13

The Nile: Evolution, Quaternary River Environments and Material Fluxes

Jamie C. Woodward1, Mark G. Macklin2, Michael D. Krom3 and Martin A.J. Williams4

1School of Environment and Development, The University of Manchester, Manchester M13 9PL, UK 2Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Aberystwyth SY23 3DB, UK 3School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK 4Geographical and Environmental Studies, University of Adelaide, Adelaide, South Australia 5005, Australia

13.1 INTRODUCTION et al., 1980; Williams et al., 2000; Woodward et al., 2001; Krom et al., 2002) as well as during previous interglacial The Nile Basin contains the longest river channel system and interstadial periods (Williams et al., 2003). The true in the world (>6500 km) that drains about one tenth of the desert Nile begins in central Sudan at Khartoum (15° African continent. The evolution of the modern drainage 37′ N 32° 33′ E) on the Gezira Plain where the Blue Nile network and its fl uvial geomorphology refl ect both long- and the White Nile converge (Figure 13.1). These two term tectonic and volcanic processes and associated systems, and the tributary of the Atbara to the north, are changes in erosion and sedimentation, in addition to sea all large rivers in their own right with distinctive fl uvial level changes (Said, 1981) and major shifts in climate and landscapes and process regimes. The Nile has a total vegetation during the Quaternary Period (Williams and catchment area of around 3 million km2 (Figure 13.1 and Faure, 1980). More recently, human impacts in the form Table 13.1). of land use change over the last few millennia and large The Nile Basin supports a vast range of ecosystems dam construction over the last hundred years or so, (Rzoska, 1976) and has played a central role in the devel- have had major impacts on the hydrology and sediment opment of a rich diversity of cultures (Wendorf et al., budget of the Nile Basin. The river basin straddles 35° of 1976; Welsby, 1998, 2001; Shaw, 2003). It has been an latitude (4° S to 31° N) encompassing a wide variety of important corridor for the movement of people and animals climates, river regimes, biomes and terrains – from the throughout the Quaternary and the lower Nile and delta Equatorial lakes plateau of the White Nile headwaters to saw the rise of the Kingdoms of Egypt (Davies and Fried- the delta complex in the Eastern Mediterranean Sea man, 1999) and their great irrigation-based civilization (Figure 13.1). It is unique among the large exotic rivers (Hassan, 1988). Today over 180 million people live within of the world in that today it fl ows for almost 2700 km the basin and about half this number are almost totally through the Sahara Desert without any signifi cant peren- dependent on Nile fl ows for economic and domestic nial tributary inputs. Of all the world’s rivers with catch- needs. While much of the basin is sparsely populated, ments greater than 1 million km2, the Nile has the lowest rapidly growing urban centres, most notably Cairo (popu- specifi c discharge, 0.98 litres s−1 km−2 at Aswan (Shahin, lation >16.5 million) and greater Khartoum (population 1985) – although base fl ows and the summer fl ood were >6 million) – and extensive tracts of agricultural land on signifi cantly higher in the early Holocene (Adamson the valley fl oor and delta – are heavily or totally reliant

Large Rivers: Geomorphology and Management, Edited by A. Gupta © 2007 John Wiley & Sons, Ltd 262 Large Rivers

Mediterranean Sea Dams A Aswan B Jebel Aulia C Khashm el Girba Cairo D Sennar E Roseires F Owen Falls Nile 1 Cataracts Marshes

EGYPT LIBYA 1 A Lake Nasser

R Lake Nubia 2 Wadi Halfa e d

S Nile ea 3 5 Dongola 4 Atbar

Wadi Howar 6 a CHAD ERITREA Khartoum C B Blue Nile

Wadi el Melk SUDAN White Nile D Lake Tana E

Malakal Sobat ETHIOPIA

CENTRAL AFRICAN Bahr

REPUBLIC el

Jebe Juba

l UGANDA Lake CONGO Lake Turkana Albert F Lake Kampala Edward Lake KENYA Victoria N RWANDA 0 500 km TANZANIA

Figure 13.1 Map of the Nile Basin showing the drainage network, basin states, and major dams The Nile 263

Table 13.1 Geographical, hydrological, and water resource data for each of the 10 countries of the Nile Basin (based on sources in UNEP, 2000) Country Area of country Average rainfall Internal Actual The dependency Population in the within the Nile across the basin in renewable renewable ratio = the Nile Basin in 1990 basin (km2) and its each country for water water percentage (millions) with the percentage of the minimum and resources resources difference percentage of the total basin area (in maximum years (IRWR) (ARWR) between the country’s population parentheses) (mm) (km3 year−1) (km3 year−1) ARWR and the in the basin (in IRWR parentheses) Burundi 13 260 (0.4) 895 to 1570 3.6 3.6 0 3 204 (58) DR Congo 22 143 (0.7) 875 to 1915 935 1019 8.2 1 838 (4) Egypt 326 751 (10.5) 0 to 120 1.7 58.3 96.9 47 599 (85) Eritrea 24 921 (0.8) 240 to 665 2.8 8.8 68.2 0.918 (30) Ethiopia 365 117 (11.7) 205 to 2010 110 110 0 19 454 (35) Kenya 46 229 (1.5) 505 to 1790 20.2 30.2 33.1 9 129 (32) Rwanda 19 876 (0.7) 840 to 1935 6.3 6.3 0 5 731 (72) Sudan 1 978 506 (63.6) 0 to 1610 35 88.5 77.3 20 893 (74) Tanzania 84 200 (2.7) 625 to 1630 80 89 10.1 4 878 (16) Uganda 231 366 (7.4) 395 to 2060 39.2 66 40.9 15 999 (75) Total 3 112 369 (100) 129 643

on Nile fl ows for irrigation and power generation. The of the basin. Local water resources are minimal in the arid population of Egypt (>65 million in 2001) is expected to and semi-arid regions of Egypt and Sudan (Table 13.1) double by 2050 and it already uses the largest share of the and evaporation and seepage losses are high in this part annual water budget. The spatial pattern of rainfall inputs of the basin (UNEP, 2000). The Merowe Dam near the across the Nile Basin is highly variable and because Sudan fourth cataract in northern Sudan is expected to be com- and Egypt abstract the greatest volumes for irrigation – pleted in 2008 and it will treble the nation’s hydropower and have the highest water resource dependency ratios capacity. Hydroelectric output from the Aswan High Dam (Table 13.1) – the allocation and use of Nile waters con- never lived up to expectations owing to seepage losses tinues to be a highly charged political issue within the 10 and serious underestimates of evaporation losses. The basin states (Howell and Allan, 1994). Merowe project will create the fi rst dam on the main Nile The dominant water resource management and fl ood in Sudan and will involve the resettlement of 50 000 control strategy of the twentieth century saw the develop- people as well as the loss of many archaeological sites ment of major river impoundments in the basin (Figure (Askouri, 2004). 13.1). Over the same period, as the population of Egypt This chapter reviews the evolution of the river and its and its irrigation schemes grew apace, the allocation and present day hydrology and fl uvial geomorphology. It also use of Nile waters emerged as a source of major interna- summarises the response of the Nile Basin to Late Pleis- tional political tension (Collins, 1990; Howell and Allan tocene and Holocene climatic changes as this provides 1994) and Nile Waters Treaties were signed in 1929 and important context for the contemporary fl uvial environ- 1959. The fi rst phase of construction of the Aswan High ments of the basin. The suspended sediment budget of Dam was completed in 1964 (the fi nal phase ended in the basin and offshore regions is also discussed and the 1971) and other major twentieth century dams include the impacts of the major water resource management develop- Jebel Aulia (1937) on the White Nile and the Roseires ments on water and sediment fl uxes are assessed. Dam (1966) on the Blue Nile (Figure 13.1). Since the long-term fl ow records for the Nile exhibit considerable 13.2 NILE BASIN RIVER ENVIRONMENTS annual variability (Hassan, 1981; Sutcliffe and Lazenby, 1994), the major water storage schemes hold considerable According to Said (1981) the Nile Basin encompasses fi ve strategic importance for water resource management, major regions that differ from one another in structure and power generation and fl ood control. These schemes are geological history (Figures 13.1 and 13.2). These are partly a response to the fact that most of the fl ows are listed below and the characteristic fl uvial environments of generated in headwater areas that cover only about 20 % each region are briefl y described: 264 Large Rivers n le of the Nile from White headwaters to Mediterranean Sea (after Said, 1994). Note that Blue is not show Long profi Long 13.2

