Morpho-tectonic analysis of the Tekeze River and the Blue Nile drainage systems on the Northwestern...

Morpho-tectonic analysis of the Tekeze River and the Blue Nile drainage systemson the Northwestern Plateau, Ethiopia

Elamin H. Ismail, Mohamed G. Abdelsalam ⇑Missouri University of Science and Technology, Department of Geological Sciences and Engineering, Rolla, MO 65409, USA

a r t i c l e i n f o

Article history:Received 11 February 2012Received in revised form 27 April 2012Accepted 28 April 2012Available online 8 May 2012

Keywords:Morpho-tectonicsEthiopian PlateauTekeze RiverBlue Nile

a b s t r a c t

This study examines the morpho-tectonic evolution of the drainage system in the Northwestern Plateauin Ethiopia dominated by the Tekeze River and the Blue Nile. The Northwestern Plateau is underlain byPrecambrian crystalline rocks, followed by Mesozoic sedimentary section and topped with Oligocene–Quaternary volcanic rocks. The plateau is bounded in the east and southeast by the Afar Depressionand the Main Ethiopian Rift, respectively. Digital Elevation Models (DEMs) from the Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) and the Shuttle Radar Topography Mission(SRTM) data are analyzed to extract morpho-tectonic parameters including the Normalized SteepnessIndex (Ksn), the Concavity (h) and the Regression Fit (r2) between the observed and predicted channel pro-files from the sub-basins and the tributaries of the Tekeze River and the Blue Nile. Analysis of these mor-pho-tectonic parameters has shown that the evolution of the drainage systems on the NorthwesternPlateau was influenced by three tectonic and geological events. The first event resulted in a broad andregional uplift of the plateau, most likely due to the rise of the Afar mantle plume �30 Ma. This regionaluplift was accompanied by moderate incision rate of the Tekeze River and the Blue Nile drainage systemswithin the entire Northwestern Plateau. The second event, which was in the form of shield volcanoesbuild-up, occurred at �22 Ma and resulted in localized increase in the incision rate around these volca-noes. The third event is manifested by rift-flank uplift at �11 Ma on the western escarpments of the AfarDepression and the northwestern escarpments of the Main Ethiopian Rift. This event resulted in anincrease in the incision rate of the Tekeze River and the Blue Nile drainage systems, but this increaseseems to diminish towards the west and northwest leaving the drainage systems in the lower reachesof the two rivers relatively tectonically undisturbed, hence allowing for the establishment of a long-livedhydrological stability.

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1. Introduction

Significant research has been conducted to understand theinterplay between geomorphology and tectonics in continentalcollision regimes such as the Alps, the Himalayas, the Tibetan pla-teau, and the Colorado plateau (e.g. Pederson et al., 2002; Clarket al., 2004; Schoenbohm et al., 2004; Whipple, 2004; McMillanet al., 2006; Wobus et al., 2006; Cyr et al., 2010; Ferraris et al.,2012). However, less research effort has been devoted to under-stand the morpho-tectonic evolution of drainage systems in activecontinental extensional regimes such as the Ethiopian Plateau. TheEthiopian Plateau (Fig. 1) being one of the largest uplifted regionson Earth (the Wall of Africa) was formed during extensional tec-tonics associated with the Afar mantle plume and the opening ofthe East African Rift System which is represented in Ethiopia bythe Afar Depression and the Main Ethiopian Rift (Fig. 1). This region

provides the geoscientific community a unique opportunity toexamine drainage evolution within continental extensional tec-tonic regimes and to examine the role of tectonic uplift in the evo-lution of regional drainage systems.

The Ethiopian Plateau is underlain by Precambrian crystallinerocks and Mesozoic sedimentary rocks, and topped by 1–3 kmthick Oligocene–Quaternary volcanic rocks (Fig. 2). Through time,rivers have incised deeply into the Ethiopian Plateau forming insome places spectacular canyons that can be as deep as 1.5 km.The plateau is dissected by the Afar Depression and the Main Ethi-opian Rift into the Northwestern Plateau, dominated by the TekezeRiver and the Blue Nile drainage systems and the SoutheasternPlateau occupied by the Wabi Shebelle and Ter Shet drainagesystems (Figs. 1 and 2).

Few studies have been carried out to examine drainage evolu-tion in the Ethiopian Plateau (McDougall et al., 1975; Weisselet al., 1995; Pik et al., 2003; Gani and Abdelsalam, 2006; Ganiet al., 2007). These studies have only focused on the Blue Nile andits gorge, the Gorge of the Nile (Fig. 2) which is arguably the rival

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⇑ Corresponding author.E-mail address: [email protected] (M.G. Abdelsalam).

Journal of African Earth Sciences 69 (2012) 34–47

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of the Grand Canyon of the US. Two contrasting end-member mod-els have emerged from these studies: (1) Steady and constant rateof incision of the Blue Nile on the Northwestern Plateau since thebeginning of the incision �25–29 Ma ago (Pik et al., 2003); and(2) significant increase in the rate of incision of the Blue Nile at�10 and �6 Ma (Gani et al., 2007).

