The Lake Chad catchment has an area 2,500,000 km2. In the past it contained a giant lake known as Lake Megachad. The last long-lived Lake Megachad high stand occurred during the early and middle Holocene, with a shoreline at an altitude of about 329m and a lake area of 361,000 km2 (Figure 2.1). In its heyday it was fed by a number of large river systems that rose in the Ahagar, Tassili, Air, Tibesti and Edeni mountains of what is now the Sahara Desert (Figure 2). The river in the southern part of the catchment, the Komadugu, Chari and Longwe, still contains significant flow today and feeds the (much smaller) modern day Lake Chad.
Figure 2.1. The rivers, lake area and catchment area of Lake Megachad overlaid on the DEM of the Chad basin. Lakes are marked in blue, rivers in black and marshlands in green. When rivers flow into sand seas they often disappear in the dunes fields, and thus appear to terminate abruptly. This is probably because the channels are obscured by moving sand. Intermittent channel reaches can often be detected amongst the dunes in both the DEM and Landsat TM imagery suggesting they were once much more extensive (image by Nick Drake)
Lake shoreline landforms were discovered in the Chad Basin early last century when the existence of giant palaeolake Megachad was first postulated and described as an African Caspian Sea (Tilho1925). Initial estimates of the lake area by Tilho (1925) were 200,000 km2 although this was later revised 320,000 km2 (Schneider 1967) with lake levels fluctuating due to climatic change during the Holocene (Maley 1977; Servant and Servant-Vildary 1980; Gasse 2000).
However, soon after the last of these studies, the existence of a Holocene Lake Megachad was questioned and it was suggested that the lake shoreline beach ridges were in fact fault scarps (Durand 1982). Further investigation of the archaeology and sedimentology of the ridges (Thiemeyer 1992) led to the resurrection of the megalake theory, and these results were supported by interpretation of the topography of the Chad Basin using the TOPO 6 and GLOBE digital elevation models (DEMs) that allowed recognition of a very extensive wave cut shelf interpreted as a lake shoreline feature(Ghienne, et al. 2002 Schuster et al., 2003).
Most recently, Drake and Bristow (2006) and Schuster et al. (2005) interpret higher resolution (900 m) SRTM-30 DEM data and Landsat TM imagery to provide the first detailed overview of the sedimentary systems around palaeolake Megachad. They describe the palaeolake shoreline geomorphology at Goz Kerki, Kanem, Angamma and Chari palaeodeltas.
Drake and Bristow (2006) show that Landsat TM false colour composites, the SRTM DEM, show clear evidence of coastal landforms of numerous types including cuspate forelands, tombolos, sand spits, deltas, beach terraces and ridges. The profusion and complexity of coastal landforms is illustrated well by the Angamma palaeodelta (Northern Chad, Fig 2.1) which shows many lake shoreline features (Figure 2.2) (Schuster et al, 2005; Ergenzinger 1978).
Figure 2.2. Landsat TM false colour composite of the Angamma Delta. The sites discussed in the text are marked (image by Nick Drake)
The Angamma Delta has a cuspate morphology typical of wave modified deltas. It has prograded beyond a beach ridge with shoreline elevations of 345m and in doing so has also incorporated cuspate forelands on the eastern side of the delta at 343m (Figure 2.2, site 1) and 340 (Figure 2.2, site 2) and 335m (Figure 2.2, site 3). The orientation of the preserved beach ridges and the morphology of these cuspate forelands indicates wave approach from the southwest and from the northeast indicating palaeowinds blowing from both directions (Drake and Bristow in 2006).
The prominent palaeoshoreline that defines the southern edge of the delta is clearly defined by a beach ridge at 333m (Figure 2.2, site 4) sitting atop the eroded delta front (Figure 2.3).
Figure 2.3. Exposure of delta front sediment at the Angamma palaeodelta, the delta prograded from left to right into the basin from the north. The outcrop which is 20m high is part of a coarsening upwards succession produced by delta progradation. This section has been logged and samples collected for luminescence dating (Photograph by Dr Charlie Bristow)
A sample (Figure 2.4; Sample CH 44) collected for luminescence dating from this shoreline give an age of 5.9 ± 0.5 ka, in close agreement with other estimates of a lake highstand around 6,000 years BP (Thiemeyer1992). This shoreline extends along the whole delta front and there is no evidence of fluvial incision during the later lake regression and lowstand. The lack of incision indicates that fluvial discharge has ceased before the lake level fell suggesting that the Lake Chad catchment dried from north to south.