Figure The Nile 265

(1) The Equatorial lake plateau in the White Nile head- history of the Nile Basin and neighbouring regions and waters with densely forested, tropical catchments with have related these geological phenomena to the organiza- perennial fl ow regimes. tion of the drainage network, to the location of major (2) The Sudd region and central Sudan with low sedimentary basins, and to major changes in the former gradient fl oodplains, extensive swamplands and extent of the catchment. They argue that tectonic events broad channel belts and low suspended sediment of great antiquity – probably dating to the Pan-African concentrations. orogenic event (∼550 Ma) or earlier – have been infl uential (3) The Ethiopian Highlands forming the headwaters of in shaping the Nile Basin and the courses of the large the Blue Nile and the Atbara with deep ravines, un - river channel systems that form the modern catchment. stable slopes, entrenched steep gradient gravel bed The watershed of the present Nile and its relationship to streams with highly seasonal fl ows and high sediment the major rift zones and other large river systems to the loads. east and west of the upper and middle Nile is shown (4) The great bends and cataracts of the desert Nile with in Figure 13.3. The courses of large rivers including the ephemeral tributary wadi systems and alluvial fans in Bahr el Jebel, the Blue Nile, the Atbara, the main Nile the arid lands of Sudan and Egypt and the cataracts and many tributaries are infl uenced by dominant linear and vast alluvial reaches of the main Nile with a structures including dykes, joints, faults and major geo- strongly modifi ed fl ow and sediment regime down- logical contacts (Adamson and Williams, 1980; Adamson stream of the Aswan High Dam. et al., 1993). (5) The Egyptian region including the low gradient delta complex with its large distributaries, promontories 13.3.1 The Late Miocene Nile Canyon in Egypt and lagoons, heavily modifi ed drainage and dense network of irrigation canals. The severing of the connection between the North Atlantic Ocean and the Mediterranean Sea at the end of the Miocene Marked contrasts are evident in the hydrology and fl uvial (around 6.5 million years BP) led to the onset of the Mes- geomorphology of each of the fi ve regions. Distinctive sinian Salinity Crisis – arguably one of the most dramatic large river valley-fl oor environments are present in each series of environmental changes recorded in recent Earth region and these refl ect the strong regional variations in history (Hsu et al., 1973). The evaporation of the Mediter- topography, tectonic setting, climate, hydrology, vegeta- ranean Sea and the shift to saline desert conditions across tion, sediment supply and other factors. The legacy of its former marine basins had profound impacts on the Quaternary sediments and landforms on the valley fl oor is large river systems of the region including the Nile. The also apparent in many parts of the basin and these may fall in base level in the Eastern Mediterranean led to large- exert an important infl uence on contemporary fl uvial pro- scale fl uvial incision and, according to Hsu et al. (1973), cesses and on the chemical and physical properties of the Nile carved a deep bedrock canyon to ca. 570 m below alluvial soils, especially soil permeability and soil salinity present sea level at Cairo and to −170 m at Aswan – some (Williams et al., 2000). These contrasts in river basin 1000 km upstream from the present coastline. This fall in dynamics pose a wide range of catchment management elevation of about 400 m between Aswan and Cairo for issues and the nature and extent of the human impact and the Late Miocene Nile Valley compares with a fall of use of river-related resources varies markedly across the around 70 m for the modern Nile over the same distance. Nile basin. Figure 13.4 shows a block diagram of the Eonile Canyon from the area of present-day Cairo to the dried out basin of the Late Miocene Mediterranean. Said (1981) has 13.3 EARLY ORIGINS AND THE LATE called this Late Miocene river The Eonile. Recently, Le MIOCENE AND PLIOCENE NILE Pichon et al. (2005) have estimated that the canyon was Several detailed reviews of the evolution of the Nile drain- about 1100 km long and 1900 m deep. Further downstream age basin have been published in recent decades (Adamson the Late Miocene Nile created a series of fans in the region and Williams, 1980; Williams and Williams, 1980; Said, of the North Delta Embayment (more than 4000 m below 1981, 1993; Adamson et al., 1993). These document the modern sea level) where evaporites accumulated in the development of the modern drainage confi guration fol- distal areas of these fan complexes under arid climatic lowing the infl uence of major tectonic phenomena – conditions (Figure 13.4). Said (1981: 99) has argued that including the rifting of East Africa – climatic changes and the course of this river was determined by the tectonic other factors. Adamson and Williams (1980) and Adamson framework of the region at the time and he observes that et al., (1993) have examined the structure and tectonic this was 266 Large Rivers

Figure 13.3 The drainage network of central and northeastern Africa showing the Congo, Chad, Niger and Nile river basins and the present and former extent of the Nile watershed. The major zones of Tertiary rifting are also shown (after Adamson and Williams, 1980)

‘the fi rst time in the history of the drainage of the rado River (see Chapter 10; Table 13.2) and this phase of elevated lands of Egypt that the river followed a fl uvial erosion generated an estimated 70 000 km3 of sedi- defi ned and deep gorge which distinguishes it from ment in less than 3 million years (Said, 1981). This repre- the alluviating and shallow rivers of the pre-late sents about 20 % of the total mass of sediment in the Nile Miocene drainage systems which preceded the Nile Cone (Said, 1981). Estimates vary, with McDougall et al. system.’ (1975) suggesting a maximum volume for the Nile cone of ∼200 000 km3. Other large river systems draining to the It is interesting to note that the Late Miocene Nile Canyon Mediterranean Sea including the Po and Rhone also cut was longer and deeper that the Grand Canyon of the Colo- deep canyons at this time (Ryan and Cita, 1978). The Nile 267

zone moved northwards towards the modern Mediterra- nean coastline.

