Journal of African Earth Sciences 44 (2006) 135–150
www.elsevier.com/locate/jafrearsci
Remote sensing analysis of the Gorge of the Nile, Ethiopia
with emphasis on Dejen–Gohatsion region
Nahid DS Gani *, Mohamed G. Abdelsalam
Department of Geosciences, University of Texas at Dallas, P.O. Box 830688, FO 21, Richardson, TX 75083-0688, USA
Received 1 October 2004; received in revised form 5 March 2005; accepted 7 October 2005
Available online 5 January 2006
Abstract
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 subhorizontal 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 in producing a diverse drainage network.
2005 Elsevier Ltd. All rights reserved.
Keywords: Gorge of the Nile; SRTM; ASTER; RADARSAT; Geologic controls; Geomorphology
1. Introduction
The Blue Nile or Abay River is one of two main
branches (the White Nile forms the other branch) of the
River Nile System (Fig. 1). The Blue Nile (Figs. 2A and
B) on the Ethiopian Plateau starts as a small stream known
*
Corresponding author. Tel.: +972 883 2401; fax: +972 883 2537.
E-mail address: [email protected] (N. DS Gani).
1464-343X/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jafrearsci.2005.10.007
as Little Abay or Gilgel Abay (Figs. 2B and 3A) which
originates from the springs of Sakala near Mt. Gish
(Fig. 3A). Little Abay starts at 2890 m above Mean Sea
Level (MSL) near Mt. Gish and drains into Lake Tana
at 1794 m above MSL (Figs. 2B and 3A). Lake Tana
receives most inflow from the Little Abay rather than from
its perimeter. Only a small proportion of the precipitation
on the plateau enters the lake from N, E and NE directions,
most flowing into Setit-Tekeze River and lower reaches of
the Blue Nile (Fig. 3A). This is the consequence of the
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N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
(B)
21˚E
31˚E
41˚E
Jerusalem
Egyptian Region
Mediterranean
Mediterrean seaSea
N
Jordan
Cairo
30˚N
400 km
Libya
Egypt
Saudi
Arabia
20˚N
Chad
tb
ar
Khartoum
a
Ra
Central Sudan
Region
ha
ile
Bahr
Lake Tana
el Ar
ab
Malakal
Central
African
Republic
Ethiopian Nile
Region
d
White Nile
Sudd Region
A
eN
Blu
Lake Plateau Region
Sudan
ea
dS
Re
Cataract Region
Aswan
10˚N
Addis Ababa
Ethiopia
Uganda
(A)
Lake Albert
Zaire
0˚N
Lake Edward
Lake Kivu Rwanda
Burundi Lake
Lake Tanganikya
based on fractal analysis of topography. Beaulieu and
Gaonac’h (2002) examined erosional versus depositional
surfaces in the drainage network of the Ethiopian Plateau
using Landsat TM and European Remote Sensing (ERS1) data covering the same area as that examined by Weissel
et al. (1995). Ayalew and Yamagishi (2004) studied landslides and rockfalls in the Gorge of the Nile using aerial
photographs, DEMs, and field studies.
The main objective of this work is to discuss the possible
geological controls on the Gorge of the Nile by utilizing
remote sensing data including the Advanced Spaceborne
Thermal Emission and Reflection Radiometer (ASTER),
RADARSAT, DEMs extracted from Shuttle Radar
Topography Mission (SRTM) and ASTER data, and field
studies. This work also aims at demonstrating the usefulness of the integration of optical (ASTER) and radar
(RADARSAT) remote sensing data, and DEMs (SRTM
and ASTER) for geoscientific studies. We have used quantitative geomorphic analysis of DEMs, optical and radar
remote sensing data analysis, optical-radar data fusion,
and draping optical images over DEMs, to achieve these
objectives. We have first used SRTM DEMs for geomorphic analysis of the entire Gorge of the Nile. Subsequently,
we focused on the Dejen–Gohatsion region (Figs. 2B and
3A) to illustrate the usefulness of our approach for geoscientific studies in the Gorge of the Nile and possible structural controls on this portion of the Gorge of the Nile.
3000 km
Victoria
Fig. 1. (A) Map of Africa showing the location of the Nile System. (B)
The location of the Gorge of the Nile within the Nile System. The Nile is
divided into five regions including the Lake Plateau Region, the Sudd
Region, the Central Sudan Region, the Cataract Region, and Egyptian
Region. The eastern part of the Central Sudan Region includes the
Ethiopian Nile Region where the Gorge of the Nile is situated. (Modified
after Said, 1993).
Tertiary–Quaternary tectonics. Little Abay flows into Lake
Tana and out of it as the Blue Nile, which flows SE and S
then SW before assuming a NW-flowing direction until it
joins the White Nile at Khartoum (Figs. 1 and 3A). Hence,
the Blue Nile forms a 150 km semi-circular loop around
Tertiary–Quaternary shield volcanoes (Mts. Choke,
Yacandach and Gish; Figs. 2B and 3A) referred to here
as the Blue Nile Bend. The river runs through a series of
rapids and carves the 1.6 km deep Gorge of the Nile on
the Ethiopian Plateau which is bordered in the E and SE
by the flanks of the Afar Depression and the Main Ethiopian Rift (Figs. 3A and B). The Gorge of the Nile is a geographical wonder, said to be the rival of the USA Grand
Canyon in Arizona. It is still unclear what is controlling
the carving of the Gorge of the Nile.
Few remote sensing studies have been carried out aimed
at understanding the geological controls on the Gorge of
the Nile. Weissel et al. (1995) used digital elevation models
(DEMs) and Landsat Thematic Mapper (TM) data for
studying erosional development on the Ethiopian Plateau
2. Tectonic settings and geology of the Gorge of the Nile
The Gorge of the Nile is situated on the Ethiopian
Plateau 200 km NW of Addis Ababa, the capital city of
Ethiopia and is bounded between longitude 3630 0 E and
3950 0 E and latitude 900 0 N and 1150 0 N (Figs. 2A
and B). The Ethiopian Plateau is dominated by the NW
and SE plateaux which are separated by the NE-trending
Main Ethiopian Rift (Fig. 2A). A 1200 m thick section
of Mesozoic sedimentary rocks is exposed along the Gorge
of the Nile. These are overlain by Mid-Oligocene to Early
Miocene volcanic flows (Fig. 2B) known as the Trap Series,
the thickness of which reaches in some places 2000 m
(Coulie et al., 2003). Neoproterozoic basement rocks
underlie the Mesozoic sedimentary rocks and are encountered along the Gorge of the Nile as the Blue Nile flows
NW towards the lowlands of Sudan (Fig. 2B).
The Blue Nile on the Ethiopian Plateau is divided here
into SE-, S-, SW- and NW-flowing segments (Fig. 2B).
Many tributaries join different flowing segments of the Blue
Nile suggesting a spiral drainage divide line (Fig. 3A). The
Megech, Ribb and Gumara rivers flow westward and drain
into Lake Tana (Fig. 3A). The Beshlo river comes from the
NE and joins the Blue Nile as it bends to flow S (Fig. 3A).
The westward flowing Beto and Jema rivers join the Blue
Nile at its S-flowing segment (Fig. 3A). The Muger river
flows NW to join the SW-flowing Blue Nile (Fig. 3A).
