more to the south where
it cuts through the Choke Mountains. It leaves the field of view just
below the center of the left edge.
The mountains on both side of the gorge are more than
11,000 feet high. The river leaves the lake at about 5,865 feet, and is
down to below 4,000 feet in-between these high peaks.
This part of Ethiopia is on the northwest side of the
African rift zone. The rifts themselves are off the lower edge of the
picture, but uplift just to the northwest of the rift is producing the
mountains in the foreground portion of the picture.
Uplift is much greater in the foreground and much less in
the distant portion of the picture. The result is a
“tipping” of the terrain.
The Blue Nile River was in place before this
uplift/tipping began. The first effect of the uplift was to dam the
river. The initial Lake Tana was closer to the foreground. As the
mountains rose the Blue Nile immediately began to erode into them as a
natural process to try to maintain its path. The erosion process
started to cut the gorge. Also the “tipping” process is
causing the lake to migrate toward the northwest. (Toward the distance
in this view.)
While you can’t see it in this view, a closer Google
Earth view of the southeast side (near side) of the lake shows
ancestral beach terraces extending up to 8 miles inland from the
current lake shore. As the lake slowly tips down toward the northwest,
these ancestral beaches are left behind.
Lake Tana is destined to disappear in a few million
years. If erosion by the Blue Nile is strong enough to offset the
tipping, the gorge will eventually cut back upstream into the lake and
thus drain it. If this “headstream erosion” doesn’t
destroy the lake, the tipping will.
Tipping is forcing the lake to migrate northwestward.
However, this motion is driving the northwest shore of the lake ever
closer to the rim of the plateau, and there is a steep downhill
gradient on the far side of this rim. The lake level is within 300
vertical feet of this rim on the lake’s west side, and if/when
the rim is breached, the river can easily rip out a new path to the
west. When that happens, the current Blue Nile River will be history,
and there will only be local drainage left in the canyon that we see in
the picture.
Congo River,
Democratic Republic of the Congo and the Republic of the
Congo
In terms of water volume, the Congo River is the second
biggest river in the world. (First place goes to the Amazon River). In
this picture it forms the border between the Democratic Republic of the
Congo (right side of the river) and the Republic of the Congo (left
side of the river). The respective capitals of Kinshasa and Brazzaville
are about 35 linear miles downstream off the lower edge of the picture.
In the picture we can see that the Congo has cut a narrow
gorge down through a very flat plateau. Elevations of the top of the
plateau range from 2,150 to 2,250 feet above sea level while the river
is about 850 feet above sea level.
If we measure the narrowest rim-to-rim width (elevations
greater than 2,150 feet above sea level), the gorge is only 3 miles
wide. What is interesting is the huge upstream area that is lower than
the 2,150-foot rim. If you built a rim-to-rim dam across the gorge, you
would flood many hundreds of thousands of square miles upstream. The
lake would back up over 1,000 miles. It would cover an area several
times larger than the combined 5 Great Lakes. There are several possible alternate exit
elevations below 2,000 feet including an approximate 1,700-foot
possible exit
that would overflow to the north to inundate Lake Chad on the southern
edge of
the Sahara
Desert.
The Congo River was here long before the plateau was
uplifted. The elevation of the river has been below 1,000 feet above
sea level for many tens of millions of years. The Congo Gorge is
relatively narrow which indicates it has been cut recently. The plateau
has been uplifted over the last ten million years. As the plateau rose,
the Congo River was quite capable of eroding away material that kept
trying to get in the way.
Lufira River, Upemba
Natl. Park, Democratic Republic of the Congo
The picture above looks northwestward across a plateau in
Upemba National Park in the southern portion of the Democratic Republic
of the Congo. The largest of the distant lakes is Lake Upemba.
The Lufira River enters the picture from the lower left
edge and cuts across the middle of a long northeast to southwest
plateau that extends 75 miles to the right and 100 miles to the left of
the deep gorge. The top of the plateau is uniformly between 5,000 and
6,000 feet above sea level while the river in the bottom of the gorge
is about 2,500 feet. Thus the short, sharp canyon is over half a mile
deep.
Uplift history of the area is not known. The area is not
too far west of the African Rift Zone, and recent uplift may be related
to the rift. However the steep sides of the canyon indicate that it has
been cut from scratch in the last 10 million years. In any case the
implication is that the river was in place first, and as the area was
uplifted, the river eroded away a rising mountain mass that kept trying
to get in the way.
There are two adjacent indentations in the plateau that
are of interest. One is a short distance to the left of the gorge while
the second one is further to the right. Both of these indentations in
the plateau look like they were formerly tributary systems to the
Lufira River. As the plateau rose neither one of these tributaries had
the erosion power of the Lufira. Both of these former tributary systems
cut down a short distance into the rising plateau, and then rerouted to
join the Lufira off the lower left edge of the picture before the
Lufira cuts across the plateau. Basically it was easier to let
“The Big Guy” do the erosion work.