13.3.2 The Integrated Nile Much debate has surrounded the antiquity of the inte- grated Nile Basin that exists today and the nature and timing of the connections between the Egyptian Nile and the major headwater basins of the White Nile and Blue Nile (Figure 13.3). Adamson (1982) has reviewed the development of ideas on this topic during the 1960s and 1970s. Williams and Williams (1980) point out that while there is some evidence for a Late Cretaceous drainage link between Sudan and Egypt, the earliest fi rm evidence of Nile fl ow from the Ethiopian Highlands to the Mediterra- nean Sea (via Sudan and Egypt) dates to the Early Pleis- tocene. Tertiary tectonic events associated with the rifting Figure 13.4 Schematic reconstruction of the Nile Canyon of Africa and the separation of Arabia created the high showing the main channel and tributary gorges cut by the Egyp- elevation sub-basins of the Nile and determined its north- tian Nile drainage during the Late Miocene (Messinian) salinity ward course to the Mediterranean Sea (Adamson, 1982; crisis. See Table 13.2 and text for estimates of the main canyon’s Adamson et al., 1993). Williams and Williams (1980) and dimensions. Reprinted from The Geological Evolution of the River Nile, Said, R., page 151, Copyright (1981). With kind per- Adamson (1982) have argued that the Nile has been trans- mission of Springer Science and Business Media porting water and sediment from the Ethiopian Highlands to the Mediterranean since ‘well back into the Tertiary’ and they point to the microfossil and mineralogical evi- Table 13.2 A comparison of the modern Grand Canyon in dence in the sediments of the Nile Cone – as well as the Arizona with estimates for the size of the Late Miocene Nile huge volume of those sediments – as evidence for a very Canyon. Reprinted from the Geological Evolution of the River early integration of sub-Saharan drainage with the Nile, Said R., pp 151. Copyright (1981). With kind permission Egyptian Nile. Part of their argument was based on the of Springer Science and Business Media rough equivalence in the volume of rock eroded from the Grand Canyon, Eonile Canyon, Ethiopian headwaters of the Nile (McDougall et al., 1975) Arizona, USA, Egypt (Late and that of the Nile Cone. Adamson and Williams (1980: Colorado River Miocene) 251) have argued that, while climatic and tectonic events may have periodically interrupted the connection, the inte- Width (km) 10–20 10–20 Length (km) 320 1300 grated Nile Basin is as old as the Tertiary rifting of Africa. Depth (m) 2080 2500 Butzer and Hansen (1968: 6), however, contend that the Gradient 1 : 625 1 : 400 Nile hydrographic system as we know it today is of com- Age Early Miocene Late Miocene paratively recent age – they have argued that the White to present Nile and the Blue Nile catchments did not connect with Formation time (years) ca. 20 million ca. 3 million the Saharan Nile before the Early Pleistocene. It is diffi - cult to reconcile this latter view with the presence of sap- ropels in the pre-Quaternary marine record of the Eastern Higher sea levels during the Pliocene resulted in the Mediterranean Sea as most workers agree that elevated creation of a long narrow marine gulf in the Nile Valley fl ood fl ows from the Blue Nile Basin are a key ingredient that reached as far south as Aswan. The marine sediments for their formation (Rossignol-Strick et al., 1982). of the Early Pliocene fi lled about one third of the depth of More recent work has shown that the lower White Nile the Eonile canyon (Said, 1981). The Late Pliocene Nile is relatively young, and fl ows across the fl oor of a very saw a shift from marine to brackish conditions with a load large lake that occupied its valley around 400 000 years of fi ne-grained sediments derived from local wadi systems BP (Williams et al., 2003). The age of the Blue Nile is – the Egyptian Red Sea Hills were an important source of harder to estimate. Some 2 km of uplift in the past 8 Ma sediment at this time. This period saw the complete infi ll- initiated or accelerated the incision that gave rise to the ing of the Eonile canyon (Said, 1981) as the freshwater Blue Nile Gorge, which dissects horizontal Trap Series 268 Large Rivers

basalts of Upper Oligocene age (McDougall et al., 1975). Holocene records in the Nile Delta (Krom et al., 2002) It is clear from a wide range of sources that the Nile Basin and in northern Sudan (Woodward et al., 2001) are briefl y watershed has expanded and contracted throughout its summarized below. Note that all ages in the following history (Figures 13.3 and 13.4). However, while the exis- section are uncalibrated radiocarbon ages unless otherwise tence of several large, separated river basins at some stage stated. in its early development is now widely accepted, reliable geochronological and stratigraphical data on their age and 13.4.1 20 000 to 12 500 14C Years BP extent are not yet available for periods before the Late Pleistocene, apart for the lower White Nile (Williams The early part of this period coincides with the global et al., 2003). LGM of Marine Isotope Stage 2. This was a dry period across much of the Nile Basin with colder conditions than present and rapid erosion in the Ethiopian Highlands 13.4 THE LATE PLEISTOCENE AND where the tree line was probably about 1000 m lower than HOLOCENE NILE at present during the coldest phases of the Pleistocene. In Fluctuations in the climate and hydrology of tropical very general terms, during the global LGM the main chan- Africa during the Quaternary exerted an important infl u- nels of the Blue Nile and desert Nile were characterized ence on the behaviour of the Nile sediment system. During by net aggradation and sediments were deposited on the the cold stages of the Pleistocene low lake levels refl ect valley fl oor in a braided river environment dominated by a decrease in precipitation as the expanded ice sheets sandy bar complexes with some fi ne gravel. Adamson cooled the global oceans and monsoon intensity fell et al. (1980) have argued that discharges at this time were (Hulme, 1994). These changes were driven by Milankov- much reduced – perhaps amounting to only about 10–20 % itch forcing of global climate and the precession signal of the modern Nile fl ows. The increased sediment supply was an especially important infl uence on tropical rainfall to the fl uvial system at this time resulted from a decrease dynamics. The infl uences of eustatic changes in Quater- in vegetation cover in the Ethiopian Highlands leading to nary sea level driven by changes in ice volume were not slope instability and gully and sheet erosion. The hillslope propagated upstream of the lower Nile in Egypt. environments and fl uvial geomorphology of the Blue Nile The Nile Basin contains a wide range of sedimentary Basin during the global LGM are shown in Figure 13.6. archives that preserve information about Pleistocene and Small glaciers were present on the highest mountains in Holocene environments. Lacustrine sequences from lake Ethiopia and periglacial activity was widespread in these basins of various sizes form the most detailed and well uplands. It is likely that the LGM fl ow regime in the Blue dated of these records (e.g. Gasse et al., 1980; Stager Nile Basin was even more seasonal than at present (Wil- et al., 1997). The alluvial sedimentary record is also liams and Adamson, 1980). important, but most of the work in the Nile Basin has been During this period the regime of the White Nile was conducted in Sudan and Egypt and, in general, the depo- also highly seasonal and with very low fl ows and even sitional records tend to be much more fragmented and less periods of complete drying out for all or part of the year well dated than the lake sediment records (Butzer, 1980; (Williams et al., 2000). This is a key contrast with the Said, 1981). An exception is the lower White Nile, where modern White Nile system. This fl ow regime was the an alluvial record extending back at least 240 ka is reason- product of lower Equatorial rainfall during the Late Pleis- ably well preserved due to very gentle gradients and tocene and the absence of a connection with Lake Victo- minimal erosion (Williams et al., 2003). In some areas the ria. Thus, the White Nile in Sudan became isolated from Late Pleistocene and Early Holocene fl uvial geomorphol- its Equatorial headwaters in Uganda, and, as noted above, ogy is well preserved and large-scale valley fl oor land- it is the White Nile that is presently responsible for main- forms point to the existence of very different fl uvial taining the perennial fl ow of the main Nile, especially environments at these times. Figure 13.5 shows the chang- during drought years. With greater aridity at this time the ing fl uvial geomorphology of the lower White Nile valley valley fl oor of the White Nile was characterized by exten- during the global Last Glacial Maximum (LGM) (ca. 18– sive active aeolian dune systems derived from the sandy 20 000 14C years BP), the Early Holocene (ca. 7000 14C fl uvial bed load and these were large enough to block the years BP) and the present day. In a series of papers, channel zone in some reaches (Figures 13.5 and 13.7). The Adamson et al. (1980), Williams and Adamson (1980) dunes are typically aligned north to south and it is likely and Williams et al. (2000) have reviewed the Late Qua- that some of the discharge may have continued to fl ow ternary history of the Nile and its main tributaries and their north through the dune complexes (Williams and Adamson, fi ndings, along with the outcomes of recent work on the 1980). The Late Pleistocene White Nile in central Sudan The Nile 269