The Guder and Finchaa rivers coming from the S, and
Dura, Fattom, Birr and Timchaa rivers coming from the
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
137
Fig. 2. (A) Distribution of Mesozoic sedimentary rocks in the Horn of Africa (Modified after Assefa, 1991). (B) Generalized geological map of the Gorge
of the Nile (Modified after Mangesha et al., 1996).
N join the Blue Nile where it flows NW (Fig. 3A). These
tributaries meet the Blue Nile almost perpendicular to its
flow directions (Fig. 3A).
Various aspects of the Gorge of the Nile have been
described by Krenkel (1926), Stefannini (1933), Jepsen
and Athearn (1961a,b, 1964), Mohr (1962), Ficcarelli
(1968), Dainelli (1970), Kazmin (1973, 1975), Merla et al.
(1973), Beauchamp and Lemoigne (1974), Kalb and
Oswald (1974), McDougall et al. (1975), Canuti and
Radrizzani (1975), Beauchamp (1977) and Wood et al.
(1997).
Assefa (1979, 1980, 1981, 1991), Chernet (1988), and
Mangesha et al. (1996) divided the stratigraphy of the
Gorge of the Nile region into five units including from base
to top, Neoproterozoic basement rocks, Early–Middle
Jurassic Adigrat Sandstone, Middle Jurassic Abay Limestone, Shale and Gypsum (Gohatsiyon Formation),
Middle-Late Jurassic Antalo Limestone, Late Cretaceous
Amba Aradam Sandstone, and younger volcanic rocks.
Informal names of lithological units are used in this study.
Based on field studies and previous geologic maps
(Chernet, 1988; Mangesha et al., 1996), seven stratigraphic
units are proposed for the Gorge of the Nile (Table 1).
These are, from stratigraphically older to younger: (1)
Neoproterozoic basement rocks composed of metamorphic
rocks including highly deformed and sheared gneisses,
schists, migmatites, granites and igneous intrusions. In
some places the Neoproterozoic rocks are overlain by
sub-horizontal to shallowly dipping Paleozoic sedimentary
rocks or Cenozoic Trap Series. However, at the Gorge of
the Nile, the Neoproterozoic rocks are unconformably
overlain by the Triassic-Middle Jurassic Lower Sandstone
(Adigrat Sandstone). (2) Triassic-Middle Jurassic Lower
Sandstone (Adigrat Sandstone) capped by a glauconitic
sandy mudstone unit. (3) Middle Jurassic Lower Limestone
(Abay Limestone). (4) Middle-Late Jurassic Upper
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Fig. 3. (A) Drainage pattern of the Gorge of the Nile (Modified after Collins, 2002). (B) Regional 3D perspective view of the Gorge of the Nile, the Main
Ethiopian Rift and the Afar Depression created from GTOPO30 DEM with 0.5 vertical exaggerations.
Limestone (Antalo Limestone). (5) Late Cretaceous Upper
Sandstone (Debre Libanos Sandstone). (6) Top volcanic
rocks (Middle Oligocene-Early Miocene; Coulie et al.,
2003). These are mostly flood basalts with subordinate
trachytes and rhyolites. (7) Quaternary Volcanic rocks.
The depositional evolution of the Gorge of the Nile
including Mesozoic sedimentary rocks can be modified
from Russo et al. (1994) as follows: (1) Peneplanation
of the Neoproterozoic basement outcrops; (2) Post-rift
stage (related to Gondwana rifting), in which fluvial sedimentary rocks of the Triassic-Middle Jurassic Lower
Sandstone covered the Neoproterozoic basement rocks;
(3) First marine transgression indicated by the glauconite
bearing sandy mudstone beds deposited on top of the
Lower Sandstone; (4) Early flooding stage and deposition
of the Middle Jurassic Lower Limestone which includes
gypsum layers indicating drying of the basin; (5) Second
phase of marine transgression when the Middle-Late
Jurassic Upper Limestone has been deposited; (6) Regression of the sea leading to the deposition of the Late Cretaceous Upper Sandstone in a fluvial condition; (7)
Basaltic lava extrusion during the Middle Oligocene to
Early Miocene related to deep mantle plume activity
(Marty et al., 1996; Ritsema and Heijst, 2000) and regional tectonic activity associated with the East African
Rift System; and (8) Second volcanic event leading to
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
139
Table 1
Stratigraphy of the Gorge of the Nile
Stratigraphy of the Gorge of the Nile
Age
Formation
Quaternary
Middle Oligocene to Early Miocene
Lithologic Units
(informal name)
Volcanic Rocks
Volcanic Rocks
Late Creataceous
Upper Sandstone Unit
Debre Libanos
Sandstone
Middle to Late Jurassic
Upper Limestone Unit
Antalo Limestone
Middle Jurassic
Lower Limestone Unit
Abay Limestone
Triassic to Middle Jurassic
Glauconitic Sandy
Mudstone Unit
Lower Sandstone Unit
Neoproterozoic
Lithology
Adigrat Sandstone
Basement Rocks
Mostly alkaline basalts and trachytes
Mostly flood basalts with extensive
columnar joints and dialation fractures,
rare intercalation of pyroclastic deposits,
minor trachyte and rhyolites
Coarser-grained than Lower sandstones,
dune cross stratified, more pebbles along
foresets, rare small-scale channels, both
fluvial and alluvial fan deposits
Bedded to massive limestones with
alternating layers of lime mudstones,
fossiliferous, fossils include mostly
brachiopods, gastropods, bivalves
Alternating Limestone and gypsum where
limestone increases upward, bedded
limestone at base sometimes bioturbated
Upper part interbedded with hummocky
cross-Stratified sandstone
Coarse to very coarse sandstone, dune
trough, less interbedded mudstone,
channelized with large accretion surface,
granule or pebble pockets, silicified wood,
mostly fluvial
Mostly gneissic and schistose rocks with
veins and dike intrusions, granites and
migmatites
Thickness (m)
500 to 1600+
170 to 280
350 to 400
400 to 450
20–30
280 to 300
800 900+
Modified after Chernet (1988), Mangesha et al. (1996), Coulie et al. (2003) and Pik et al. (2003).
emplacement of the Quaternary volcanic rocks, which is
one of the important Quaternary tectonic events in the
Blue Nile region. Most of the southern part of the Tana
basin was covered by basaltic lava flows of Quaternary
age (Jepsen and Athearn, 1961b) and Lake Tana acquired
its present form by damming of a 10,000 years old lava
flow (Grabham and Black, 1925) which helped Little
Abay to reflow from Lake Tana towards the SE as the
present-day Blue Nile.
3. Remote sensing data
ASTER, RADARSAT, and SRTM data (Table 2) are
used in this study.
(1) ASTER is an optical imaging system aboard the
satellite Terra. ASTER has 14 spectral bands (Jensen,
2000) included in three subsystems (Table 2): (1) Three
bands (bands 1–3) in the visible and near infrared (VNIR)
region of the electromagnetic spectrum with 15 m spatial
resolution. Band 3 has a nadir (3N) and also has a backward-looking (27.7 off-nadir; 3B) detector for stereoscopic
data acquisition and DEM extraction (Fig. 4); (2) Six
bands (bands 4–9) in the short wavelength infrared (SWIR)
region with 30 m spatial resolution; and (3) Five bands
(bands 10–14) in the thermal infrared (TIR) region with
90 m spatial resolution. Radiometrically and geometrically
corrected Level 1B ASTER data are used in this study.