Lukuga River, Lake
Tanganyika, Democratic Republic of the Congo
The picture above looks south over the western edge of
Lake Tanganyika and the Lukuga River in the southeastern part of the
Democratic Republic of the Congo. The Lukuga River is the outlet for
Lake Tanganyika (left edge), and the river flows across the center of
the field of view to exit off the middle of the right edge. In the
process, the river cuts through a broad area of mountains and plateaus
(in the center of the picture) that are 1,500 to 2,000 feet higher than
Lake Tanganyika.
Lake Tanganyika is “a work in progress”.
Convection currents in the earth’s mantle are beginning to raft a
large portion of eastern Africa away from the rest of the continent. A
“rift” is developing between the two sections. Bedrock
directly in the rift is being faulted downward in an attempt to fill in
the rift. The technical name for this type of structure is a
“graben”. Run-off water from ordinary rain plus
accompanying silt drain into this widening rift, but the rift is
continuing to get wider and deeper. Lake Tanganyika is nearly 5,000
feet deep. It is the second deepest and second most voluminous lake in
the world. Lake Baikal in Russia, another rift lake, is first in both
categories. If the rifting process continues for a few more tens of
millions of years, eastern Africa will move away from the rest of
Africa, and a new ocean will fill the gap.
The earth’s crust is floating on top of the
mantle. For any given area, the weight of the crust pushing down is
exactly offset by the upward pressure in the mantle that is holding the
crust up. If you decrease the weight over a given area by any process,
then the two forces will become unbalanced, and the mantle will lift
the whole region back up again until the forces are once again in
equilibrium.
The rifting that has opened up the earth’s crust has
reduced the total net weight of the crust in the vicinity of Lake
Tanganyika. When the forces in the earth’s mantle lift the crust
to reestablish equilibrium, the areas adjacent to Lake Tanganyika are
included. This lifting produces new mountain ranges adjacent to the
lake including the area seen in the picture.
The ancestral Lukuga River was in place before the
mountains near the lake began to rise. As the mountains were uplifted,
ordinary erosion by the river removed material that kept trying to get
in the way. Today we see the result of this erosion. The Lukuga River
has cut a gorge through the rising mountain range.
Luvua River, Lake
Mweru, Democratic Republic of the Congo & Zambia
The view above extends from the north end of Lake Mweru
northward over the southeastern corner of the Democratic Republic of
the Congo. A small piece of Zambia is included in the lower right
corner. The Luvua River is the outlet for the lake and immediately cuts
into 4,600+ foot high mountains on the left side of the picture. There
are multiple other exit possibilities for the lake that stay under
3,600 feet. The lake itself is just over 3,000 feet.
Lake Mweru and the Luvua River are part of the African
rift system. Lake Tanganyika (see above) is only about 105 miles to the
northeast.
Uplift history of the 4,600-foot mountains on the left
side of the picture is not known. A speculative analysis implies the
mountains are bounded by one fault that forms the left lakeshore and
two nearly parallel faults on the far side of the mountains. It appears
the uplift has taken place in the last 20 million years.
In any case, this looks like another example of
antecedence. The river was in place first, and as the mountains rose,
the river was able to erode away rising material that kept trying to
get in
the way.
Zambezi River, Kariba
Dam/Reservoir, Zambia & Zimbabwe
The picture below overlooks the Kariba Dam/Reservoir on
the Zambezi River. The Zambezi River forms the border between Zambia on
the left and Zimbabwe on the right. The gorge through the mountains was
an ideal place to build one of the largest hydroelectric projects in
Africa. The downside was that many tens of thousands of people were
displaced by the resulting reservoir, and left to fend for themselves
in what has been described as “the worst dam-resettlement
disaster in African history.”
The picture above shows where the Zambezi River has cut a
1,500-foot deep gorge through the uplifted mountains/plateau in the
center portion of the picture. There is a much easier, 900-foot
lower alternate path (near the left edge) that logically, the river
should have taken. Why does the river flow through today’s high
elevations when a low elevation path is available?
The Zambezi established is path when the lowest available
path was its current path. The mountains/plateau had not been uplifted
yet. If you look at the picture, you can see a sharp boundary between
the highlands in the top center portion of the picture and the lowlands
in the upper left corner. The boundary is an obvious fault system.
Over the last 10 to 20 million years, the land to the
right of the fault has been lifted upward. If the Zambezi had not been
able to erode down through the rising land mass, it would have been
dammed and would have
been forced to take the left hand route. However, the erosion
ability of the Zambezi was equal to the task. As the land mass rose,
the Zambezi abraded away material that kept trying to get in the
way.
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