Figure 13.5 The geomorphology of the middle and lower reaches of the White Nile during the global Last Glacial Maximum, the Early Holocene and present day. Note that the dates are in radiocarbon years before present. The geomorphology of the reaches demarcated by the boxes on the present Nile is shown in more detail in Figure 13.7. Reprinted from Global and Planetary Change, 26, M.A.J. Williams et al., Late Quaternary . . . White Nile, pages 305–316, Copyright (2000), with permission from Elsevier 270 Large Rivers

Figure 13.6 Geomorphological processes in the Blue Nile Basin during (a) the global Last Glacial Maximum and (b) the Early Holocene (after Williams and Adamson, 1980) saw a range of large scale fl uvio-aeolian interactions with waters and suspended sediments from the Blue Nile were each process regime providing sediment for aeolian or conveyed via a series of distributaries across the Gezira fl uvial transport and there is a good deal of evidence for fan (upstream of the confl uence at Khartoum) and into the aeolian modifi cation of fl uvial landforms and vice versa. reaches of the lower White Nile (Figure 13.5). This has These process interactions would have been in evidence produced complex alluvial sedimentary records in this on a seasonal basis and in the longer term. Williams et al. large confl uence zone. (2000: 310) have argued that fl ood discharges were highly variable in the White Nile valley during this period, and 13.4.2 12 500 to 5000 14C Years BP seasonal ponding of runoff behind dunes and mobile sand bars would have created a fl uvial landscape similar to that Shortly after about 12 500 14C years BP enhanced rainfall of the central Niger valley today. It is also important to in the tropics led to the overfl ow of Lake Victoria and appreciate that for much of the Late Pleistocene, fl ood Lake Albert (Figure 13.1 and Table 13.4) leading to ‘dra- The Nile 271

Figure 13.7 Geomorphological maps of the lower White Nile fl oodplain showing large-scale fl uvial landforms relating to higher discharges in the Early Holocene and aeolian dunes representing more arid periods and much lower fl ows. (a) Reaches near Esh Shawal. (b) Reaches near El Geteina. See Figure 13.5 for locations. Reprinted from Global and Planetary Change, 26, M.A.J. Williams et al., ‘Late Quaternary . . . White Nile’, pages 305–316, Copyright (2000), with permission from Elsevier.

matic and enduring changes in the lower White Nile and led to the deposition of fi ne-grained sediments over valley’ (Williams et al., 2000: 310). The onset of the an extensive area of the lower White Nile valley (Williams modern discharge regime in the Nile Basin can be traced et al., 2000). The reaches near Esh Shawel record a back to the beginning of this Late-glacial and early Holo- sequence of extensive Late Pleistocene linear sand dunes cene wet phase around 12 000 14C years BP (Talbot et al., that were partly reworked by fl uvial processes and partly 2000). An increase in regional humidity and a shift to a buried beneath the fl uvio-lacustrine sediments that were perennial fl ow regime led to stabilization of the dune laid down across much of the valley fl oor when these systems on the White Nile valley fl oor by vegetation. In reaches were inundated by the extensive terminal Pleisto- some areas the dunes were buried beneath low energy cene White Nile lake (Figure 13.7a) (Williams et al., fi ne-grained fl uvio-lacustrine clays (Figures 13.5 and 2000). Further north on the western margins of the Gezira 13.7). By about 11 500 to 11 000 14C years BP, the White Plain near El Geteina, well-preserved and highly sinuous and Blue Niles conveyed very large fl ood discharges and palaeochannels of terminal Pleistocene and Early Holo- this produced a very extensive inundated area in the lower cene age are evident (Figure 13.7b). White Nile valley (Figure 13.5). This inundated area was After ca. 12 000 14C years BP, the climate became much about 4 to 5 m higher and 20 to 40 km wider than present wetter in the Ethiopian Highlands as the summer monsoon 272 Large Rivers