A stereo correlation approach is used to generate ASTER
DEMs (Fig. 4) by converting ASTER 3N and 3B bands into
quasi-epipolar images which have pixel displacement in the
satellite flight direction that is proportional to pixel elevation. This correlation method transforms the displacement
into relative elevation values with 15 m x–y resolutions
and 1 m z resolution and within 10–30 m root mean square
error (RMSE) z accuracy (Lang and Welch, 1999).
(2) RADARSAT, which is equipped with Synthetic
Aperture Radar (SAR) sensor, collects C-band radar data
with a wavelength of 5.6 cm (Table 2) and in horizontally
transmitted and horizontally received (HH) polarization
(Raney et al., 1991). RADARSAT has different imaging
modes with different spatial resolutions, geographical coverages, and incident angles. The Standard Beam RADARSAT
data with geographical coverage 100 · 100 km, spatial resolution of 25 m and incident angle of 45 are used in this study.
(3) SRTM is the only spaceborne single-pass interferometric SAR imaging system, that collected data in February 2000. The technology allowed construction of 3D
DEMs of 80% of the Earth’s surface (between 60 N
and 60 S). Elevation computation for DEMs includes a
phase difference which is measured by: (1) co-registration
of two SAR images within a fraction of a pixel; and (2)
complex conjugate multiplication of the registered image.
Every pixel of the registered image carries phase information from which topographic information of the image is
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Table 2
Comparison between spatial and spectral characteristics of ASTER, RADARSAT and SRTM data
Parameters
Optical Imaging System (ASTER)
Radar Imaging System
RADARSAT
SRTM
Launched date
Nationality
Orbital platform
Life time
Organization
Swath width
December, 1999
USA/Japan
Satellite
5 years
NASAa/MITIb
60 km
November, 1995
Canada
Satellite
5 years
CSAc
50–500 km
Altitude
Class
Data collecting region in electromagnetic spectrum
Bands
750 km
Optical
Visible to Thermal Infrared
VNIR bands – 1 N, 2N, 3N, 3B
SWIR bands – 4, 5, 6, 7, 8, 9
TIR bands – 10, 11, 12, 13, 14
1N = 0.52–0.60
2N = 0.63–0.69
3N = 0.76–0.86
3B = 0.76–0.86
4 = 1.6001.700
5 = 2.145–2.185
6 = 2.185–2.225
7 = 2.235–2.285
8 = 2.295–2.365
9 = 2.360–2.430
10 = 8.125–8.475
11 = 8.475–8.825
12 = 8.925–9.275
13 = 10.25–10.95
14 = 10.95–11.65
VNIR bands – 15 m
SWIR bands – 30 m
TIR bands – 90 m
None
798 km
Radar
Microwave
C-band
February, 2000
USA
Space shuttle
11 days
NASAa/NIMAd/DLRe/ASIf
X band – 50 km
C band – 225 km
233 km
Radar
Microwave
X-band
C-band
Wavelength
Spatial resolution
Polarization
a
b
c
d
e
f
C band = 5.6 cm
X = 3 cm
C = 5.6 cm
10–100 m
30 m
HH
X band – VV
C band – HH, VV
NASA—National Aeronautics and Space Agency.
MITI —Japan’s Ministry of International Trade and Industry.
CSA—Canadian Space Agency.
NIMA—National Imagery and Mapping Agency.
DLR—German Space Agency.
ASI—Agenzia Spaziale Italiana.
calculated after phase unwrapping and by using cosine rule
to calculate heights (Jensen, 2000; NASA JPL, 1999). This
method is repeated for every pixel within the image (Fig. 5).
SRTM-3 data with 90 m x–y resolution and ±30 m RMSE
z accuracy are used in this study.
4. Remote sensing analysis
4.1. SRTM data analysis
SRTM data covering the entire Gorge of the Nile
(Fig. 6) have been used for quantitative river geomorphic
analysis. Fifty topographic profiles perpendicular to the
river’s flow direction, starting from headwater to downstream, are extracted for analyzing channel geometry
(Fig. 7). These profiles contain relative z-values in meters
whereas the horizontal x-distances are recalculated from
digital number (DN) values to meters.
4.2. ASTER data analysis
ASTER data are used to generate a Red-Green-Blue
(RGB) 7-3-1 color combination image (Fig. 8A) for lithological mapping of the Dejen–Gohatsion part of the Gorge
of the Nile. The 7-3-1 ASTER image better differentiates
the boundary between basalts and Mesozoic sedimentary
rocks. Adding a SWIR band (band 7) with better spectral
characteristics to bands 3 and 1 enhances the lithological
discrimination. Band-ratio images in which the DN value
of one band is divided by the DN value of another band
is sometimes effective in lithological mapping (Ramadan
et al., 2001) because it provides useful reflectance and
absorption information of individual minerals. ASTER
band-ratio image 4/5-3/1-3/4 (Fig. 8B) is generated for
the purpose of enhancing the spectral information. This
band-ratio image is subsequently used for mapping lithological boundaries in the Dejen–Gohatsion region.
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
141
Fig. 4. (A) The configuration of ASTER system for generating 15 m x–y resolution DEMs from the photogrammetry of bands 3N and 3B. ASTER 3N
and 3B bands calculate elevation using stereo configuration from the algorithm Dh ¼ x1 x2 = tan a ¼ Dp=tanaÞ H Dp=B. Dh = elevation, Dp = parallax
difference, a = orientation angle of camera, Dt = required time interval for recording top and bottom of the object, B = baseline, H = height, x1 and x2 are
the distances, and t1, t2 and t3 are the time. In the stereo pair Dt is represented by (x1 x2) = Dp. B is equal to x1 (Modified from Lang and Welch, 1999).
(B) Band 3A image of part of the Gorge of the Nile. (C) Band 3B for the same area covered by 4B. (D) Gray scale DEM generated from bands 3N and 3B.
(E) Color coded DEM of 4D. Locations of the ASTER images and DEMs are shown in Fig. 2B.
Fig. 5. (A) Geometric relationship between two SAR systems used for the creations of DEMs from single-pass radar interferometry of SRTM data
(Modified from Jensen, 2000; NASA JPL, 1999). Elevation information are extracted using the cosine equations h ¼ r1 cos h; ðr2 Þ2 ¼ ðr1 Þ2 þ B2 2r1 B cosð90 þ h aÞ ¼ ðr1 Þ2 þ B2 2r1 B sinðh aÞ where r1 and r2 are the range between each radar antenna to the object on the ground surface, B = the
baseline between two radar antennas, a = baseline angle with horizontal, h = height, h = angle between vertical and r1. (B) Color coded SRTM DEM with
90 m spatial resolution covering the same area of the Gorge of the Nile covered by 4E.
4.3. RADARSAT data analysis
Standard Beam RADARSAT scenes covering the
Dejen–Gohatsion region are enhanced by Frost adaptive
filtering for speckles reduction (Fig. 8C; Shi and Fung,
1994). This filtering replaces the DN value of the image
pixel by a DN value that is calculated on the basis of the
distance from the center of the filter, damping factor and
variance. Radar images can be interpreted in terms of tone,
texture and pattern (Abdelsalam et al., 2000). The tone and
texture are controlled by the surface roughness of the ter-
rain. On the other hand, patterns in radar images are controlled by the slope angle and orientation. Bright areas
represent regions where the slope is nearly perpendicular
to the radar wave propagation direction and reflects almost
all the incident energy. Radar shadow-illumination which
depends on depression angle, slope and aspect of morphologically-defined structures is reflected as bright and dark
stripes in the radar image. The greater the depression angle,
the stronger the signal return, and therefore the brighter
the image. This helps in mapping structural features such
as lineaments and their geometrical relationships.
