Sheet nº 240
May 2006
Remote sensing reveals Megalake Chad features
Drought has been affecting the Sahel for
more than 30 years and during this time
the media have frequently been announcing the imminent disappearance of
Lake Chad (1). The lake has indeed
shrunk dramatically, regressing from its
status as a Great Lake to that of a Small
Lake in only very few years. Yet such
rapid fluctuations have already occurred
over the past centuries. They are linked
to the strong climate variability that prevails in tropical Africa. Further back in
time, on the scale of several thousands
of years, fluctuations of much greater
amplitude have taken place. Remotesensing data used by a team including
IRD scientists confirmed the existence of
a gigantic lake, Megalake Chad, in the
Holocene (more than 6000 years BP) and
helped define its main features.
he Lake Chad Basin is the world’s most
extensive endorheic basin, in other words
a region with internal drainage that does
not flow into the ocean but remains and dissipates inland. With a surface area of 2.5 million
km2, its hydrological regime is highly sensitive to
climate modifications. Semi-arid conditions,
which currently prevail, are causing Lake Chad
to diminish by evaporation and infiltration, so
much so that it now occupies less than 1% of the
basin’s surface area. The existence of a vast
Megalake Chad, further back in the past, has
been the subject of intense debate throughout
the XXth century. Multiple data from satellites
(Landsat, Modis) or other remote-sensing
systems (SRTM (2)), has now enabled IRD
researchers, working with Monash University
(Australia (3)) , to establish definitively the existence of the megalake in the Middle Holocene
and to identify its characteristics.
T
Shoestring sand formation marking the palaeoshore of
Megalake Chad. In the foreground, fertile clay soils from the
floor of the Megalake (photograph M. Leblanc, August 2004,
IRD field mission to Niger, 12°20E, 13°37'N). The waters of
Lake Chad are currently situated 100 km to the east.
©IRD/ Marc Leblanc
The palaeoshores of the megalake, represented by an almost uninterrupted shoestring sand
formation more than 2 300 km long, have been
identified. They delimit a surface area of more
than 340 000 km2 in the centre of the basin. By
comparison, the area of the world’s largest
currently existing lake, the Caspian Sea, is only
8% greater.
The maximum depth of the megalake, as determined by reconstruction, reached 160 metres, as
opposed to the current figure of less than 10 m,
for a volume of 13 500 km3. This makes it the 4th
largest lake in the world after the Caspian Sea
and Lakes Baikal and Tanganyka.
The input of water into such a megalake
would imply palaeoclimatic and palaeohydrological conditions strikingly different from the current
period, with in particular much more intense
monsoon rainfall. The present-day lake is fed by
../...
An exhaustive re-examination of the body
of 14C datings obtained from the Lake Chad
Basin over a period of more than 30 years,
notably by the IRD, made it possible to
assign the latest series of episodes of the
megalake to the Middle Holocene, between
8500 and 6300 years cal. BP (4) .
In real terms, several episodes could well
have followed one another during this
period, in quite rapid succession, as such a
lake was necessarily highly sensitive to the
slightest change of climatic parameters like
evaporation and rainfall, as it is at present,.
Megalake Chad thus provides the most
spectacular example of the consequences
of climate changes in tropical Africa. In the
SRTM data on the Lake Chad Basin. The Holocene palaeoshore (black outline)
current context of global warming, analysis
corresponds to an altitude of 324 ± 5 m, a level similar to that of the threshold
communicating with the Niger River Basin.
of such a significant palaeohydrological
©IRD/ Marc Leblanc
model is therefore particularly important for
understanding the mechanisms at work and
only two large rivers, the Logone and the Chari, who for finding out the feedback effects that could develop.
find their sources in the most humid regions of the Predictive climate simulation models aimed at several
southern part of the basin. The palaeolake was sustai- centuries in advance currently yield conflicting results
ned by a multitude of small rivers and deltas that func- concerning the future conditions of the basin’s hydrolotioned round the whole of its perimeter. The relicts of gical regime. They need further refining if they are to
these water-courses were detected even in the indicate methods for sustainable management of the
Saharan sector to the north, which is at present subjec- water resource, an objective that is crucial for the deveted to the harshest desert conditions. The data collec- lopment of the countries bordering the shores of Lake
ted indicate also that the basin was not permanently Chad.
endorheic over the whole Holocene. At its maximum
level, Megalake Chad reached stillstand conditions, ______________________
stabilized by a natural hydraulic threshold. Any overflow (1) Bordered by Chad, Niger, Cameroon and Nigeria
Shuttle Radar Topographic Mission, carried out by the space shuttle
found spillover points, then flowed towards the Bénoué, (2) Endeavour
(Nasa, USA) in February 2000
a left-bank tributary of the River Niger, and would in this (3) As part of a French research programme “ECCO/PNRH (Ecosphère
way have found an outlet to the Atlantic.
Continentale / Hydrologie ; http://www.insu.cnrs.fr)” financed over the
Redaction – IRD : Guillaume Favreau
Translation : Nicholas Flay
period 2003-2005 and coordinated by the IRD (research unit UMR
HydroSciences, Montpellier).
(4) Age obtained by the 14C dating method. BP = Before Present:
counted before the reference year 1950. Cal.= Calibrated to correct
the bias of the 14C method.
For futher information
CONTACTS:
Guillaume FAVREAU, IRD - UMR HydroSciences,BP 434, 1004 El Menzah 4, Tunis, Tunisie.
Tel : (216) 71 750 009 ; Email:: Guillaume.Favreau@ird.fr ; http://www.hydrosciences.org
Christian LEDUC, IRD - UMR G-EAU, BP 434 - 1004 El Menzah 4, Tunis, Tunisie.
Tel (216) 71.750.009 ; Email: : Christian.Leduc@ird.fr ; http://www.mpl.ird.fr/hydrologie/divha
Marc LEBLANC, Monash University, School of Geosciences, Clayton, Victoria 3800, Australie.
Tel : +61 (0)399051782 ; Email:: marc.leblanc@sci.monash.edu.au ; http://www.earth.monash.edu.au/
IRD Communication :
Aude Sonneville (editor), Tel.: +33 (0)1 48 03 76 07, Email: fichesactu@paris.ird.fr ;
Sophie Nunziati (press officer), Tel.: +33 (0)1 48 03 75 19, Email: presse@paris.ird.fr
REFERENCES
LEBLANC, FAVREAU ET AL., (2006A) – Reconstruction of Megalake Chad using Shuttle Radar Topographic Mission data, Palaeogeography,
Palaeoclimatology, Palaeoecology, (in press).
LEBLANC, LEDUC ET AL., (2006B) – Evidence for Megalake Chad, north-central Africa, during the late Quaternary from satellite data, Palaeogeography,
Palaeoclimatology, Palaeoecology 230, 230-242.
ILLUSTRATIONS
Contact Indigo Base, IRD picture library, Claire Lissalde or Danièle Cavanna, Tel.: +33 (0)1 48 03 78 99, Email : indigo@paris.ird.fr
The illustrations can be viewed on: www.ird.fr/us/actualites/fiches/2006/fiche240.htm