increased in intensity and water fl uxes from the Blue Nile fl oor geomorphology and human use of the river environ- headwaters increased. This resulted in an increase in ve- ment during the Neolithic, Kerma (ca. 2500 to 1500 cal- getation cover and widespread slope stability and soil for- endar years BC), and later periods (Woodward et al., 2001; mation and a net decrease in sediment fl ux from the Blue Welsby et al., 2002). Extensive systematic survey of the Nile Basin. The geomorphology of the early Holocene valley fl oor has recorded over 450 new archaeological Blue Nile is summarized in Figure 13.6. In the lower sites and many of these are closely associated with former reaches of the Blue Nile the main channel incised into the channels of the Nile up to 18 km to the east of the modern valley fl oor leaving a series of perched palaeochannels on Nile. Geomorphological research and OSL dating of the the surface of the Gezira fan complex (Figure 13.5). On Holocene alluvial record has shown that these palaeochan- the main Nile in northern Sudan, over 15 km to the east of nel belts were active during the Neolithic and Kerma the modern channel, large palaeochannel belts have been periods (Macklin and Woodward, 1998, 2001). Decreas- dated by OSL to the Early Holocene with the oldest ages ing Nile fl ows after ca. 2500 calendar years BC resulted in >7000 calendar years BP (Woodward et al., 2001). Some major changes in the settlement pattern on the valley fl oor of these palaeochannels are associated with Neolithic sites with all archaeological sites located close to the palaeo- in the Northern Dongola Reach (Welsby, 2001) and rep- channel systems. However, by the middle of the fi rst resent high discharges at this time. During the early Holo- millennium BC, all permanent settlements had become cene humid phase, the northern front of monsoonal rainfall concentrated in the west along the banks of the course of extended about 700 km further north than present produc- the modern river (Welsby, 2001; Welsby et al., 2002). ing lake basins in the Sahara (Ritchie et al., 1985), recharg- Krom et al. (2002) have shown that the sedimentary ing groundwaters and feeding seasonal fl ows in local wadi record in the delta can provide a more or less continuous systems in northern Sudan. More extensive river systems record of changing catchment behaviour for much of the also supplied water and sediment to the Nile in the Early Holocene. Figure 13.9 is a strontium isotope record from and Middle Holocene including the Wadi Howar Basin a sediment core taken from Manzala Lagoon in the Nile that drained a large catchment in western Sudan (Figure Delta. Sediments from the Blue Nile Basin derived from 13.1) until it dried out around 4500 14C years BP (Pachur young volcanics and other rocks contain low strontium and Kropelin, 1987). isotope ratios. The ancient shield terrains of the White This general model is supported by work in the Nile Nile headwaters yield sediments with higher strontium Delta by Foucault and Stanley (1989). They showed that isotope ratios and this contrast provides a valuable basis the contrasts in lithology between the White Nile and Blue for establishing a long-term record of the changing prov- Nile headwaters produces suspended sediment loads in enance of fl uvial sediment deposited in the delta. The form these rivers with distinctive heavy mineral assemblages. of the strontium isotope curve refl ects changing environ- Analysis of sediments from delta cores spanning the last ments in the Nile headwaters and, in general terms, pro- 40 000 years or so have shown that the heavy mineral suite vides a proxy for Nile fl ows and sediment fl ux for the last refl ects changes in the relative contributions of sediment 7000 years or so (Krom et al., 2002). The lower part of from the Blue Nile and the White Nile and provides a the record shows high fl ows in the Early Holocene Nile more or less continuous record of sediment source varia- that correlate with some of the large palaeochannel forms tions in the major Nile tributaries. in the White Nile Valley (Figure 13.7). This part of the record refl ects a signifi cant fl ux of sediment from the White Nile Basin. The steep fall in the curve down to ca. 13.4.3 5000 14C Years BP to Present 4200 calendar years BP refl ects a progressive increase in Over the last fi ve millennia or so the climate in much of sediment fl ux from the Ethiopian Highlands as Nile fl ows the Nile Basin has become increasingly dry with the devel- declined following a decrease in monsoon intensity and a opment of conditions that are comparable with the present. reduction in vegetation cover in the Blue Nile headwaters. Detailed records of environmental change from this period Krom et al. (2002) and Stanley et al. (2003) have argued in the arid zone of northern Sudan have been recovered that these strontium isotope minima provide the fi rst geo- from former lake basins in the Sahara Desert (e.g. Ritchie logical data showing a catastrophic decline in Nile fl ows et al., 1985). This period saw the establishment of the that may have led to the demise of the Old Kingdom (Bell, exotic Nile that fl ows through the arid lands of northern 1971). The central part of the record (after 4200 calendar Sudan and Egypt without any signifi cant tributary inputs years BP) shows fl uctuating Nile fl ows up to around 1500 downstream of the Atbara. calendar years BP when the record may be partly infl u- Recent work in the Dongola Reach of northern Sudan enced by enhanced sediment yields from Ethiopia follow- (Figure 13.8) has revealed dramatic changes in valley ing deforestation. In general terms the record shows a long The Nile 273

Figure 13.8 Geomorphology of the Nile channel complex in the Dongola Reach in northern Sudan at low fl ow. (a) A large sand bar and thick channel margin fl ood deposits. (b) A fl ood channel to the east of the main Nile. View looking downstream in each case. Photographs taken by Jamie Woodward in February 1997

term drying trend during the course of the Holocene and a freshwater layer across the surface of the Eastern Medi- one with the tantalizing prospect of linking it to the long terranean. Indeed, Rossignol-Strick et al. (1982: 3) have historical fl ood records. argued that if the discharge of the Late-glacial Nile was 2.5 times greater than today (similar to the present Zambezi), it could have produced a 25 m deep freshwater 13.5 RECORDS OF NILE RIVER BEHAVIOUR IN layer across the surface of the Levantine and Ionian THE EASTERN MEDITERRANEAN SEA basins in only 15 years. Recent work by Williams et al. The marine sedimentary record in the Eastern Mediter- (2003) has shown that times of very high fl ow in the ranean provides a valuable record of the long-term White Nile over the past 240 ka that have been dated by behaviour of the River Nile (Rossignol-Strick et al., OSL show a good correlation with the dated sapropel 1982) because long-term changes in the fl ux of water and record in the Mediterranean. sediment from the Blue and White Nile Basins have exerted an important infl uence on oceanographic and 13.6 THE MODERN NILE: HYDROLOGY sedimentation dynamics. The sedimentary record in the AND GEOMORPHOLOGY Eastern Mediterranean is punctuated by the occurrence of dark, organic rich layers known as sapropels (Stanley, The climate and hydrology of the modern Nile Basin 1978; Rohling, 1994; Thomson et al., 1999). These dis- ranges from Equatorial in the White Nile headwaters to tinctive sediments range in thickness from a few milli- hyperarid in the desert region of northern Sudan and metres to tens of centimetres and represent episodes of Egypt. Thus, for much of its course, the Nile is an exotic stratifi cation and deep-water stagnation (when organic river. Despite its large area, on average, runoff from the material was not broken down) and a net increase in bio- Nile Basin accounts for only around 2 % of the total runoff logical productivity – which increases the fl ux of organic from the African continent (UNESCO, 1978). In fact it matter to the sea fl oor. Periods of enhanced discharge ranks fi fth behind the Congo, Niger, Ogooue and Zambezi from the Nile have been shown to correspond with Rivers for mean annual runoff (Table 13.3). Data pre- periods of sapropel deposition as the fl oodwaters created sented in UNESCO (1978) show that the Congo River is 274 Large Rivers

Figure 13.9 The strontium isotope record from the Nile Delta (Manzala Lagoon) showing the fl uctuations in Nile fl ows for much of the Holocene. Note that the dates in the far right column are given in calibrated radiocarbon years before present. See text for dis- cussion. Reproduced from Geoarchaeology: An International Journal, 18 (3), Stanley, D.-J., Krom, M.D., Cliff, R.A. and Woodward, J.C., Nile fl ow failure at the end of the Old Kingdom Egypt: strontium isotopic and petrologic evidence, 395–402, Copyright (2003), John Wiley & Sons., Inc

the most signifi cant of the large African rivers in terms Table 13.3 Runoff data for the largest rivers on the African continent (after UNESCO, 1978 and Walling, 1996) of water fl ows with the highest mean annual runoff of 1414 km3 which compares with 73 km3 for the Nile River Mean annual Contribution to total (Table 13.3). runoff external runoff from Africa (%) (mm) (km3) Congo 370 1414 38.0 13.6.1 The White Nile Basin Niger 128 268 7.2 The southern headwaters of the White Nile lie in the Ogooue 732 149 4.0 Equatorial lakes region of east-central Africa (Figures Zambezi 80 106 2.8 13.1 and 13.2). This part of the basin is formed in ancient Nile 25 73 2.0 Sanaga 503 68 1.8 uplifted and rifted rocks of Precambrian age in Kenya, Volta 103 41 1.1 Uganda and the Democratic Republic of Congo (DRC). The Nile 275