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Fig. 6. (A) Drainage network with stream orders extracted from the 90 m x–y resolution SRTM DEMs (Location of the image is shown in Fig. 6B). Basin
area = 89340.8 km2; Basin relief = 2.12 km; Strahler order = 6 with a threshold value of 6; Drainage network magnitude = 145704; Network
diameter = 2029; Longest channel length = 769.99 km; Total channel length = 152100.2 km; Drainage density = 1.70 per km. (B) Shaded relief map of the
Gorge of the Nile extracted from SRTM DEMs.
Fig. 7. The geometry of the Gorge of the Nile from 50 topographic profiles extracted from SRTM DEMs perpendicular to the Blue Nile at different
segments. (A) Depth vs. Cumulative distance from headwater (Lake Tana). (B) Width vs. Cumulative distance from headwater. (C) Depth/width ratio vs.
Cumulative distance from headwater. (D) Asymmetry Index (the ratio between large flank and small flank of the valley) vs. Cumulative distance from
headwater. (E) Elevation vs. Cumulative distance from headwater.
4.4. Optical-radar data fusion
ASTER and RADARSAT images are fused to convey
information that cannot be obtained when ASTER
and RADARSAT are used individually. The ASTER-
RADARSAT fused image preserves the spectral characteristics (included in the ASTER image) as well as the surface
roughness and slope angle and orientation of the terrain
(embedded in the RADARSAT image). Color Normalization Transformation (CNT) method (Vrabel, 1996) is used
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
143
Fig. 8. (A) 7-3-1 ASTER image covering part of the Gorge of the Nile. (B) 4/5-3/1-3/4 ASTER band-ratio image. (C) Enhanced Standard Beam
RADARSAT image. The arrow indicates illumination direction. (D) Fused image generated by applying CNT technique to 7-3-1 ASTER and enhanced
RADARSAT image. (E) 3D perspective view of part of the Gorge of the Nile generated by draping 7-3-1 ASTER image over ASTER DEM. (F)
Simplified lithological map of the area generated by the interpretation of remote sensing images. The locations of images are shown in Fig. 2B.
for the fusion of ASTER and RADARSAT data covering
the Dejen–Gohatsion region. The CNT is a mathematical
manipulation where the DN value of each pixel in each
band in the RGB color combination image is multiplied
by the DN value of each pixel of the grayscale image and
then divided by the sum of the DN value of each pixel of
the three bands of the RGB color combination image.
The CNT technique is applied to fuse the 7-3-1 ASTER
image (Fig. 8A) and the Standard Beam RADARSAT
image (Fig. 8C). The spatial resolution of the ASTER
image is controlled by band 3 which has 15 m spatial resolution. The ASTER-RADARSAT fused image (Fig. 8D)
proved effective in showing the 2D geometrical relationship
between lithological units and morphologically-defined
structures.
4.5. Draping ASTER images over ASTER DEMs
The 7-3-1 ASTER image covering the Dejen–Gohatsion
region is draped over the ASTER DEMs (Fig. 8E) in order
to generate a 3D perspective view that helps in understanding the 3D geometry of geological features and the extraction of details of geological features such as interfaces
between lithological units (Fig. 8F). In addition, spatial
relationships between lithology, structure and geomorpho-
logical features are well illustrated in the perspective view
(Fig. 8E). This has been of significant importance in tracing
the interfaces between sub-horizontal units of the Tertiary–
Quaternary volcanic rocks and the Mesozoic sedimentary
rocks.
5. Results
5.1. Quantitative geomorphic analysis
5.1.1. Drainage network analysis
SRTM DEMs are used for analyzing the drainage network of the Gorge of the Nile including the extraction of
drainage basin statistics and drainage network (Fig. 6A).
The drainage network pattern of the Gorge of the Nile
(Fig. 6A) revealed a sixth order stream suggesting a mature
drainage network. Using Strahler’s (1952) hierarchization
of drainage network, NW–SE, NE–SW, and N–S oriented
streams are mostly the highest order streams (fourth, fifth
and sixth order) corresponding to the main flow direction
of the Blue Nile (Fig. 6A). Drainage network analysis
shows that third and fourth order streams with more dendritic pattern tributaries join fifth order streams, and that
third and fourth order streams with less dendritic pattern
and less tributaries connect directly with fifth order streams
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or sixth order streams (Fig. 6A). First and second order
streams are more evenly distributed throughout the basin
indicating that these are of more ephemeral type. Hence,
distribution of these tributaries with respect to the Blue
Nile resulted in the formation of a spiral drainage divide
(Fig. 3A).
The shaded relief map of the Gorge of the Nile has been
generated from SRTM DEMs (Fig. 6B) in order to show
the erosional pattern of the Blue Nile and its tributaries.
This DEM shows how the spatial erosional pattern of
streams changes with regional topography. As the E and
SE parts of the Gorge of the Nile are bounded by the
uplifted flanks of the Afar Depression and the Main Ethiopian Rift (Figs. 2A, 3A and B), the streams on these parts
are on more actively eroding bedrock than other sides of
the Gorge of the Nile. Because of a steeper slope towards
the W due to the uplifted flanks (Weissel et al., 1995) these
streams incised deeply to the W to join the Blue Nile. The
nature of the erosional pattern of these streams is more
dendritic indicating mature erosion resulting in inhomogeneous topography and rugged relief (Fig. 6B). On the other
hand, in the W and SW part, the erosional pattern is more
homogenous and young resulting in an even topography
(Fig. 6B). In the central part of the Gorge of the Nile,
the spatial pattern of erosion is more radial and erosion
is more immature, which might be associated with the
growth of shield volcanoes (Fig. 6B).
5.1.2. Channel geometric analysis
Quantitative analyses of topographic profiles of the
Gorge of the Nile (Fig. 7) show that the geometrical characteristics of the Blue Nile vary from headwater (Lake
Tana) to downstream (Lowlands of Sudan). The depth of
the Gorge of the Nile increases from 665 m to 1600 m
and its width increases from 7 km to 50 km (Figs. 7A
and B). The depth/width ratio generally decreases with distance from source (Fig. 7C). The symmetry of the Gorge of
the Nile also decreases with distance from source (Fig. 7D).
The gradient is inversely proportional to distance from
source and decreases from 4 m/km to 0.42 m/km
(Fig. 7E).
The Blue Nile flows SE from Lake Tana through Middle
Oligocene-Quaternary volcanic rocks (Fig. 2B). The river is
confined to a narrow symmetrical valley where the width of
the gorge in this segment is 7 km to 23 km and its depth
increases from 665 m to 1150 m (Figs. 7A, B and D).