The Pik et al. (2003) steady–steady model relied on resultsobtained from Apatite/He ages of samples collected from the Pre-cambrian crystalline rocks exposed along the lower reaches ofthe Blue Nile. Pik et al. (2003) concluded that the upper Blue Niledrainage system went through a long-term stability since the

beginning of its incision �25–29 Ma. Additionally, Pik et al.(2003) suggested that the current hydrological characteristics ofthe Blue Nile drainage system were established early in the Oligo-cene time. This implies that the uplift of the Northwestern Plateauand its current physiographic features that control most of thepresent-day Blue Nile hydrology were established since theOligocene.

Alternatively, Gani et al. (2007) suggested a model that involvesan increased incision rate through time driven by pulsed plateauuplift. Gani et al. (2007) results were based on Geographic Informa-tion System (GIS) modeling of the Shuttle Radar Topography

Fig. 1. Hill-shade Digital Elevation Model (DEM) extracted from the Global 30 Arc Second Elevation Data (GTOPO30, 1 km spatial resolution) showing the location of theTekeze River and the Blue Nile catchment area in the Northwestern Plateau in Ethiopia. MER = Main Ethiopian Rift.

Fig. 2. Geological map of Ethiopia showing the Ethiopian Plateau underlain by Precambrian crystalline rocks and topped with Oligocene–Quaternary volcanic rocks withMesozoic sedimentary rocks between them. 1 = Blue Nile; 2. = Tekeze River; 3 = Wabi Shebelle and Ter Shet Drainage System. Red lines represent the approximate trends ofthe Afar Depression and Main Ethiopian Rift border faults. Modified after Tefera et al. (1996). (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

E.H. Ismail, M.G. Abdelsalam / Journal of African Earth Sciences 69 (2012) 34–47 35

Mission (SRTM) Digital Elevation Model (DEM) data and ages ofvolcanic rocks that occur at different elevations in the Northwest-ern Plateau. In this model, Gani et al. (2007) proposed three phasesof incision driven by tectonic uplift with an increased rate of inci-sion through time. The first phase of incision, which dominated theperiod between 29 and 10 Ma, was characterized by slow and long-term incision that gradually increased from 53 mMa�1 to80 mMa�1. The second phase which occurred between 10 and6 Ma was characterized by moderate rate of incision that increased

from 80 mMa�1 to 120 mMa�1. The most recent incision phase oc-curred between 6 Ma and present where the long-term incisionrate increased rapidly from 120 mMa�1 to 320 mMa�1. Ganiet al. (2007) concluded that much of the increase in the incisionrate can be attributed to tectonic uplift, possibly associated withthe rise of the Afar mantle plume, rather that climate change. Thisconclusion is based on that the timing of the increase of the inci-sion rate of the Blue Nile in the Northwestern Plateau having beenout-of-phase with any enhanced precipitation in the region. Gani

Fig. 3. Hill-shade Digital Elevation Model (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data showing the Tekeze River and the Blue Nile drainagesystems, and geological and tectonic features in the region.

36 E.H. Ismail, M.G. Abdelsalam / Journal of African Earth Sciences 69 (2012) 34–47

et al. (2007) based this conclusion on that during the late Mioceneto Pliocene (8–3 Ma) east Africa was undergoing aridification asdocumented by a shift from wood-dominated vegetation tograss-dominated vegetation (Cerling et al., 1997), paleo-climaticdata (Cane and Molnar, 2001) and numerical modeling on the linkbetween the rising topography of east Africa during the late Mio-cene and aridification (Sepulchre et al., 2006).

This work aims at understanding the morpho-tectonic evolu-tion of the drainage systems in the Northwestern Plateau by focus-ing on the Tekeze River and the Blue Nile. It relies on GIS analysis ofDEMs extracted from the Advanced Spaceborne Thermal Emissionand Reflection Radiometer (ASTER) and SRTM data, and publishedgeological information. Specifically, this work: (1) Presents the

geological context of the Tekeze River and the Blue Nile drainagesystems; (2) Use morpho-tectonic analysis including: (A) Thedelineation of sub-basins of the two rivers and the calculation oftheir Normalized Steepness Index (Ksn) to evaluate the effect of tec-tonic uplift on the evolution of these sub-basins; (B) the extractionof longitudinal profiles and the computation of the Concavity (h) toevaluate the relative effect of tectonic uplift on the incision rates ofthese tributaries. (C) The calculation of the Regression Fit (r2) be-tween the observed and predicted channel profiles of the TekezeRiver and the Blue Nile tributaries from the longitudinal profilesand the h values to evaluate the maturity of these channels. (3)Use these results to discuss the evolution of the Tekeze River andthe Blue Nile drainage systems on the Northwestern Plateau.

Fig. 4. The Tekeze River and the Blue Nile Horton–Strahler Streams Order showing the two drainage systems as sixth order streams. The streams order are obtained from theShuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) using RiverTool software which is an extension for the Environment for Visualizing Images (ENVI)software.