The uplifted terrains of the western rift form the watershed toum and 800 mm year−1 near Malakal. The western moun- with the eastern headwaters of the Congo River (Figure tains of the Lake Plateau are extremely wet with around 13.3). The highest point in the Nile basin is Mount Stanley 360 rain days and over 5000 mm of precipitation each year (5109 m) in the Ruwenzori Mountain range between Lake (Said, 1993). The hydrology of the White Nile headwaters Edward and Lake Albert on the border between the DRC is strongly infl uenced by the presence of large lakes and and Uganda. The geological evolution of this area is swamps and the main lakes and swamps in the Nile basin complex and it was subjected to tectonic movements are listed in Table 13.4. These water bodies cover an area related to the African Rift system during Tertiary and approaching 130 000 km2. Lake Victoria, which is the Quaternary times (Said, 1981). The drainage of the Lakes second largest freshwater lake in the world, is by far the Plateau region into the Nile Basin followed regional uplift largest (ca. 67 000 km2) and exerts an important control on and the formation of Lake Victoria. Prior to this the White Nile discharge (Tate et al., 2001). Annual fl ows region’s drainage appears have formed part of the eastern into Lake Victoria of about 20 km3 year−1 are typical. This headwaters of the Congo River (Said, 1981) (Figure 13.3). infl ow is made up from 7.5 km3 year−1 from the Kagera Figure 13.2 shows the long profi le of the White Nile with River (which forms the border between Rwanda and Tan- its steep gradient reaches in the Lake Plateau region with zania), 8.4 km3 year−1 from north-east Kenya, 3.2 km3 from large waterfalls and rapids between Ripon Falls (1135 m) the Serengeti of Tanzania and 1–2 km3 year−1 from the and Juba (455 m) – a distance of some 874 km. Down- swamplands of north-west Uganda (Howell et al., 1988; stream of Juba the White Nile is a very low gradient FAO, 1997). The Ripon Falls in Uganda form the only system with an average slope of 1 : 13 900 in the Sudd outlet from Lake Victoria. The volume of the outfl ows Region and 1 : 101 000 between Malakal and Khartoum in (ca. 40 km3 year−1, Table 13.4) – which feed the Victoria its middle and lower reaches where it fl ows across the Nile – is very sensitive to precipitation inputs to the Lake aggraded fl oor of the ∼400 ka mega-lake (Williams et al., Victoria Basin as evaporation rates do not vary greatly 2003) (Figures 13.2 and 13.5). The Sudd overlies a vast from year to year. The Victoria Nile fl ows into Lake Albert alluvial fan, hence its relatively steep gradient. The exten- via Lake Kioga (Figure 13.1). sive low-lying alluvial plain of the Sudd region extends A dominant characteristic of the White Nile fl ow regime for over 220 km on either side of the modern river channel is that the seasonal variability of fl ows is highly damped complex (Said, 1981). by storage in lakes and swamps. The Sudd Region down- Rainfall in the White Nile Basin varies markedly stream of Juba contains a huge area of swamplands (Figure between the Lake Victoria catchment (with annual rainfall 13.10) and evaporation from these water bodies is a very of <2000 mm) and the arid and semi-arid environments in signifi cant component of the water balance. Typical evapo- central Sudan with values of about 175 mm year−1 at Khar- ration losses from this part of the White Nile Basin are

Table 13.4 Long-term water balance data for lakes and swamps in the Nile Basin for various periods of the twentieth century (after Sene et al., 2001). The monitoring periods are approximations in some cases Water body Area (km2) Monitoring Active Nile Local runoff Outfl ow Direct rainfall Evaporation period storage infl ow (km3 year−1) (km3 (km3 year−1) (km3 year−1) (km3) (km3 year−1) year−1) Kagera swamps 3 000 1940–1973 3.8 0.0 8.1 6.3 3.0 4.8 Lake Victoria 67 000 1925–1990 208.4 0.0 25.9 40.0 121.3 107.2 Lake Kioga 6 270 1970–1973 23.8 40.0 3.0 42.1 9.1 10.0 Lakes George and 2 500 0.8 0.0 3.6 2.6 3.5 4.5 Edward Lake Albert 5 400 1970–1973 28.1 42.1 6.2 45.1 5.4 8.6 Albert-Sudd reach 380 swamps Sudd swamps 22 400 1961–1980 60.5 50.3 0.0 21.4 19.3 48.2 Machar marshes 3 200 1950–1955 0.0 1.4 0.1 6.0 7.3 Bahr el Ghazal <16 600 1942–1986 0.0 11.3 0.3 9.5 20.5 swamps Lake Tana 3 000 1928–1937 0.0 3.9 4.0 3.8 3.7 Totals <129 750 63.4 180.9 214.8 276 Large Rivers

Figure 13.10 Geomorphological map of the Blue Nile and Atbara River basins and the middle and lower reaches of the White Nile. Note the large-scale areas of fan sedimentation in the lower reaches of each of these basins (after Williams and Adamson, 1980). The Sudd region is also shown upstream of Malakal on the White Nile (see Figure 13.2) The Nile 277

shown in Table 13.5. According to Howell et al. (1988), those parts of the Ethiopian Highlands lying above 2000 m between 1905 and 1980 the mean annual water loss to and the deeply dissected drainage network that feeds the evaporation and evapotranspiration in the Sudd region main tributaries of the Blue Nile and Atbara rivers. Inci- was 16.9 km3 and this represents about 51.2 % of the sion of these uplands was initiated by regional uplift fl uvial input to the area as recorded on the White Nile at during the Late Oligocene to Miocene, the Late Pliocene Mongalla. The Sudd region has an extremely low gradient and the Late Pleistocene (McDougall et al., 1975). This and contains a series of natural channels that fl ow through created a superimposed drainage network from a cover of permanently waterlogged swamps fl anked by seasonally Oligocene to Miocene age basalts into Mesozoic lime- fl ooded grasslands. The Sudd stretches over 900 km from stone and sandstone and granite and metamorphic rocks Juba to Malakal (Figures 13.2 and 13.10) and hydrological of Precambrian age (Williams and Williams, 1980). Note assessments have shown that the evaporation losses are that several tributaries of the Sobat River drain the south- greatest in those years when infl ows are highest and the ern part of this region before traversing low gradient inundated areas expand to cover a much greater area swamplands in central Sudan and eventually joining the (Howell et al., 1988). High levels of rainfall in the lakes White Nile upstream of Malakal (Figure 13.10). region of the White Nile headwaters in the 1960s and Downstream of Khartoum the fl ood hydrology of the 1970s led to a signifi cant increase in the perennial swamp main Nile is dominated by runoff from the Blue Nile and area in the Sudd from 2700 km2 in 1952 to 16 200 km2 in the Atbara (Table 13.6) and the climate and hydrology of 1980. Downstream of the Sudd Region, between Malakal their catchments are dominated by the infl uence of the and Khartoum, the Nile falls only 8 m over a distance of summer monsoon. Rainfall and runoff in the Upper Blue about 809 km with a mean average gradient of less than Nile catchment (176 000 km2) are strongly seasonal and 1 cm km−1. The climate becomes drier to the north and many tributary channels do not convey fl ows in the dry these reaches do not contain swamps and permanent water season from November to May (Conway, 2000). The bodies. Downstream of the Sobat River junction there are Upper Blue Nile has a mean annual discharge of 48.5 km3 no major tributary inputs south of Khartoum (Figure 13.1). (1912–1997) and inter-annual variability in fl ows is con- The White Nile below Malakal is gently incised forming trolled by the intensity of the monsoon and by ENSO, with a broad and shallow alluvial channel that commonly bifur- low fl ow during years of strongly negative Southern cates around the mid-channel bars and islands that are Oscillation Index, at least in the past hundred years, and common in these reaches (Williams et al., 1982). The conversely (Whetton et al., 1990). Precipitation dynamics channel planform of the White Nile upstream of the in the Blue Nile and Atbara headwaters are infl uenced by ∼400 ka mega-lake bed differs from that of the river where three main mechanisms (Conway, 2000): it fl ows along the former lake fl oor but has yet to be studied in detail. (1) the summer monsoon; (2) tropical upper easterlies; (3) local convergence in the Red Sea coastal region. 13.6.2 The Blue Nile and Atbara Basins The headwaters of the Blue Nile and the Atbara River lie About 70 % of the annual rainfall in the Blue Nile head- in the highlands of Ethiopia and these upland fl uvial waters of Ethiopia falls between June and September systems contain steep river channels with coarse boulder declining from 2000 mm in the southwest to less than beds that fl ow within deep gorges. Figure 13.10 shows 1000 mm in the northeast (Conway, 2000). The lower