The gradient is steep in the volcanic rocks (4 m/km)
and becomes shallower (0.7 m/km) as the river flows
south through the sedimentary rocks (Fig. 7E). As the distance from headwaters increases, the river incises deeply
and exposes a thick section of the Upper Sandstone and
Upper and Lower Limestone (Fig. 2B) with an increase
in asymmetry index (Fig. 7D). Bedrock lithology might
have played a significant role in changing the geometry
of the Gorge of the Nile in which the fragile and easily erodable Mesozoic sedimentary section presented a favorable
condition for deep incision. The width of the gorge in the
Upper Sandstone changes from 12 km to 27 km and
its depth increases from 1150 m to 1245 m (Figs. 7A
and B). The average river gradient in this segment is
0.7 m/km (Fig. 7E). The gorge becomes more asymmetrical with the steeper flank occurring in the W (Fig. 7D). The
numerous tributaries that drain into the Blue Nile might
have contributed to shaping the geometry of the Gorge
of the Nile in this segment. In the Upper and Lower Limestone, the width varies from 19 km to 30 km and from
13 km to 22 km, respectively (Fig. 7B). Further downstream, the Blue Nile carves its valley deep enough to
expose the Lower Sandstone as the river changes its flow
direction to SW (Fig. 2B). The river deepens its valley from
1280 m to 1570 m (Fig. 7A). In addition, the valley widens up to 40 km and the gradient drops to 0.52 m/km
(Figs. 7B and E). In this segment the gorge becomes symmetrical and assumes a broad U-shape. The river exposes
Neoproterozoic basement rocks as it flows NW towards
the lowlands of Sudan. The river’s gradient in this segment
drops to 0.42 m/km, the depth of the gorge reaches
1.6 km, and it widens to 50 km (Fig. 7).
5.2. Lithological mapping
The Gorge of the Nile exposes a thick section of Mesozoic sedimentary rocks that includes Lower Sandstone,
Lower Limestone, Upper Limestone, and Upper Sandstone
overlain by Middle Oligocene-Early Miocene and Quaternary volcanic rocks (Fig. 2B). These rocks have distinctive
spectral characteristics and/or surface roughness. Remote
sensing analysis, discussed above, together with field studies are used to generate a detailed lithological map for the
region between Dejen and Gohatsion (Figs. 2B and 8F)
with the help of published geological maps (Stefannini,
1933; Merla et al., 1973; Mangesha et al., 1996). The interpretation of the 7-3-1 and 4/5-3/1-3/4 ASTER images, 3D
model (Figs. 8A, B and E), and field studies are used to
generate a lithological map of the region (Fig. 8F). The
boundary between different units is established as follows:
(1) The boundary between the volcanic rocks and the
sedimentary succession is obvious in the ASTER image
draped over ASTER DEM (Fig. 8E). This boundary is
traced on both sides of the Gorge of the Nile as well as
in canyons associated with tributaries draining into the
Blue Nile.
(2) The sub-horizontal nature of bedding of sedimentary
rocks in the Gorge of the Nile has made it difficult to trace
boundaries on the 7-3-1 ASTER or the fused images (Figs.
8A and D). These boundaries are characterized by significant differences in slope morphology that are obvious in
the 3D perspective views (Fig. 8E), which proved to be extremely helpful in tracing these boundaries. The volcanic rocks
are characterized by sharp and steep cliffs that are different
from the underlying sedimentary rocks, which are generally
characterized by gentler slopes (Fig. 8E). The sub-horizontal contact between the Lower Sandstone and Lower Limestone is obvious and can be traced throughout the 3D
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
model. This is further enhanced by the high reflectance of
the Lower Sandstone in band 7 and high reflectance of the
Lower Limestone in band 3. In addition, there are significant differences in cliff steepness, slope breaks and benches
between the two formations (Fig. 8E). The limestone units
are associated with curved slopes that are less steep than
the Lower Sandstone (Fig. 8E). Chemical weathering of
the limestones might be responsible for slope degradation.
This might also be the reason why the Gorge of the Nile
has widened to 30 km when the Blue Nile encountered
these units. The change in slope between the limestone and
sandstone units has produced in some places bench morphological features (Fig. 8E).
(3) It is difficult to discriminate between the Upper and
Lower Limestone from ASTER and RADARSAT images
or slope morphology. Therefore, ASTER band-ratio
image 4/5-3/1-3/4 (Fig. 8B) that enhances the spectral difference between different rock types has been effective in
resolving this contact. The Upper Limestone appears yellowish-orange in the band-ratio image whereas the Lower
Limestone which is gypsum-bearing appears red due to
dominance of band-ratio 4/5. The contact between the
Lower Limestone and the upper part of the Lower Sandstone is defined by glauconite-bearing sandy mudstone.
The glauconite-bearing unit appears green in the bandratio image indicating dominance of the band-ratio 3/1
in the RGB color combination. This is because of the
145
high differences in spectral reflectance and absorption features of glauconite minerals in ASTER bands 3 and 1,
respectively.
The Gorge of the Nile is characterized by sub-horizontal
layering of sedimentary and volcanic rocks partially covered (less than 10%) with a thin veneer of colluvial deposits.
These deposits are, in most cases, the products of the same
corresponding lithology. Hence, the colluvial deposits have
the same spectral characteristics as the parent rocks. Therefore, their presence does not mask the underlying lithological units.
Using ASTER bands and band-ratio images together
with ASTER DEMs shows significant improvement of lithological mapping in the Gorge of the Nile compared to existing geological maps (Stefannini, 1933; Merla et al., 1973;
Mangesha et al., 1996). Various units in the Dejen–Gohatsion region have been mapped in better detail than any existing geological map. In addition, the glauconite-bearing unit
is mapped for the first time in this study using its spectral
characteristic. This unit can be traced throughout the Gorge
of the Nile using this technique, hence providing valuable
information on marine transgression surfaces.
5.3. Structural mapping
ASTER, RADARSAT, ASTER-RADARSAT fused
image and ASTER images draped over ASTER DEMs
Fig. 9. Criteria for extracting lineaments from the remote sensing images: (A) Radar shadow-illumination on RADARSAT image. (B) Drainage deflection
on ASTER-RADARSAT fused image. (C) Bedding truncation on ASTER image. (D) 3D geometry of structural features on the ASTER image draped
over ASTER DEM. The locations of the images are shown in Fig. 8. The dashed lines in each of the image indicate lineaments.
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N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
(Figs. 8A, C, D and E) are used to extract lineaments in
the Dejen–Gohatsion region, that might be related to
faults associated with the Afar Depression and the Main
Ethiopian Rift. The orientation of these lineaments might
have contributed to the geological controls on this portion of the Gorge of the Nile. Four criteria are used to
identify lineaments in the remote sensing data: (1) Radar
shadow-illumination that reveals morphologically-defined
structures as an enhanced array of brightness (Fig. 9A);
(2) Drainage deflection (Fig. 9B); (3) Truncation of bedding and layering, and cross-cutting relationship
(Fig. 9C); (4) Using the ASTER image draped over the
ASTER DEM to view these structures in 3D and ensure
that they have significant along—strike extent (Fig. 9D).
The main trends of the lineaments that have been
extracted from the remote sensing data and verified in
the field are mostly NE and NW (Figs. 10A, B and C).
These are present as both map-scale and mesoscopic normal faults (Figs. 10B and C). Zewde et al. (1997) concluded that the major faults and fractures in the Gorge
of the Nile are N–S and NE–SW trending, based on seismic refraction surveys, resistivity profiling and vertical
electrical soundings aimed at locating dam sites.
N-trending lineaments are characterized by long escarpments defined by radar shadow-illumination in the
RADARSAT image (Figs. 8C and 9A). The NW-SE and
NE-trending lineaments form long morphological features
obvious in the ASTER images draped over ASTER DEMs
(Figs. 8E and 9D). In addition, lineaments of these trends
form short segments that are defined by drainage deflection
(Fig. 9B). Moreover, the NE–SW trending lineaments
(Fig. 9A) are emphasized by the presence of ridge lines that
are aligned in this direction, obvious in RADARSAT and
ASTER-RADARSAT fused images (Figs. 8C and D).