2. Overview of the Tekeze River and the Blue Nile

The Tekeze Rive and the Blue Nile drainage systems are locatedbetween latitudes �8�300 and �15�000N and longitudes �36�000

and �40�000E (Fig. 3). The length of Tekeze River in Ethiopia is�608 km and its average elevation is �1850 m above sea level(asl). The river flows in a north direction from its southernmostreaches, then circles Mount Ras Dashen in a semi-circular loopby flowing northeast, then north, then northwest before descend-ing towards the low lands of the Sudan, (Fig. 3). The Tekezedrainage system spans an area of �69,000 km2 bounded by theAxum–Adigrat lineament in the north, the western escarpment ofthe Afar Depression in the east and the Simen ranges in the west,with an elevation ranging between 2500 and 4639 m (Figs. 3 and4). The Simen ranges extend from Mount Ras Dashen (4639 m asl)in the north to Mount Guna (4231 m asl) in the southeast (Fig. 3).These ranges continue to the east to Mount Abuna Yosef(4190 m asl) and merge into the western escarpment of the AfarDepression. These elevated mountain ranges separate the TekezeRiver drainage basin from the Blue Nile drainage basin (Fig. 3).

The Blue Nile, which is �900 km long in Ethiopia, originatesfrom small rivulets in the Springs of Sakala (Fig. 3). These rivuletsjoin each others to from Gilgil Abay which flows northward toempty into Lake Tana (Fig. 3). The river emerges from Lake Tanaas Abay or the Blue Nile where it makes a �150 km radius looparound Mount Choke (4070 m asl) by flowing first southeast, then

south, then southwest before ultimately traveling northwestwardtowards the low land of the Sudan (Figs. 3 and 4). The total catch-ment area of the Blue Nile drainage system is estimated to be�250,000 km2 (Fig. 4) bounded between the elevated ranges ofMount Guna in the north, the western escarpment of the AfarDepression and the northwestern escarpments of the Main Ethio-pian Rift in the east and southeast, respectively (Fig. 3).

3. Tectonic and uplift history

Three tectonic and geological events have shaped the presentmorphology of the Northwestern Plateau. The oldest of theseevents was the eruption of the �30 Ma volcanic rocks which cov-ered an area of �500,000 km2 and it is inferred to mark theappearance of the Afar mantle plume (Mohr and Zanettin, 1988;Hofmann et al., 1997; Sengor, 2001; Kieffer et al., 2004). Thisevent results in the burial of whatever drainage system that dom-inated the region prior to the Oligocene. It is likely that the erup-tion of the �30 Ma volcanic rocks was on a peneplain as indicatedby the relatively flat base of the volcanic pile extending for hun-dreds of kilometers (Fig. 5A and B). It is suggested that the erup-tion of the plume-related volcanic rocks was accompanied by aregional uplift resulting in the formation of the Afar dome(Sengor, 2001). This event was followed at �22 Ma by shield vol-cano-building episodes (Kieffer et al., 2004). Plume-related uplift

Fig. 5. (A) Geological map of the Tekeze River and the Blue Nile catchment area modified after Tefera et al. (1996). (B) Geological section across the Northwestern Plateauencompassing the catchment area of the Tekeze River and the Blue Nile.


caused deep-seated faults within the Ethiopian lithosphere, lead-ing to the collapse of the Afar dome; a collapse that formed theAfar Depression at �24 Ma (Beyene and Abdelsalam, 2005). Thiswas followed by opening of the Main Ethiopian Rift at �11 Maas suggested by Wolfenden et al. (2004). However, Bonini et al.(2005) argued that extension forming the Main Ethiopian Riftstarted between 6 and 5 Ma. Another phase of shield volcano-building episodes took place at �11 Ma (Kieffer et al., 2004) sug-gested to be related to the early phase of the Main Ethiopian Riftformation (Wolfenden et al., 2004).

Geological observation indicates that the plateau had witnesseda significant amount of tectonic uplift. The presence of the lime-stone of the Late Jurassic Antalo formation which is overlain bythe continental fluvial sandstone of the Cretaceous Amba Aradomformation (Fig. 5A and B) suggests that the plateau was belowsea level �150 Ma ago (Gani et al., 2007). At the Gorge of the Nile,this contact today is at an elevation of 2.2 km indicating that therock uplift of the Ethiopian Plateau since �150 Ma is over 2 km.Gani et al. (2007) suggested that much of this uplift occurred inthe past 30 Ma through the combined effects of the rising Afarmantle plume and rift-flank uplift of the Afar Depression and theMain Ethiopian Rift. This is in good agreement with suggestionsby Sengor (2001) that the Afar mantle plume reached the base ofthe African lithosphere by the Middle Eocene resulting in regionaluplift dominated by the Afar dome which reached a radius of�1000 km. This was followed by outpouring of extensive(1–3 km thick) flood basalts erupted at �30 Ma over less than�1 Ma period of time (Hofmann et al., 1997). It is worth mention-ing here that not all researchers agree on that doming occurred be-fore volcanic eruption. For example, Drury et al. (1997) suggestedthat extension related to the Red Sea opening was followed byvolcanism as well as doming and uplift. However, this debate isbeyond the scope of this work.