Table 13.5 Water balance of the Sudd region for various Table 13.6 Mean annual discharges for the major basins of periods of the twentieth century (after Howell et al., 1988 and the Nile as reported by Williams et al. (1982) FAO, 1997) River basin Discharge Peak fl ow Low fl ow Period Annual Annual Annual % (km3) (%) (%) discharge outfl ow from losses White Nile 27.5 10 83 at Mongalla swamps (km3) Blue Nile 51.0 68 17 (km3) (km3) Atbara 12.5 22 0 1905–1960 26.8 14.2 12.6 47.0 Main Nile 91 1961–1980 50.3 21.4 28.9 57.5 Ethiopian 1905–1980 33.0 16.1 16.9 51.2 Highlands 63.5 90 17 278 Large Rivers

Blue Nile has a catchment area of about 324 530 km2 13.10) – a body of sediment forming the large low-angle (Shahin, 1985) and its fl ow regime is also characterized inland alluvial fan of radius ∼300 km that lies upstream of by highly seasonal fl ows and marked inter-annual vari- Khartoum between the lower reaches of the Blue and ability. On average about 85 % of annual fl ow of the Blue White Niles (Williams et al., 1982). Extensive areas of Nile is concentrated into one third of the year between fan deposits in the lower gradient reaches of the Atbara July and October. The 1900 to 1982 gauging station and Blue Nile Basins are shown in Figure 13.10 and these records for the Blue Nile at Khartoum show a mean represent signifi cant long-term sediment sinks. In its lower monthly range in discharge from 9 to 8834 m3 s −1 with a reaches the Blue Nile is a narrow and deep, single channel mean of 1513 m3 s −1. The fl ow regime of the Atbara exhib- system incised into the Holocene and Pleistocene alluvial its even greater seasonality and the river is commonly dry sediments of its former fl oodplain. from December to May with very low fl ows in November and June. Between 1912 and 1982 the mean discharge in 13.6.3 The Confl uence Zone and the Desert Nile the Atbara at the Kilo 3 station (69 000 km2) for the month − from Khartoum to the Mediterranean of August ranged from 533 to 4928 m3 s 1 with an average fi gure of 2002 m3 s −1. A key function of the major water bodies in the Upper The Ethiopian Highlands are strongly dissected and the White Nile Basin is the maintenance of the perennial base Blue Nile fl ows in a narrow, steep-sided valley for much fl ow of the desert Nile from November to June when the of its course. Between its source at Lake Tana (1829 m) Blue Nile and Atbara are not in fl ood (Figure 13.11a). The and the Sudan border – a distance of about 850 m (Figure ratio of peak monthly fl ow to minimum monthly fl ow is 13.10) – the Blue Nile falls 1350 m at a mean gradient of typically 40 : 1 in the Blue Nile and 5 : 2 in the White Nile. 0.0016. From the border to Khartoum (374 m), a distance The Blue Nile fl ood reaches its peak in late August to early of some 900 km, the gradient is much more subdued September with a rise in stage of about 10 m above its low (0.00013) (Williams et al., 1982). This change in channel season fl ow at Khartoum. The White Nile fl ood peak gradient indicates that much of the coarser bed load mate- arrives later and is much lower (ca. +2 m) – but lasts from rials derived from the granites, basalts and sandstones of late September until January (Williams et al., 2000) the Ethiopian Highlands now probably lie below the fi ne- (Figure 13.11a). Before the construction of large dams this grained sediments of the Gezira Fan Formation (Figure contrast in fl ood stage at the confl uence resulted in the

Figure 13.11 (a) Typical annual fl ow regime of the pre-impoundment Nile at Aswan showing the contributions from the three major tributary basins. Reproduced from The Nile: a General Account of the River and the Utilization of Its Waters, H.E. Hurst, Discharge of the Nile at Aswan, 1952, Constable and Company, London. (b) The discharge and suspended sediment budget of the Nile. Reprinted by permission from Macmillan Publishers Ltd: Nature, 339, 6219, Alain Foucault, Daniel Jean Stanley, Late Quaternary palaeoclimatic oscillations in East Africa recorded by heavy minerals in the Nile delta, pages, 44–46, Copyright (1989) The Nile 279