6. Discussion
6.1. Geological controls on the Gorge of the Nile
This study shows a number of possible geological
factors controlling the Gorge of the Nile. The Ethiopian
Plateau is dominated by an active drainage system including the Blue Nile and its tributaries (Beaulieu and
Gaonac’h, 2002). The Gorge of the Nile is situated on an
uplifted plateau and flanks of tectonically active rift
systems. The Blue Nile and its tributaries have been deeply
incised towards the lowlands of Sudan.
The first order control on the Gorge of the Nile is the
base-level adjustment as in the case of every river where
uplift is significant. Humphrey and Konrad (2000)
proposed that a river’s sediment flux and tectonic uplift
are the most important variables for the river’s incision.
The Ethiopian Plateau was uplifted due to the Afar mantle
plume followed by rifting (Hofmann et al., 1997). The
uplift of the Ethiopian Plateau was initiated 30 Ma years
ago (McDougall et al., 1975). The rate of uplift was
Fig. 10. (A) Structural map of the Gorge of the Nile around Dejen and Gohatsyion towns generated from the interpretation of 7-3-1 ASTER,
RADARSAT, ASTER-RADARSAT fused image and 3D perspective views, and field studies. For location see Fig. 8. Starmarks indicate fault location in
Figs. 8B and C. (B) Map-scale normal fault which resulted in the drop of the Tertiary basalt against the Upper Limestone at the same elevation. (C)
Mesoscopic normal faults deforming sandstone unit. The faults are emphasized by a 50–70 cm thick thinly bedded mudstone.
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
0.1 mm/yr in the Middle Tertiary to Quaternary times
(McDougall et al., 1975). The rate of uplift became more
rapid in the Pliocene-Pleistocene (Mohr, 1967) and reached
0.5–1.0 mm/yr during the Late Pleistocene (McDougall
et al., 1975). Due to the continuous uplift, the Blue Nile
and its tributaries cut deeply into the plateau to maintain
the equilibrium of its baseline, which has become successively lowered in course of time by first eroding the volcanic rocks, and then incised deep into the sedimentary
rocks possibly due to rapid base-level fall (Ayalew and
Yamagishi, 2004; Fig. 11), and subsequently into basement
rocks. This is similar to the Grand Canyon where uplift
of the Colorado Plateau forced the Colorado River to
incise deeply into sedimentary rocks (Chernicoff and
Venkatakrisnan, 1995).
The spatial distribution of the shield volcanoes which
are aligned approximately N–S might have also played a
role in controlling the Gorge of the Nile. Mt. Gish,
Yacandach, Choke, Guna, and Guguftu are five shield volcanoes in the vicinity of the Gorge of the Nile (Figs. 2B and
6B). These volcanoes present positive relief that rises up to
4000 m above MSL. The age of these volcanoes ranges
between 10.7 and 22.3 Ma (Kieffer et al., 2004). The Blue
Nile circles around Mts. Gish, Yacandach and Choke as
it flows SE, S and SW and, it is bounded in the NE by
Mt. Guna and in the E by Mt. Guguftu (Figs. 2B and
6B). This suggests that the build-up of some of these volcanoes might have forced the Blue Nile drainage to migrate
towards the NE, E and SE through time. In addition, the
building of Mts. Gish and Guna might have forced the
Blue Nile to flow SE as it emerged from Lake Tana
147
(Fig. 2B). Mts. Yacandach, Choke and Guguftu also might
have forced the Blue Nile to flow to the S (Fig. 6B). Subsequently the uplift of the western flank on the Afar Depression and the Main Ethiopian Rift might have forced the
Blue Nile to flow to the SW (Weissel et al., 1995).
Obstruction and diversion due to faulting represents a
second order control on the Gorge of the Nile as exemplified by the Dejen–Gohatsion region where the Blue Nile
flows SW. The geological map that has been produced
from remote sensing, DEMs and field data (Fig. 11A)
reveals the second order controls on the Gorge of the Nile
in the form of morphologically- and lithologically-defined
faults (Fig. 11A). Many of these faults have been mapped
for the first time. This region is dominated by NW- and
NE-trending normal faults (Figs. 10A and 11A). The Blue
Nile course in this segment coincides with a major NEtrending normal fault (Figs. 10A and 11A) with the region
SE of the river representing the footwall and that to the
NW the hanging wall (Fig. 11B). The throw of the fault
is in the order of 70 m measured using the glauconite
layer as a marker horizon (Fig. 11B). This major fault is
associated with numerous NE- and NW-striking faults that
also play a significant role in controlling the Gorge of the
Nile. These faults might be conjugate sets related to the
NW-trending faults and E-W transverse faults associated
with the Afar depression and the Main Ethiopian Rift.
The third order controls on the Gorge of the Nile
include differential incision with respect to spatial distribution of lithological units and channel geometry that influenced the drainage network. In the Tertiary–Quaternary
basalts, the width and depth of the Gorge of the Nile
reaches 23 km and 1150 m respectively, and the river
gradient is 4 m/km (Figs. 7A, B and D). The overall
drainage pattern is dendritic (probably due to uniform
resistance of aerially extensive basaltic bedrocks) and
radial (due to localized uplift). Locally, the drainage patterns are parallel to trellis-like and rectangular in design
(due to steep gradient, presence of faults, fractures and
joints). Within the Upper Sandstone the drainage pattern
is trellis-like and controlled by faults and joints that might
have facilitated incision to a depth of 1245 m (Figs. 6A
and 7A). In the Lower Sandstone, the depth of the gorge
increases to 1570 m with the development of trellis and
dendritic drainage patterns (Figs. 6A and 7A). Finally in
the Neoproterozoic basement rocks, the river incised to a
depth of 1.6 km and increases its width to 50 km, facilitating development of dendritic drainage patterns (Figs.
6A and 7A).
6.2. The usefulness of remote sensing data integration for
geoscientific studies
Fig. 11. (A) Geological map of Dejen–Gohatsion part of the Gorge of the
Nile generated from the interpretation of the remote sensing data, DEMs
and field studies. (B) NW–SE geological section across the Gorge of the
Nile. The topographic profile is extracted from the ASTER DEMs.
SRTM DEMs proved useful for geomorphology and
river geometric analyses of the Gorge of the Nile (Figs. 6
and 7). SRTM data are useful for detailed drainage network analysis of vast regions. Regardless of the poorer spatial resolution of SRTM compared to ASTER DEMs,
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valuable geomorphological information can be extracted
from the former. The spatial resolution of SRTM DEMs
is sufficiently adequate and superior over the GTOPO30
DEMs (1 km spatial resolution) which have been routinely used for such studies.
Analysis of the 7-3-1 ASTER image proved significantly
useful for lithological mapping (Fig. 8F). Moreover,
ASTER band-ratio image shows better results than
ASTER band image for mapping lithological units because
the band-ratio image enhances subtle variation in spectral
characteristics. The 4/5-3/1-3/4 ASTER band-ratio image
has been useful in mapping different lithological units
(Fig. 8B) because many rocks in the Gorge of the Nile have
characteristic absorption and reflection features in these
bands. However, band-ratio images sometimes suppress
albedo variation. Some geological materials have similar
spectral characteristics but different albedo, hence might
be difficult to identify in the band-ratio images.
