EVOLUTION OF THE GORGE OF THE NILE, ETHIOPIA, FROM INTEGRATION OF OPTICAL AND RADAR REMOTE SENSING DATA AND DIGITAL ELEVATION MODELS

The Blue Nile on Ethiopian Plateau, west of the Main Ethiopian Rift and Afar Depression starts as a small stream (Little Abay) which originated from the Springs of Sakala located 1850 m above sea level and flows about ~96 km northwest until it drains into Lake Tana at ~1829 m above sea level. The river flows SE (gradient ~4m/km) after it leaves Lake Tana, then S and SW before flowing NW (gradient ~0.4 m/km) towards the lowlands of Sudan where it increases its depth and asymmetry. Hence, the river forms a near-circular loop (the Blue Nile Bend) by carving ~1.5 km deep gorge (the Gorge of the Nile) around Tertiary-Quaternary shield volcanoes with steep cliffs and exposes a thick section of Mesozoic sediments with gentler slopes and bench morphology. Optical-Radar Remote Sensing and DEMs Data integration is effective in determining geologic evolution of Gorges including better understanding of the drainage basin in terms of river engineering and slope stability. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and RADARSAT remote sensing data fusion aids in mapping spatial and spectral relationship between lithology and structural features. Shuttle Radar Topography Mission (SRTM) and Global Topography 30 Arc Second (GTOPO30) Digital Elevation Models (DEMs) data are integrated with ASTER and RADARSAT in order to understand geological controls on the Gorge of the Nile and the formation of the Blue Nile Bend as well as the geometrical characteristics of the gorge. Remote sensing analysis suggests that the Gorge of the Nile and the Blue Nile Bend have been controlled by a number of factors: (1) Miocene to Pliocene shield volcanoes (2) NW-, S- and SW- trending fractures (3) Flexural uplift of the western escarpment of the Main Ethiopian Rift and the Afar Depression and (4) Normal faults associated with E-W transverse faults related to the Main Ethiopian Rift. This study also concluded the evolution of the Gorge of the Nile and the Blue Nile bend as follows: (1) Initiation of the Blue Nile at ~8 Ma ago where it flowed SE (2) Deflection of river flow direction was due to uplift on the western flank of the Main Ethiopian Rift and the Afar Depression occurred somewhere between ~8 and ~1 Ma (3) Deflection of the river to a SW and NW-flowing directions at ~1 Ma was due to uplift on the northern flank of Ambo lineament.