GEOLOGIC CONTROLS OF THE GORGE OF THE NILE IN ETHIOPIA: AN INTEGRATED APPROACH FROM ASTER, RADARSAT, DEMS, AND FIELD STUDIES

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and RADARSAT images in conjunction with Digital Elevation Models (DEMs) extracted from the Shuttle Radar Topography Mission (SRTM) and ASTER, coupled with field studies have been used to extract geological and geomorphological information in order to understand the geological controls on the Gorge of the Nile in Ethiopia. The Blue Nile on the NW Ethiopian Plateau forms a ~150 km diameter loop by carving the ~1.6 km deep Gorge of the Nile, and exposes ~1200 m thick section of Mesozoic sediments sandwiched between Tertiary-Quaternary volcanics and Neoproterozoic basement rocks. The river decreases its gradient from ~4 m/km to ~0.4 m/km as it bends around Tertiary-Quaternary shield volcanoes. Our work shows that: (1) 3D perspective views generated by draping ASTER images over ASTER DEMs are effective in mapping sub-horizontal lithological units in the Gorge of the Nile. These perspective views are also effective in highlighting lithologically-defined structures. (2) The side-looking geometry of the Standard Beam RADARSAT data is effective in mapping morphologically-defined structures due to radar shadow-illumination effect. (3) Fusion of ASTER and RADARSAT data using Color Normalization Technique (CNT) enhances mapping because the fused image preserves the spectral information of ASTER data and incorporates terrain morphology from RADARSAT. (4) ASTER band-ratio (4/5 – 3/1 – 3/4) image which reduces the topographic effect between multi-spectral bands better resolves lithological units and lithologically-defined structures. (5) SRTM DEMs are effective for characterizing the 3D spatial relationships between the river’s course and regional geomorphological features such as shield volcanoes, the Main Ethiopian Rift and Afar Depression. These DEMs are also useful in extracting the river’s geometric properties and the analysis of drainage network. This study also shows a number of possible geologic controls in the evolution of the Gorge of the Nile. Base-level adjustment due to regional uplift, obstruction and diversion due to bedrock structures, and differential incision due to varying lithology play significant roles in deep carving the Gorge of the Nile, deflecting the course of the Blue Nile and producing a diverse drainage network.