RADARSAT images are useful in mapping morphologically-defined structures because of normal reflection, double bounce and edge effect of faults and escarpments
(Abdelsalam et al., 2000). In addition, RADARSAT data
proved viable to aid structural mapping in inaccessible
remote areas (Robinson and Kusky, 1999). Moreover,
radar data are also effective in lithological mapping on
the basis of surface roughness (Zebker et al., 1996;
Abdelsalam et al., 2000).
Fusing ASTER with RADARSAT data sharpens the
edges that are not evident in the ASTER images
(Fig. 8D). Combining ASTER images with RADARSAT
images using the CNT data fusion maximizes the amount
of extractable geological information. One disadvantage
of data fusion is that the fused image will have poorer spatial resolution compared to ASTER RGB color composite
images, because the intensity in the ASTER image (15 m
spatial resolution) is replaced by RADARSAT gray-scale
image whose spatial resolution is 30 m.
Combining ASTER remote sensing data with 3D DEMs
advances mapping of lithological units compared to when
only ASTER data are used (Bedell, 2001; Rowen and
Mars, 2001; Lang and Abdelsalam, 2002). Draping
ASTER images over ASTER DEMs in order to generate
3D perspective views advances lithological mapping, especially in regions where sub-horizontal bedding and layering
dominate in slightly deformed series of sedimentary or effusive volcanic rocks (Fig. 8E). Such remote sensing image
processing allows incorporating spectral information with
topographical and morphological information, providing
3D geometrical and spatial relationships of inaccessible
areas such as the Gorge of the Nile (Fig. 8E). One of the
advantages of ASTER DEMs is their high spatial resolution for detailed visualization and analysis. ASTER
multi-spectral and DEM data are acquired by the same
instrument, hence the two datasets can easily be georeferenced and used together. However, ASTER data lack complete geographic coverage and sometimes can be affected
by cloud cover.
7. Conclusions
The possible geological controls on the Gorge of the
Nile include: (1) Base level adjustment with regional uplift
of the Ethiopian Plateau and flexural uplift of the western
escarpment of the Main Ethiopian Rift and the Afar
Depression; (2) spatial distribution of the shield volcanoes;
(3) Drainage deflection and obstruction by faulting that
might be associated with Main Ethiopian Rift and Afar
Depression; and (4) Preferential incision due to interplay
of lithology and drainage network and channel geometry.
SRTM and ASTER DEMs covering the Gorge of the
Nile are a significant improvement in analyzing the relationship between geological and geomorphological features
because these DEMs show greater topographic details that
are useful for detailed drainage studies of geographically
extended areas. Such DEMs allow us to characterize the
spatial relationship between the river and regional geomorphological features such as obstacle relief formed by
Tertiary–Quaternary shield volcanoes whose accumulation
might have contributed to the formation of the Gorge of
the Nile and the Blue Nile Bend.
Integration of optical-radar remote sensing data and 3D
DEMs has proven useful in understanding possible geological controls on the Gorge of the Nile. Optical-radar data
fusion (ASTER-RADARSAT) helped in better understanding the spatial distribution of lithological units and
geological structures. Draping of the ASTER images over
ASTER DEMs enabled visualization and analysis of the
3D geometry of geological and geomorphological features.
Acknowledgements
This work is funded by National Science Foundation.
The authors would like to thank the National Aeronautics
and Space Administration (NASA) Jet Propulsion Laboratory (JPL) for SRTM data and the Earth Resources Observatory System (EROS) for ASTER data. RADARSAT
data are acquired through a NASA grant. Thanks are also
due to the Geological Survey of Ethiopia for co-operation
during the field work of this project. Special thanks to S.
Gerra, R. Gani, and L. Smith for their assistance in the
field. We would like to thank R. Stern for reviewing an
early version of the manuscript. We thank S. Drury, G.
Kozminski and an anonymous reviewer for their detailed
review of the manuscript. This is the University of Texas
at Dallas, Department of Geosciences Contribution number 1067.
References
Abdelsalam, M.G., Robinson, C., El-Baz, F., Stern, R.J., 2000. Application of orbital imaging radar for geologic studies in arid regions: The
Saharan testimony. Photogrammetric Engineering and Remote Sensing 66, 717–726.
Assefa, G., 1979. Clay mineralogy of the Mesozoic sequence in the Upper
Abay (Blue Nile) River Valley region. Ethiopian Journal of Science
(Sinet) 3, 37–65.
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
Assefa, G., 1980. Stratigraphy and sedimentation of the type Gohatsion
Formation (Lias-Malm) Abay River basin, Ethiopia. Ethiopian
Journal of Science (Sinet) 3, 87–110.
Assefa, G., 1981. Gohatsion Formation: A new Liasmal lithostragraphic
unit from the Abby River basin, Ethiopia. Geoscience Journal 2,
63–88.
Assefa, G., 1991. Lithostratigraphy and environment of deposition of the
Late Jurassic-Early Cretaceous sequence of the central part of
Northwestern Plateau, Ethiopia. Neues Jahrbuch Fur Geologie Und
Palaontologie-Abhandlungen 182, 155–284.
Ayalew, L., Yamagishi, H., 2004. Slope failures in the Blue Nile basin, as
seen from landscape evolution perspective. Geomorphology 57,
95–116.
Beauchamp, J., 1977. Paleofloral dating of volcanics on the Ethiopian
plateau (Shoa). Bull. Geophys. Obs., Addis Ababa 15, 125–131.
Beauchamp, J., Lemoigne, Y., 1974. Presence d’un basin de subsidence en
Ethiopie centrale et essai de reconstruction paleogeographique de
l’Ethiopie durant le Jurassique. Bulletin of the Geological Society of
France 7, 563–569.
Beaulieu, A., Gaonac’h, H., 2002. Scaling of differentially eroded surfaces
in the drainage network of the Ethiopian Plateau. Remote Sensing of
Environment 82, 111–122.
Bedell, R.L., 2001. ASTER satellite remote sensing; a significant leap in
factual geologic mapping. Abstracts with programs. Geological
Society of America 33, 348.
Canuti, P., Radrizzani, C.P., 1975. Microfacies from the upper portion of
the Antalo limestone (Blue Nile sequence, Ethiopia). Rev. Esp.
Micropal. 7, 131–147.
Chernet, T., 1988. Hydrogeological map of Ethiopia (1:2,000,000).
Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia.
Chernicoff, S., Venkatakrisnan, R., 1995. Geology: An Introduction to
Physical Geology. Worth Publishers, 593 pp.
Collins, R.O., 2002. The Nile. Yale University Press, 384 pp.
Coulie, E., Quidelleur, X., Gillot, P.Y., Coutillot, V., Lefevre, J.C.,
Chiessa, S., 2003. Comparative K-Ar and Ar/Ar dating of Ethiopian
and Yemenite Oligocene volcanism: implication for timing and
duration of the Ethiopian traps. Earth and Planetary Science Letters
206, 477–492.
Dainelli, P.A., 1970. Photogeology of the Debre Marcos, Blue Nile area,
southern Gojam, Ethiopia. Mem. Soc. Geol. It 9, 779–791.
Ficcarelli, G., 1968. Fossili giuresi della serie sedimentaria del Nilo
Azzurro. Riv. It. Paleont. Strat. 74, 23–50.
Grabham, G.W., Black, R.P., 1925. Report of the Mission to Lake Tana
1920–21, Government Press, Cairo.