Gani et al. (2007) used a 1 km average thickness of the 30 Mabasaltic flow, an average density of 2800 kg m�3 for the basalt,and 3300 kg m�3 density for the mantle to estimate that such avolcanic pile would result in �0.85 km isostatic subsidence of theEthiopian Plateau in the absence of continued upwelling hot mate-rial to sustain topography. Gani et al. (2007) also estimated thatthe removal of geological material from the Ethiopian Plateauthrough erosion (estimated to be �93,200 km3) would result in�0.3 km of isostatic rebound of the plateau. Hence, the net rockuplift of the Ethiopian Plateau since 30 Ma is �1.75 km. The com-bined effect of the isostatic subsidence and rebound might haveresulted in leaving the base of the volcanic rocks �0.55 km lowernow than �30 Ma ago.

4. Geology

Fig. 5A and B are a generalized geological map of the TekezeRiver and the Blue Nile (Tefera et al., 1996) and a generalizedgeological section across the two rivers. The NorthwesternPlateau, the focus of this study, is underlain by Precambriancrystalline rocks which are regionally overlain by a Mesozoicsedimentary section with various thicknesses constituting theJurassic Adigrad sandstone, the Late Jurassic Abay and Antalolimestone, and the Cretaceous Amba Aradom sandstone. How-ever, in places such as the Mekele outlier, the Precambrian crys-talline rocks are overlain by a Paleozoic sedimentary sectionconstituting the Ordovician Edaga Arbi glacials and the Entichoformation (Fig. 5B; Beyth, 1972). Much of the plateau is coveredwith the Cenozoic flood basalts of the Afar mantle plume formingthe Ashangi, Aiba, Makonnen, and Alage formations (Fig. 5A andB). In places, the flood basalts are overlain by Oligocene–Mioceneshield volcanoes (dominantly basalts and referred to as TarmaberGussa formation) forming the topographically elevated MountChoke, Mount Guna and Mount Ras Dashen (Fig. 5B). The discon-tinuous basaltic exposure of the Middle Miocene TamaberMegezez formation defines portions of the western and north-western escarpments of the Afar Depression and the Main Ethio-pian Rift, respectively (Fig. 5A). Quaternary basalt and trachyte,referred to as the Plateau Basalt, crop out in the southwesternpart of the plateau (Fig. 5A).

Previous work conducted in the Blue Nile sedimentary basin issummarized in Gani et al. (2009). Gani et al. (2009) proposed thatthe Blue Nile is underlain by a NW-trending sedimentary basinmost likely the continuation of the Blue Nile rift in the Sudan. Ganiet al. (2009) proposed that the basin has geologically evolvedthrough three main phases. The first phase (pre-sedimentationphase) occurred before rifting and was characterized by thepeneplanation of the Precambrian crystalline rocks, possiblyduring the Paleozoic time. The second phase dominated the Juras-sic to early Cretaceous and was characterized by the deposition ofMesozoic clastic and marine sedimentary formations (Adigrad,Abay, Antalo, and Amba Aradom; Fig. 5A). The third phase (post-sedimentation phase) was characterized by the extrusion of Oligo-cene to Quaternary volcanic rocks, dominantly basalts.

Less work has been conducted in the Tekeze River and the sur-roundings compared to the Blue Nile. Beyth (1972) studied theMekele outlier to the east of the Tekeze River and documentedstratigraphic succession similar to that of the Blue Nile sedimen-tary basin with the exception of the presence of a Paleozoic sedi-mentary section (the Ordovician Edaga Arbi glacials and theEnticho formation) below the Mesozoic sedimentary section(Fig. 5B). However, the geology of the Tekeze River differ from thatof the Blue Nile and the Mekele outlier in that the Mesozoicsedimentary section is much thinner and most of the formationsare missing with the exception of the Adigrat formation (Fig. 5B).

Table 1The Normalized Steepness Index (Ksn), the Concavity (h), and the Regression Fit (r2) ofthe sub-basins and the tributaries of the Tekeze River and the Blue Nile drainagesystems.

