## 4 Paleolakes and Groundwater
### Hypothetical Paleolake between Kharga and Toshka  
While the GSS in North-Western Egypt has recently appeared, at first glance, improbably as a (paleo)lake or a river system or a combination of both, the \"lowland\" (depression, valley, long and relatively narrow bowl) south of Kharga appears to be a very logical candidate (Figure 17a). During wet periods, water would be gathered by gravitation. Archaeologists anticipate that there was a lake (see below). Figure 17: (**a–d**) _ETOPO 1_ topography (m) of the area mainly between Kharga (town) and Toshka (depression) (**a**), _Ag_ [mMGal] (**b**), _T\({}_{zz}\)_ [E] (**c**), and _xd_ [–] (**d**). The two lowermost red points on (**b**) indicate the location of Kiseiba and Nabta discussed in this study. A Middle Pleistocene (approx. 0.5 Ma) overflow of the Nile to the west through Wadi Toshka was proposed to account for the findings of lake remnants, fossil fish and paleochannel terminations at \(\sim\)250 m al to the south (Selima) and west (Bir Tafawi, Bir Sahara) of our research area [52]. Note that, now, we do not include water input from the Nile increased by the dam (Lake Nasser) via Toshka for the hypothetical (and recent) paleolake (and lakes). The Toshka endorheric lakes at \(\varphi\) = 23–23.5\({}^{\circ}\) (see Google Earth for partly independent check and for time series) with the height of around 150 m asl are new and artificial. Water started to flow here in the late 1990s through a concrete canal. Since 2006, the lakes have been shrinking rapidly. It is very probable that in the intervals of the “green Sahara”, the lowlands between Kharga and Toshka were filled, most likely repeatedly, with water. In Figure 17a-d, we show the _ETOPO 1_ topography, then \(\Delta g\), \(T_{zz}\) and \(vd\) for the area between Kharga and Toshka, where we seek another paleolake (TK). There is a topographic bowl; there are also local minima of \(\Delta g\) and \(T_{zz}\) and compression in \(vd\). There was evidently ample space for a lake/inner sea/river system some time ago when the Sahara was green; the TK lowland was filled, probably repeatedly, by water. How deep and large might a hypothetical paleolake be? We will repeat the task carried out for the GSS from [3] now for the TK. With the relationship between the free air gravity anomaly \(\Delta g\) and the relevant height difference \(\Delta h\) (e.g., [53]), we obtain:  
\[\Delta h_{[m]}=\Delta g/0.2667=3.75\ \Delta g_{[\rm{mGal}]},\]  
where we take the densities of dry sand \(\rho=1.4\)-\(1.7\) g/cm\({}^{3}\) and of the typical non-volcanic rock density \(\rho=2.6\)-\(2.7\) (g/cm\({}^{3}\)). We seek to identify the known prehistoric settlements (squares in Figure 18e,_f_) at a paleoshore. We read the difference \(\Delta g\) between these dots and the largest negative anomaly in the assumed bowl in TK. We obtain \(\Delta g=30\)-\(50\) mGal, which corresponds to \(\Delta h\approx 110\)-\(190\) m. This would be the maximum possible depth (the basin fully filled by water). In reality, we may expect depths in the order of tens of meters. Then, we add \(20\) m for the height above sea level of the deepest parts of the bowl and speculative \(20\) m for the sand layers. Figure 18: _Cont._  
Figure 18: **(a–d)** The _ETOPO 1_ with selected contour lines for the heights 150, 200 and 250 m asl (**a–c**). We recall that the estimated external accuracy in the heights in the _ETOPO 1_ model is \(\pm\)20 m. The last figure (**d**) shows only the selected contour lines at 200, 170, 150 and 120 m asl (red for 200 m, pink for 120 m). (**e**,**f**) The position and extent of the huge hypothetical paleolake(s) between Kharga and Toshka, Southern Egypt (TK), versus _ETOPO 1_ topography and \(\Delta g\) (EIGEN 6C4). The squares show archaeological sites from Table 1. The blue dots on (**e**) are for the main modern Toshka lakes (or their present remnants).