WAS THERE A QUATERNARY MEGALAKE IN THE BASIN OF THE WHITE NILE IN SUDAN AND SOUTH SUDAN? NEW EVIDENCE PRO AND CON

In South Sudan, the White Nile stagnates in the Sudd swamp (elev. ca. 400 m). It has long been suggested that the Sudd is the remnant of a very large Quaternary lake, here called “Megalake Sudd” (MLS). A problem with the Megalake-Sudd hypothesis (MLSH) is that there is no obvious site around the lake’s northern perimeter, north of Khartoum, where a natural dam could have impounded such a lake; the Sudan Basin opens widely northward. This problem is so serious that the MLSH seems almost forgotten.

The present study cites evidence, some recently published by a Sudanese group and some apparently new, that supports the MLSH and offers a possible explanation for the missing dam site. But it also reports evidence that questions the MLSH. The Sudan Basin is presently occupied by the Quaternary (?) Umm Ruwaba fm. El Shafie et al. (2011, J. Afr. Earth Sci.) report lacustrine diatoms up to a probable shoreline at 515 m, providing new evidence to support an enormous MLS. The present abstract reports discovery, using SRTM elevation data, of an inactive spillway in the Central African Republic, leading from the Sudd Basin to the Chad Basin. This “Oulou Spillway” is flat bottomed, about 200 km long, slopes gently westward from a divide at 514 m elevation (as predicted by El Shafie et al.!) and leads to the Chari River, the main source feeding Lake Chad – a tiny remnant of former Megalake Chad. This suggests that the spillway controlled the height of an enormous MLS, and that Megalake Chad was fed primarily by the White Nile.

It is here proposed that a natural dam once existed but was completely eroded away by ca. 35,000 km³ of escaping lake water. This hypothesis needs to be taken seriously and tested because it appears to explain (1) the no-river “Desert Phase” of Egyptian-Nile history, during filling of Megalakes Sudd and Chad, (2) transport of coarse gravels > 600 km north through Egypt on a gradient of < 0.01 degrees, which can be understood as a “Protonile Megaflood,” (3) the absence of a dam site, (4) the presence of the Umm Ruwaba fm., at least in part lacustrine, over much of Sudan and South Sudan, (5) the Oulou Spillway, and (6) how Megalake Chad could have been filled from an otherwise small drainage basin. But the matter is not settled, because a search, using SRTM elevation data, did not find the MLS shoreline features that the MLSH would predict. Further work is needed!