Tekeze River Blue Nile

Tributary Ksn h r2 Tributary Ksn h r2

TK1 101 0.23 0.34 BN1 100 0.24 0.22TK2E 112 0.41 0.71 BN2 71.8 0.22 0.14TK2 W 73.6 0.37 0.64 BN3 63.8 0.18 0.11TK3 69.5 0.19 0.2 BN4 65.7 0.17 0.16TK4 79.3 0.33 0.67 BN5 104 0.36 0.60TK5 156 0.31 0.25 BN6 70.2 0.061 0.03TK6 59 0.31 0.46 BN7 43 0.21 0.19TK7 59.4 0.35 0.46 BN8 81 0.25 0.58TK8 25.8 0.27 0.47 BN9 103 0.29 0.65TK9 24.2 0.16 0.22 BN10 75 0.27 0.39TK10 68.8 0.32 0.62 BN11 117 0.19 0.096TK11 64.6 0.19 0.59 BN12 71.1 0.19 0.32TK12 65.8 0.33 0.86 BN13 127 0.24 0.43BN13 71.6 0.33 0.79 BN14 134 0.30 0.50TK14 24.7 0.24 0.55 BN15 108 0.27 0.37TK15 68.5 0.38 0.79 BN16 62 0.20 0.15TK16 98.5 0.53 0.92 BN17 64.4 0.15 0.00097TK17 157 0.31 0.29 BN18 84.1 0.097 0.044TK18 61.5 0.49 0.90 BN19 63.2 0.042 0.0035TK19 89.3 0.42 0.57 BN20 105 0.16 0.11TK20 162 0.34 0.79 BN21 96 0.17 0.12TK21 72.3 0.27 0.68 BN22 92.3 0.14 0.016TK22 52.9 0.34 0.94 BN23 67.2 0.20 0.10TK23 123 0.32 0.55 BN24 70.7 0.12 0.10TK24 70.2 0.38 0.94 BN25 78.1 0.32 0.65TK25 143 0.38 0.65 BN26 109 0.18 0.2

BN27 64.9 0.20 0.49BN28 76.1 0.11 0.69BN29 58 0.094 0.051


5. Methods and results

This study focuses on the extraction of morpho-tectonic param-eters of the Tekeze River and the Blue Nile including: (1) the Nor-malized Steepness Index (Ksn); (2) the Concavity (h) values of thelongitudinal profiles; and (3) the Regression Fit (r2) between theobserved and predicted channel longitudinal profiles.

5.1. The Normalized Steepness Index (Ksn)

The Ksn values are computed for 55 out of the 56 sub-basins ofthe Tekeze River and the Blue Nile (online Supplementary material1; Table 1; Figs. 6 and 7). Only sub-basin TK1W (Fig. 6) is left outbecause no Ksn value is obtained for it. This index is directlyproportional to the river’s incision rate derived by tectonic upliftproviding that the lithological and climate conditions are homoge-nous (e.g. Whipple, 2004; Wobus et al., 2006; Cyr et al., 2010). It isdifficult to insure lithological homogeneity for a large area such asthe Tekeze River and the Blue Nile drainage systems. However, it isapproximated here that the Ksn values generally reflect rock upliftbecause: (1) About 60% of the Tekeze River and the Blue Nile drain-

age systems are underlain by Cenozoic-Quaternary volcanic rocks,dominantly basalts (Fig. 5A). Other lithologies including the Pre-cambrian crystalline rocks and the Paleozoic and Mesozoic sedi-mentary rocks are present in the northern and southern parts ofthe region. (2) Only eight out of the 55 sub-basins examined in thisstudy drain lithological units other than basaltic rocks (compareFigs. 5A and 6). This constitutes only �14% of the total sub-basinsconsidered in this study.

The Ksn value represent the channel’s steepness normalized tothe upstream drainage area and the difference between a referenceh value and the observed h value of the channel (Wobus et al.,2006). It can be calculated through the equation Ksn ¼ KsA

href�hð Þ

where Ks is the channel’s steepness, A is the upstream drainagearea, href is the reference concavity and h is the observed concavityof the channel which is a mathematical representation of the cur-vature of the channel’s longitudinal profile. The Ks and h values areusually extracted from the regression models of the longitudinalprofile of the channel and A is obtained from delineating the drain-age area of the channel in the DEMs. Generally, the href values re-flect the concavity of channels that are free of knickpointsresulting from tectonic uplift, faulting or change in rock strength

Fig. 6. Color-coded Digital Elevation Model (DEM) extracted from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data of the sub-basins ofthe Tekeze River and the Blue Nile drainage systems. The elevation color ramps for each sub-basin shown with elevation values in meters. The establishment of the sub-basinswas achieved through the implementation of HydroModeling module which is an extension of ArcGIS.


and these values are close to those computed by averaging the re-gional observed h values of channels under consideration (Wobuset al., 2006). It is found that in most tectonic settings the href valuesrange between 0.35 and 0.65 (see summary in Wobus et al., 2006).The term (href � h) is needed for the consideration of channels withsegments that have different h values. In this work a href value of0.45 is adopted following the recommendation of Whipple (2004).

The Ksn values generally vary between 20 and 600 when the hrefvalue is set to 0.45 as shown for drainage systems in Nepal,California, Himalaya, New Zealand, Appalachian Mountains, Ural

Mountains, and Taiwan (Whipple, 2004). High Ksn values are indica-tive of fast tectonic upliftwhereas lowKsn values are associatedwithrivers that are not affected by tectonic uplift. However, Duvall et al.(2004) studied drainage systems in the coastal California and foundKsn values ranging between 8.75 and 30.9 for rock-only channelsusing a slightly lower href value (0.4) than the 0.45 values used inthe studies compiled in Whipple (2004). Additionally, Duvall et al.(2004) found that these Ksn values have a bimodal distribution witharithmetic means of 13.8 and 26.1. Duvall et al. (2004) concludedthat the average Ksn value of 13.8 coincides with a drainage system

Fig. 7. Color-coded Normalized Steepness Index (Ksn) values of the sub-basins of the Tekeze River and the Blue Nile drainage systems computed from the AdvancedSpaceborne Thermal Emission and Reflection Radiometer (ASTER) Digital Elevation Model (DEM). The Ksn values were calculated using the StreamProfiler 5.1 which is anextension of ArcGIS in connection with MatLab.


in a region of low uplift rate whereas the average Ksn of 26.1 is asso-ciated with a drainage system in a region of high uplift rate.