|Figure 2.4. Sample locality CH 44 beach ridge sediments including laminated beach sands with darker heavy minerals capped by well rounded pebbles are part of the prominent shoreline at 333m that defines the crest of the Angamma palaeodelta. Luminescence dating of sands from this locality give an age of 5.9 ± 0.5 ka indicating a lake highstand around 6,000 years BP which is consistent with earlier estimates of this palaeolake level. Note that the sample was collected at night to avoid exposure to sunlight and that the photograph was also taken at night after sampling (Photograph by Dr Charlie Bristow). |
Below the delta front on its eastern side the Landsat image reveals additional shorelines at 323, 305 and 286m (Figure 2.2, site 5, 6 and 7). They appear to have been formed by waves reworking delta sediments as the lake level fell from the 333m shoreline because they are uncut by fluvial activity from the delta that would be expected were they from older still stands.
Figure 2.5. Goz Kerki sand spit and beach ridges. A) SRTM-3 DEM of Goz Kerki sand spits. Three separate shorelines are evident, the higher shore at 329m, the intermediate shoreline at 320m and the lower at 304m. B) LANDSAT TM band 4 image of a single sand spit and associated beach ridges. C) Interpretation of Landsat image (image by Nick Drake and Charlie Bristow)
The morphology of the shorelines in the region of Goz Kerki (Figure 2.5), like the Angamma Delta (Figure 2.2), indicates two dominant winds. One wind is north-easterly and is consistent with the present day winds in the region. The other originated from the southwest (Figure 2.5 C). We attribute it to an enhanced monsoon caused by a precessionally driven increase in northern hemisphere insolation. Samples CH 73 and CH 74 collected from beach ridges at Goz Kerki for luminescence dating have ages of 6.7 ± 0.7 and 11.4 ± 0.7 ka respectively. The younger age is consistent with a lake highstand between 6,000 and 7,000 BP while the older age appears to be a previously unreported Younger Dryas shoreline.
Multiple shorelines are found at both Goz Kerki and the Angamma Delta. This is also the case in other locations. In total 41 shoreline sites have been located as shown in the Figure 2.6 (Drake and Bristow 2006).
|Figure 2.6 Location of the palaeolake shoreline complexes identified in this research. Details of these sites are shown in Table 1 (Image by Nick Drake). |
Table 1. Shoreline elevations extracted from the DEM, the locations are indicated by the numbers. The lake area values and associated errors have been rounded to the nearest 1,000 km2. Errors were calculated by rounding the standard deviation to the nearest meter, adding and subtracting this value from the lake area, determining the lake area, differencing it from the mean and averaging the two differences
These shorelines suggest that its peak the palaeolake Megachad had an area of at least four hundred thousand square kilometres, bigger than the Caspian Sea, the biggest lake on Earth today (Figure 2.7). Subsequent desiccation of the palaeolake is recorded by numerous regressive shorelines. Of these, there is a prominent shoreline at around 329m (Figure 2.7) where the geomorphology is preserved around the vast majority of the palaeolake that has been radiocarbon dated to 6,340±250 14C years BP (Thiemeyer1992) that calibrates with a two sigma range to 7,500-6,940 BP.
Figure 2.5. Selected Lake Megachad Shorelines overlaid on the 90m SRTM DEM (Image by Nick Drake)
The majority of the beach ridges below 329m are interpreted as being younger, having formed in a falling stage, as they appear to be unaffected by fluvial activity (eg the shorelines below the Angamma Delta). Schuster et al, (2005) has radiocarbon dated shells from a regressive shorelines at Goz Kerki to somewhere between 4,410 and 5,280 Cal BP, suggesting that the lake had started to contract by this time.
Other regressive shorelines can be dated using our knowledge of the archaeology found on the plains that were exposed by the final demise of Lake Megachad. The oldest archaeological sites found in the vicinity of Lake Chad are thought to have been first occupied soon after the desiccation of Lake Megachad and date to about 4,000 BP.
These settlement sites are found at an altitude of 298m along the margin of a once larger Lake Chad (Breunig et al, 1996) suggesting the retreat of Lake Megachad from the 305m shoreline before 4,000 BP (Figure 2.7). As the lake level continued to fall it became divided into three separate water bodies with Lake Chad and Lake Fitri in the southern basin and another lake in the Bodele depression to the north (Figure 2.7). Lake Chad initially had a shoreline altitude of 289±1.4 m and an area of 22,000 km2 controlled by a sill with overflow to the north along the Bahr el Ghazal, which fed into the Bodele Depression. The lake in the Bodele Depression was larger, with a shoreline at 285m producing an area estimate of 91,00 km2.