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River network, geomorphologic, paleohydrologic, stratigraphic and sedimentologic analyses document a dramatic reorganization of the drainage pattern in the northern part of the Main Ethiopian Rift (MER) during latest Pleistocene and early Holocene. The river network modification was induced by tectonic deformation, volcanic activity, and by the arid conditions connected with the Last Glacial Maximum (LGM). This arid phase triggered the shrinking of a Pleistocene Megalake that formerly flooded large part of the Main Ethiopian Rift. The northern tributaries (paleo-Awash and paleo-Mojo rivers) extended, following the lake shore retreat, and incised the Fesesa, Koye, and Cheleleka-Sulula Hafa paleovalleys through the Pleistocene deposits. At the beginning of the Holocene, humid conditions induced a water-level rise in the lacustrine basin (Ziway-Shala basin), supplied from the north by the large Awash-Mojo-Meki fluvial system. A well exposed cross-section of the Cheleleka paleovalley at the confluence with the Meki River and the use of paleohydrological methods allowed to infer the bankfull paleo-discharge of the larger Awash-Mojo river system. Tectonic events allowed the Awash and Mojo rivers to divert their courses to the east toward the Afar depression, depriving the Ziway-Shala lacustrine basin of large volumes of water supply. This and the further increase in aridity during the late Holocene led to the separation of the Ziway-Shala paleolake into the present four lakes (Ziway, Langano, Abjiata, Shala). This study indicates that in the Main Ethiopian Rift, climatic changes cannot be inferred from lake-level variations alone because changes in water supply are also influenced by the tectonic-induced rearrangement of the fluvial drainage networks.

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