There is no universal direct empirical relationship between theKsn values and the rate of uplift. However, Wobus et al. (2006) haveshown from a case study in the Siwalik Hills in Nepal (where theuplift rate is already estimated by Hurtrez et al. (1999)) that thereis a linear relationship between the Ksn values and the mean upliftrate. Wobus et al. (2006) computed the Ksn values for seven chan-nels ranging between �35 and �70 and found these values to be

linearly corresponding to mean uplift rates ranging between �7and �14 mm yr�1 as determined by Hurtrez et al. (1999).

The Ksn values of the Tekeze River and the Blue Nile are as highas 162 and as low as 24.2 (Table 1; Fig. 7). These values can be ex-plained as follows:

(1) The lowest Ksn values (24.2, 24.7 and 25.8) are found in thesub-basins at the northwestern end of the Tekeze River onthe northern side of the river (Fig. 7). This part of the river

Fig. 8. Hill-shade Digital Elevation Model (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data showing the locations of the Tekeze River and the BlueNile tributaries used in the calculation of the Concavity (h) and the Regression Fit (r2) values from their longitudinal profiles.


is close to the low lands of the Sudan and the tributariesdraining these sub-basins originate from a terrain that hasan elevation slightly higher than the river itself (Fig. 3).The Ksn values progressively increase (ranging between 59and 79.3) towards the east as the western escarpment ofthe Afar Depression is approached (Fig. 7). This is suggestiveof an increased tectonic uplift in the east due to rift-flankuplift of the Afar Depression.

(2) The sub-basins on the southwestern side of the Tekeze Riverare characterized by moderate Ksn values ranging between68.5 and 89.3 (Fig. 7). The Ksn values on this side of the riverbecome higher (98.5–162) in the sub-basins that are drainedby tributaries that are originating from Mount Ras Dashenand Mount Guna. The increase in the Ksn values in this partof the Tekeze drainage system is attributed to the high ele-vations and steep slopes of these two mountains (Fig. 3).

(3) The sub-basins on both sides of the Blue Nile are character-izedby similarKsn values (cluster between43 and84.1). How-ever, higher Ksn values (ranging between 96 and 134) are alsopresent and these are associated with the sub-basins drainedby tributaries that originate from Mount Choke and MountGuna (Fig. 7). As explained before, this increase in the Ksn val-ues is attributed to the high elevation and steep slopes of themountains from which the drainage systems originate.

(4) The sub-basins of both the Tekeze River and the Blue Nilethat are drained by tributaries originating from the westernescarpment of the Afar Depression are characterized by rela-tively high Ksn values (ranging between 100 and 156) (Fig. 7).However, the Blue Nile sub-basins that are drained by tribu-taries originating from the northwestern escarpment of theMain Ethiopian Rift show relatively lower Ksn values (rangingbetween 65.7 and 71.8) compared to those of the westernescarpment of the Afar Depression (Fig. 7). This suggests thatthe influence of the rift-flank uplift of the Afar Depression ishigher than that of the Main Ethiopian Rift in shaping thedrainage systems in the Northwestern Plateau.

(5) The distributionof theKsn values suggests that theyhave theirhighest values in the central part of theNorthwesternPlateau.This might be due to a combination of tectonic uplift forminglong wavelength doming (the Afar dome) and the growth ofthe plateau through the build-up of shield volcanoes such asMount Ras Dashen, Mount Guna and Mount Choke (Fig. 7).

5.2. The Concavity (h)

Analysis of the shape of a river’s longitudinal profile is a power-ful tool in determining the incision and the tectonic uplift historyof a region (e.g. Whipple, 2004). The ideal shape of the river longi-tudinal profile is the logarithmic decay curve. Deviations from theideal shape are used to infer active uplift, subsidence associatedwith change in the base level, faulting, debris flows, or stream cap-ture. For instance, a river’s longitudinal profile that has abnormallyhigh slope and non-lithological knickpoints indicates major tec-tonic uplift.

The h values vary widely in different geological settings. Never-theless, these values have been divided byWhipple (2004) into low(<0.4), moderate (0.4–0.7), high (0.7–1.0) and extremely high(>1.0). Low h values are associated with short and steep channelsthat are influenced by debris flow, downstream increase in incisionrate, or by the rock strength which is commonly associated withknickpoints. Moderate h values are associated with channels incis-ing in homogeneous lithology that is affected by homogeneous andactive uplift. High h values are associated with channels that wit-ness downstream decrease in rock uplift rate or bedrock strength.Extremely high h values (>1.0) are typical of channels that encoun-ter abrupt occurrence of knickpoints due to changes in the bedrockstrength or due to temporal differences in bedrock uplift.

