Ancestral Rivers of the World
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.
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
Luvua River, Lake Mweru, Democratic Republic of the
Congo & Zambia
Meiringspoort Gorge, Groot River, South Africa
Nile River, Sixth Cataract, Sabaloka Gorge, Sudan
Zambezi River, Kariba Dam/Reservoir, Zambia &
(“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
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
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 feet 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.
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.
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
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
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
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
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.
Gorge, Groot River, South Africa
The view above looks west across South Africa’s Swartberg
Mountains. There are several east to west mountain ranges in South
Africa that are “in the way” of river systems that flow from north
to south. The Groot River is just one of these, but nevertheless,
it cuts through the mountain range instead of going someplace
else. The Groot flows from north to south (right to left in the
picture) to produce the spectacular 2,000+ foot deep Meiringspoort
Gorge in the foreground.
The Gamka River also cuts through the Swartberg Mountains
some 50+ miles further to the west. (Near the “hazy” area in the
far distance.) A hydroelectric dam (the Gamkapoort Dam) has been
built at the entrance to its gorge.
The rock layers in the mountain range are mostly
folded Table Mountain Sandstone, but there are other sedimentary
layers mixed in. The Cango Caves are in Precambrian limestone
layers not too far from the gorge.
There have been several periods of uplift and folding
of the Swartberg Mountains over the last 250 million years. The
Groot (and other) River systems had established their north to
south paths before the most recent uplift/folding period. As the
mountains were uplifted, the Groot had enough erosive ability to
cut down through the rising mountain range as rapidly as the
mountain range was rising. Thus the Groot was ability to maintain
its former path. The result of this cutting action is the
The name “Meiringspoort”, (literally - Meiring’s
gate) originally was derived from a Petrus Johannes Meiring who
settled on the south side of the Swartberg Mountains nearly 200
years ago. Out of curiosity he followed the river upstream to see
where it came from. In the process, he discovered the gorge.
Nile River, Sixth
Cataract, Sabaloka Gorge, Sudan
The picture above looks northeastward where the Nile
River cuts through the Sabaloka Mountains to form Sabaloka Gorge.
Sabaloka Gorge is some 50 miles north of Khartoum, Sudan, in the
southeastern corner of the Sahara Desert. The mountains are a dark
complex of hard, old, mostly volcanic rock that contrasts with the
lighter color sands of the Sahara Desert.
The Sabaloka Mountains form a small plateau some 5 to
6 miles wide and about 8 miles long. The central plateau of the
mountains averages about 1,700 feet above sea level. The Nile
River here is about 1,200 feet above sea level. Thus the gorge is
about 500 feet deep.
There are easier paths for the Nile around both sides
of the mountains where the terrain stays under 1,350 feet. Why did
the Nile pick a path through the Sabaloka Mountains?
Again we have a textbook example of antecedence. The
Nile was here before the mountains existed. The mountains were
subsequently formed when “recent” (probably within the last 20
million years) intrusions of magma (molten igneous rock) pushed
upward and then sideways through subterranean layers of bedrock.
The surface layers were uplifted by the magma intrusions. (The
process is similar to how a blood blister is formed when you break
a blood vessel under your skin.) The “buzzword” name for this
particular geologic feature is the Sabaloka Pluton.
The Nile River was able to erode downward as the
mountains were uplifted. Thus it maintained its original course. A
better description might be that the Nile River stayed put while
the block of rock that became the mountains was uplifted. All the
Nile had to do was to erode away the rock layers that were being
uplifted into its path.
The hard rock of the Sabaloka Mountains has been
quarried as a source for stone tools since earliest Egyptian
history. As such it has become an area of interest for
archeologists in recent years. For example, please see: http://www.academia.edu/2404735/The_Landscape_and_Archaeology_of_Jebel_Sabaloka_and_the_Sixth_Nile_Cataract_Sudan
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
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.
main Ancestral Rivers Page
Web page generated via Sea Monkey's Composer
within a Linux Cinnamon Mint 18 operating system.