Hofmann, C., Courtillot, V., Feraud, G., Rochette, P., Yirgu, G.,
Ketefo, E., Pik, R., 1997. Timing of the Ethiopian flood basalt
event and implications of Plume birth and global change. Nature
389, 838–841.
Humphrey, N.F., Konrad, S.K., 2000. River incision or diversion in
response to bedrock uplift. Geology 28, 43–46.
Jensen, J.R., 2000. Remote Sensing of the environment. Princeton Hall,
New Jersey, 544 pp.
Jepsen, D.H., Athearn, M.J., 1961a. A general Geologic map of the Blue
Nile River Basin, Ethiopia (1:1,000,000). Department of Water
Resources, Addis Ababa.
Jepsen, D.H., Athearn, M.J., 1961b. Geological plan and section of the
left bank of the Blue Nile canyon near crossing of Addis Ababa—
Debre Markos road. Ethiopia’s Water Resources Department and US
Department of Interior Internal Report, Addis Ababa, 179 pp.
Jepsen, D.H., Athearn, M.J., 1964. Land and water resources of the Blue
Nile basin. Ethiopia’s Water Resources Department and US Department of Interior Internal Report, Addis Ababa, 221 pp.
JPL, 1999. Module 2: How radar imaging works: Interferometry,
Pasadena. Jet Propulsion Lab, CA, SIR-C educational web site.
Kalb, J., Oswald, E.B., 1974. Report documenting Mesozoic invertebrate
fossil localities in the Blue Nile Gorge, Ethiopia Monograph, 23 pp.
Kazmin, V., 1973. Geological map of Ethiopia. Geological Survey of
Ethiopia, Addis Ababa, Ethiopia.
149
Kazmin, V., 1975. Explanation of the geological map of Ethiopia.
Geological Survey of Ethiopia Bulletin 1, 1–15.
Kieffer, B., Arndt, N., Lapierre, H., Bastien, F., Bosch, D., Pecher, A.,
Yirgu, G., Ayalew, D., Weis, D., Jerram, D.A., Keller, F., Meugniot,
C., 2004. Flood and shield basalts from Ethiopia: Magmas from the
African Superswell. Journal of Petrology 45, 793–834.
Krenkel, E., 1926. Abessomalien (Absessinien und Somalien). Handbook
of Regional Geolology, Heidelberg 7, 8 pp.
Lang, H., Welch, R., 1999. ATBD-AST-08 Algorithm theoretical basis
document for ASTER digital elevation models (Standard Product AST
14). Version 3.0, University of Georgia, 69 pp.
Lang, N., Abdelsalam, M.G., 2002. Geologic mapping in arid regions with
ASTER data; an example from NW Arizona. Abstracts with
programs. Geological Society of America 34, 4.
Mangesha, T., Chernet, T., Haro, W., 1996. Geological map of Ethiopia
(1:2,000,000), Geological Survey of Ethiopia, Addis Ababa, Ethiopia.
Marty, B., Pik, R., Yirgu, G., 1996. Helium isotope variation in Ethiopian
plume lavas: Nature of magmatic sources and limit on lower mantle
contribution. Earth and Planetary Science Letters 144, 223–237.
McDougall, I., Morton, W.H., William, M.A.J., 1975. Ages and rates of
denudation of trap series basalts at the Blue Nile Gorge, Ethiopia.
Nature 254, 207–209.
Merla, G., Abbate, E., Canuti, P., Tacconi, P., 1973. Geological map of
Ethiopia and Somalia. Cons. Naz. Ricerche, Firenze.
Mohr, P., 1962. The Geology of Ethiopia. Addis Ababa University Press,
268 pp.
Mohr, P., 1967. Review of the Geology of Simien Mountains. Bulletin of
Geophysical Observatory, vol. 10. Addis Ababa University, Addis
Ababa, Ethiopia, 79–93.
Pik, R., Marty, B., Carignan, J., Lave, J., 2003. Stability of Upper Nile
drainage network (Ethiopia) deduces from (U–Th)/He thermochronometry: Implication of uplift and erosion of the Afar plume dome.
Earth and Planetary Science Letters 215, 73–88.
Ramadan, T.M., Abdelsalam, M.G., Stern, R.J., 2001. Mapping goldbearing massive sulfide deposits in the Neoproterozoic Allaqi suture,
southeast Egypt with Landsat TM and SIR-C/X SAR images.
Photogrammetric Engineering and Remote Sensing 67, 491–497.
Raney, K., Luscombe, A.P., Langham, E.J., Ahmed, S., 1991. RADARSAT (SAR imaging). Proceedings of the IEEE 79, 839–849.
Ritsema, J., Heijst, H., 2000. New seismic model of the upper mantle
beneath Africa. Geology 28, 63–66.
Robinson, C.A., Kusky, T., 1999. Results of structural mapping using
Radarsat data obtained for the Sleetmute and Medfra areas, Alaska.
Abstracts with programs. Geological Society of America 31, 176.
Rowen, L., Mars, J.C., 2001. Advances in lithologic mapping by using
optical remote sensing measurements. Abstracts with programs.
Geological Society of America 33, 347.
Russo, A., Assefa, G., Atnafu, G., 1994. Sedimentary evolution of the
Abby River (Blue Nile) Basin, Ethiopia. Neues Jahrbuch Fur Geologie
Und Palaontologie-Monatshefte 5, 291–308.
Said, R., 1993. The River Nile: Geology, Hydrology and Utilization.
Pergamon Press, 320 pp.
Shi, Z., Fung, K.B., 1994. A comparison of digital speckle filters. In:
Proceedings of International Geoscience and Remote Sensing Symposium 94, pp. 2129–2133.
Stefannini, G., 1933. Saggio di una carta geologica dell’Eritrea, della
Somalia e dell’Ethiopia, 1:2,000,000. Note Illustrative, Firenze, 195 pp.
Strahler, A.N., 1952. Hypsometric (area–altitude) analysis of erosinal
topography. Geological Society of America Bulletin 63, 1117–1142.
Vrabel, J., 1996. Multispectral imagery band sharpening method.
Photogrametry Engineering and Remote Sensing 62, 1075–1083.
Weissel, J.K., Malinverno, A., Harding, D.J., 1995. Erosional development of the Ethiopian plateau of Northeast Africa from fractal
analysis of topography. In: Barton, C.C., La Pointe, P.R. (Eds.),
Fractals in Petroleum Geology and Earth Processes. Plenum Press,
New York, pp. 127–142.
Wood, C.B., Zavada, M.R., Goodwin, M.B., Clemens, W.A., Schaff,
C.R., 1997. First palenostratigraphic dates for Mesozoic vertebrate
150
N. DS Gani, M.G. Abdelsalam / Journal of African Earth Sciences 44 (2006) 135–150
faunas in the Blue Nile Gorge region, Ethiopia. Journal of Vertebrate
Paleontology 17, 86.
Zebker, H.A., Rosen, P., Hensley, S., Mouginis-Mark, P.J., 1996. Analysis
of active lavaflows in Kilauea volcano, Hawaii, using SIR-C radar
correlation measurements. Geology 24, 495–498.
Zewde, T., Wubishet, M., Damtew, K., Mengesha, T., Belayneh, S.,
Tareke, F., 1997. Engineering geophysical investigations for Abay
River basin integrated master plan project. Ethiopian Institute of
Geological Survey, Geophysics Department Internal Report,
105 pp.