The h values are extracted from the longitudinal profiles of 55tributaries of the Tekeze River and the Blue Nile (onlineSupplementary material 1; Fig. 8). An example of such longitudinalprofiles is shown in Fig. 9A. Fig. 9B and C show the longitudinal

Fig. 9. (A) An example of a longitudinal profile from tributary BN 1 showing the observed and the predicted channel profiles. (B) The longitudinal profile of tributary TK16which has the highest Concavity (h) value of 0.53. (C) The longitudinal profile of tributary BN19 which has the lowest Concavity (h) value of 0.042.


profiles of tributaries with the highest (0.53) and the lowest(0.042) h values, respectively in the study area.

Themajority of the values h values from the Tekeze River and theBlue Nile fall within the low category (ranging between 0.042 and0.38) of Whipple (2004) classification but a few values fall withinthe moderate category (ranging between 0.41 and 0.53) (Table 1;Fig. 10). Analysis of the h values shows the following:

(1) Moderate h values seem to be associated with tributariesthat are not originating from the shield volcanoes or theescarpments of the Afar Depression and the Main Ethiopian

Rift. These moderate h values can be explained as due to theeffect of a regional and steady uplift on the NorthwesternPlateau coupled with the relatively homogeneous nature ofthe basaltic flow dominating the plateau.

(2) The lowest h values are encountered in the tributaries thatoriginate from Mount Choke (Fig. 10). These tributaries areshort and steep channels with many knickpoints.

(3) Most of the tributaries that are originating from the westernescarpments of the Afar Depression and the northwesternescarpment of the Main Ethiopian Rift have low h values(Fig. 10). These are generally long channels and are expected

Fig. 10. Color-coded Concavity (h) values of the tributaries of the Tekeze River and the Blue Nile. The h values were calculated using the StreamProfiler 5.1 which is anextension of ArcGIS in connection with MatLab. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


to have higher h values thanwhat has been observed. The lowh values in these tributaries is attributed to the presence ofmany faults occurring on the western flanks of the AfarDepression and the northwestern flank of theMain EthiopianRift (Fig. 5A) encountered by these tributaries. This is bestexemplified by the longitudinal profile displayed in Fig. 9A.

5.3. The Regression Fit (r2)

With the exception of a few tributaries, most of the tributariesof the Tekeze River and the Blue Nile show channel profiles that

deviate from the predicted channel profile obtained from the re-gressed channel, where h is derived from the regression fit (Table1; Fig. 11). These values range between 0.00097 (highest deviationfrom the predicted channel profile) and 0.94 (the observed channelprofile is almost identical to the predicted channel profile indicat-ing channel maturity).

Analysis of the r2 values shows the following:

(1) The tributaries of the Tekeze River and the Blue Nile whichoriginate from the western escarpment of the Afar Depres-sion and the northwestern escarpments of the Main

Fig. 11. Color-coded Regression Fit (r2) values of the tributaries of the Tekeze River and the Blue Nile. The r2 values were calculated using the StreamProfiler 5.1 which is anextension of ArcGIS in connection with MatLab. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


Ethiopian Rift show low r2 values (Fig. 11) suggesting thatthese river channels have been continuously modified byrift-flank uplift of the western escarpment of the AfarDepression and the northwestern escarpment of the MainEthiopian Rift.

(2) The tributaries in the lower reaches of the Tekeze River closeto the low-lands of the Sudan have high r2 values (Fig. 11)suggesting that these are mature rivers and they are notaffected by the tectonic activities to the east in the AfarDepression and the Main Ethiopian Rift.

(3) The tributaries originating from Mount Ras Dashen to jointhe Tekeze River have moderate r2 values (Fig. 11) reflectingthe influence of the build-up of this shield volcano on shap-ing the channel geometry of these rivers. The tributariesoriginating from Mount Choke to join the Blue Nile havelow r2 values (Fig. 11) suggesting an increased influence ofthe build-up of the shield volcano in shaping the channelgeometry of these rivers.

6. Discussion

Morpho-tectonic analysis of the Tekeze River and the Blue Nilesuggests that the incision history of these drainage systems on theNorthwestern Plateau is much more complicated than can be ex-plained by either the steady-state incision model of Pik et al.(2003) or the increased incision rate through time model that issuggested by Gani et al. (2007). It is suggested here that the inci-sion history of these drainage systems was influenced by a numberof tectonic and geological events including: (1) Regional uplift thatresulted in the uprising of the entire Ethiopian Plateau. This longwavelength uplift might be due to the rise of the Afar mantleplume which reached the base of the Ethiopian lithosphere shortlybefore the out-pouring of the �30 Ma flood basalts (Hofmann et al.,1997); (2) This long wavelength uplift was superimposed by anevent that produced regions of localized positive topography inassociation with the build-up of shield volcanoes such as MountRas Dashen, Mount Guna and Mount Choke. This event might haveoccurred at �22 Ma as suggested by Kieffer et al. (2004). (3) Thetopography that resulted from these two events was subsequentlymodified by the rift-flank uplift on the western escarpment of theAfar Depression and the northwestern escarpments of the MainEthiopian Plateau. This uplift might have accompanied the openingof the Main Ethiopian Rift at �11 Ma as suggested by Wolfendenet al. (2004) or at 6–5 Ma as suggested by Bonini et al. (2005). Thisuplift diminishes progressively towards the west and northwestdirections away from the Afar Depression and the Main EthiopianRift, respectively.

