Remote sensing analysis of the Gorge of the Nile, Ethiopia with emphasis on Dejen–Gohatsion region

• Published in: Journal of African earth sciences (1994) 2006-02, Vol.44 (2), p.135-150
• Main Authors: Gani, Nahid DS, Abdelsalam, Mohamed G.
• Format: Article
• Language: English
• Subjects: ASTER; Geologic controls; Geomorphology; Gorge of the Nile; RADARSAT; SRTM
• ISSN: 1464-343X; 1879-1956
• DOI: 10.1016/j.jafrearsci.2005.10.007

Digital Elevation Models (DEMs) extracted from the Shuttle Radar Topography Mission (SRTM) with 90 m x–y resolution, and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) with 15 m x–y resolution have been used in conjunction with ASTER and RADARSAT images, and field studies 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 bend and carves the ∼1.6 km deep Gorge of the Nile. The river shows a dramatic drop in gradient from ∼4 m/km to ∼0.42 m/km as it spirals around Tertiary to Quaternary shield volcanoes. A ∼1200 m thick section of Mesozoic sedimentary rocks bounded between Tertiary and Quaternary volcanic rocks and Neoproterozoic basement rocks is exposed within the Gorge of the Nile. Our work shows that: (1) SRTM DEMs are effective for the characterization of 3D spatial relationships between the river’s course and regional geomorphological features such as Tertiary to Quaternary shield volcanoes, Afar Depression and the Main Ethiopian Rift. These DEMs are also useful in extracting the river’s geometric properties and the analysis of drainage network. (2) ASTER band (7-3-1) and band-ratio (4/5-3/1-3/4) images better resolve lithological units and lithologically defined structures. (3) The side-looking geometry of the Standard Beam RADARSAT data is effective in mapping morphologically defined structures due to radar shadow-illumination effect. (4) Fusion of ASTER and RADARSAT data using Color Normalization Technique (CNT) enhances the mapping ability because the fused image preserves the spectral information of ASTER data and incorporates terrain morphological characteristics from RADARSAT data. (5) Three dimensional (3D) perspective views generated by draping ASTER images over ASTER DEMs are effective in mapping sub-horizontal lithological units such as those dominating the Gorge of the Nile. These perspective views are also effective in highlighting lithologically defined structures. 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, spatial distribution of shield volcanoes, obstruction and diversion due to bed rock structures, and differential incision due to varying lithology have significant roles in deep carving of the Gorge of the Nile, deflection of the course of the Blue Nile, and