The regional uplift of the Northwestern Plateau might havebeen accompanied by slow to moderate rate of incision of the Tek-eze River and the Blue Nile. This is evident from the relatively mod-erate Ksn values that dominate the Northwestern Plateau (exceptaround the shield volcanoes), the low to moderate h values, andthe moderate r2 values. This regional uplift is also supported bygeological observations. The top of the flood basalts which wereextruded due to the Afar mantle plume is impressively flat extend-ing at �2.2 km elevation for hundreds of kilometers throughoutthe entire Northwestern Plateau (Fig. 5B).

The events that produced the localized positive topography inassociation with the build-up of the shield volcanoes might haveresulted in an additional increase in the rate of incision. This is evi-dent from the relatively high Ksn values encountered aroundMount Ras Dashen, Mount Guna and Mount Choke. In addition,the low h r2 values characterizing the tributaries of the Tekeze Riv-er and the Blue Nile origination from these shield volcanoes furthersupport this proposition.

A significant increase of the incision rate might have occurred inassociation with the rift-flank uplift, especially on the westernmargin of the Afar Depression. This is particularly evident fromthe relatively high Ksn values, and the relatively low h and r2 valuesof these tributaries.

The Pik et al. (2003) steady-state model and the long-livedestablished hydrological stability of the drainage system in theNorthwestern Plateau, especially the Blue Nile, holds true forsome parts of the plateau. This is particularly true for the tribu-taries at the lower reaches of the Tekeze River and the Blue Nile.Morpho-tectonic parameters of these tributaries indicate thatthese drainages are mature and they have not been affected bytectonic disturbances for a long time. This is evident from thelow Ksn values and the high r2 values of many of these rivers. Itis worth mentioning here that Pik et al. (2003) study was con-ducted in the lower reaches of the Blue Nile close to the lowlands of the Sudan. Such long-lived stability can be attributedto that these tributaries were incised in the western part of theplateau far away from the disturbance of the rift-flank uplift thatoccurred further east along the western margin of the AfarDepression and the northwestern margin of the Main EthiopianRift. This stability is also suggestive of head-ward incision beforedrainage reversal.

Equally, the Gani et al. (2007) model that advocates for an in-creased rate of incision holds true, but for only parts of the drain-age system on the Northwestern Plateau. This is the case for thedrainages that emerges from the western escarpments of the AfarDepression as indicated by their high Ksn values and the low r2 val-ues. Gani et al. (2007) study identified three incision phases basedon preserved major knickpoints on the tributaries Gudar and Birr(correspond to BN4 and BN8 respectively in Fig. 8). Tributary Gudaroriginates from the northwestern escarpment of the Main Ethio-pian Rift to join the Blue Nile. Hence, it is likely that the presenceof knickpoints along the profile of this tributary reflects an in-creased rate of incision due to rift-flank uplift. On the other hand,tributary Birr emerges from Mount Choke to join the Blue Nile.Hence, the presence of knickpoints along the profile of this tribu-tary might be associated with an increased incision due to thebuild-up of this shield volcano.

7. Conclusions

Analysis of ASTER and SRTM DEMs to extract morpho-tec-tonic parameters of tributaries of the Tekeze River and the BlueNile including the Normalized Steepness Index (Ksn), Concavity(h), and Regression Fit (r2) between the observed and predictedchannel profiles suggests that the evolution of these rivers drain-age systems on the Northwestern Plateau was influenced bythree tectonic and geological events. (1) The first event whichis characterized low to moderate incision rate on the entire pla-teau was associated with long wavelength uplift of the plateau,likely due to rising of the Afar mantle plume that started�30 Ma. (2) The second event resulted in localized increase inthe incision rate in association with shield volcanoes build-upevent at �22 Ma. (3) The third event resulted in a significant in-crease of the incision rate in the eastern part of the Northwest-ern Plateau due to rift-flank uplift on the western escarpment ofthe Afar Depression and the northwestern escarpment of theMain Ethiopian Rift. This uplift diminishes westward leavingthe drainage system in the lower reaches of the Tekeze Riverand the Blue Nile relatively tectonically undisturbed, henceestablishing a long-lived hydrological stability and reaching stea-dy-state equilibrium.


Acknowledgements

This research was made possible through funding provided bythe National Science Foundation, Office of International Scienceand Engineering, International Research Experience for Students(NSF-OISE-IRES) program Grant Number 0927906. We thankS. Drury and an anonymous reviewer for detailed and constructivereviews.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jafrearsci.2012.04.005.

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