formation of a seasonal lake that extended for more than an average year (Sharaf El Din, 1977; Stanley, 1996). 300 km upstream of Khartoum along the White Nile fl ood- Approximately one third of the fl ow reaching the delta is plain (Williams et al., 2000). The width of this lake ranged lost via evapotranspiration and infi ltration to groundwater from 1 to 7 km. while another third drains slowly through irrigation chan- The six cataracts of the main desert Nile lie in the nels and canals. Thus, as Stanley (1996) has pointed out, reaches between Khartoum (378 m) and Aswan (91 m) only about one third of the discharge that passes Cairo over a distance of 1847 km where the channel falls over actually reaches the Mediterranean Sea. The last ‘normal’ 280 m (Figure 13.2). The great bends of the Nile are per- fl ood discharge in the Nile was in 1964 and the fl ux of haps the most distinctive feature of the channel between water to the Mediterranean between 1961 and 1973 is the sixth and second cataracts (Figure 13.1). Adamson and shown in Figure 13.12a. Note the marked decline in the Williams (1980) have suggested that the great bend that annual peak of the controlled releases from Aswan and incorporates the sixth, fi fth and fourth cataracts is a the shift in the timing of peak from summer (natural) to product of updoming and volcanic activity in the Bayuda winter (controlled). area where evidence for volcanic fl ows and volcanic cones Before the completion of the Aswan High Dam in 1964 is extensive. These authors also point out that the promi- the annual Nile fl ood exerted an important infl uence on nent dykes and lineaments that are oriented ENE and SSE the hydrographic conditions and the pattern of circulation correspond with the course of the Nile as it bends around in the Eastern Mediterranean Sea up to about 80 km from the Bayuda dome (Figure 13.3). The bedrock reaches the coast and to a maximum depth of ca. 150 m (Sharaf El account for about 30 % (565 km) of the channel length Din, 1977). During the fl ood season (August to October) between Khartoum and Aswan, where the steeper gradient the salinity of the Mediterranean Sea fell from about 39 cataract reaches are formed in resistant crystalline rocks to <30 ppm at the near shore surface (Sharaf El Din, 1977). (Said, 1981). Between the cataracts lie low gradient basins The huge particulate and solute loads associated with the where softer Nubian sandstones have been eroded to fl oodwaters formed a vital role in the biology of the provide accommodation space for expanses of Pleisto- marine environment. cene and Holocene alluvial sediments (Figure 13.8). One example is the Dongola Reach of northern Sudan where 13.7 THE SUSPENDED SEDIMENT BUDGET the eastern side of the valley fl oor is fl anked by a low relief Holocene fl oodplain complex with well preserved palaeo- This section examines the contemporary suspended sedi- channel belts up to 18 km from the modern channel of the ment budget of the basin upstream of the delta complex. Nile (Macklin and Woodward, 2001; Woodward et al., Suspended sediment concentrations and loads have been 2001). Extensive aeolian dune systems are present in this measured at several stations on the Egyptian Nile – espe- reach and there is evidence in some reaches of large scale cially in the decades leading up to the planning and con- wind erosion of the Holocene alluvial record (Welsby struction of the Aswan High Dam – but continuous et al., 2002). long-term records are not presently available in the scien- Many of the large African rivers have fl ow records that tifi c literature. A good deal of the early work is docu- extend back into the nineteenth century because gauging mented in Volume IX of The Nile Basin (Hurst et al., stations were established during the colonial era. Uniquely, 1959). The suspended sediment load of the Nile is rather the Nile has a long-term historical fl ood series that extends modest in comparison with its basin area. Milliman and − back to 622 AD at the Roda Island gauge at Cairo and these Syvitski (1992) quote a value of 120 million tonnes year 1 data have been analysed by many scholars (Brooks, 1927; for the average annual pre-Aswan High Dam load and a Fraedrich et al., 1997). Modern discharge measurements mean annual sediment yield of 40 t km−2 year−1 (Table on the Nile began in 1869 at Khartoum and in 1870 at 13.7). The sediment loads of many of the large rivers of Aswan. The fl ow records for the twentieth century show Africa that drain the ancient shield terrains of the tropical signifi cant variability – the maximum water yield of interior are also low by world standards (Table 13.7). 120 km3 was recorded in 1916 while 1984 registered a It is important to appreciate that sediment production minimum value of 42 km3. Between 1870 and 1984 the and sediment delivery vary markedly in the major tribu- Nile at Aswan had a mean monthly discharge of 2700– tary basins and marked spatial and temporal (annual and 2800 m3 s −1 (Figure 13.11b). El-Manadely et al. (2002) seasonal) variability in suspended sediment fl ux is a key report that the average yearly discharge of the Nile at feature of the fl uvial geomorphology of the basin. Sus- Aswan is 84 km3. It has been estimated that about half of pended sediment concentrations and loads are subject to the annual discharge released from the Aswan High Dam considerable annual variation. According to Abu El-Ata reaches the delta and this represents about 34–35 km3 in (1978) the annual sediment load of the Nile at Aswan has 280 Large Rivers

Figure 13.12 (a) The pre- and post-impoundment fl ow regime at Aswan between 1961 and 1973. Reprinted from Limnology and Oceanography, 22, 2, Sharaf El Din. Copyright (1977) by the American Society of Limnology and Oceanography, Inc. (b) Discharge and suspended sediment concentration series for summer fl oods on the main Nile at Wadi Halfa (1931) and Gaafra (1955). Reprinted from Hydrology of the Nile Basin, Developments in Water Science 21, Mamdouh Shahin, page 460, Copyright (1985), with permis- sion from Elsevier

varied between 50 and 228 million tonnes. Due to the erodible upland terrains, high sediment supply and high Table 13.7 The suspended sediment loads and yields of some runoff, much of the suspended sediment load measured at large African rivers (from Milliman and Syvitski, 1992). The Aswan derives from the highlands of Ethiopia. In fact, pre-impoundment loads are given in parentheses and these values have been used for the yield estimates together the Blue Nile and Atbara contribute on average about 97 % of the Nile’s annual suspended sediment load River Basin area Sediment load Sediment yield (with about 72 % from the Blue Nile and 25 % from the (×106 km2) (×106 t year−1) (t km−2 year−1) Atbara) and the White Nile the remaining 3.0 % (Figure Nile 3.00 0 (120) 40 13.11b) (Foucault and Stanley, 1989). Suspended sedi- Congo 3.80 43 11 ment fl ux is dominated by the summer fl ood in the Blue Orange 0.89 17 (89) 100 Nile and Atbara catchments. For example, with a peak Niger 1.20 40 33 fl ow to low fl ow ratio of 40 : 1, and typical mean August Zambezi 1.40 20 (48) 35 − and June sediment concentrations of 4000 and 100 mg l 1 Limpopo 0.41 33 80 respectively, the Blue Nile can transport up to 1600 times The Nile 281

more sediment during the summer fl oods than at the end Aswan High Dam period showed that bed load sediment of the dry season (Williams et al., 1982: 119). transport accounted for only about 1–2 % of the total load. The White Nile drains the resistant rocks of the Equato- Thus it became customary to consider the suspended sedi- rial plateau and these terrains have much lower erodibility ment load as the total load. One would expect a signifi cant and a more effective cover of protective vegetation in fl ux of coarse bed load material in the steep and fl ashy contrast to the deeply dissected Blue Nile headwaters. headwater streams of the Blue Nile and Atbara basins. Consequently, suspended sediment concentrations are Indeed, as mentioned above, Williams et al. (1982: 117) much lower in the fl oodwaters of the White Nile and the speculate that much of the coarser bed load materials lower gradient White Nile is a much less effi cient sus- eroded from the granites, sandstones and basalts of Ethio- pended sediment delivery system (Figure 13.11b). A sig- pia now probably form the base of the Gezira Formation nifi cant proportion of its suspended sediment load is also to the south of Khartoum (Figure 13.10), a surmise con- trapped in the swamps and marshes of the Sudd region in sistent with drilling records. the middle White Nile reaches. The Sudd swamps also act as an effi cient biochemical fi lter, since the solute concen- 13.7.1 Reservoir Sedimentation tration of the river on leaving the Sudd is equal to that on entering it, despite the loss of half the original fl ow volume While there are still key gaps in our understanding of through evapotranspiration and seepage (Williams and basin-wide sediment dynamics in the Nile, some of the Adamson, 1973). In the main Nile at Aswan the sediment most useful information on recent suspended sediment wave tends to peak in August before the main fl ood peak fl uxes derives from repeat bathymetric surveys of the large and several weeks may separate these two events. The artifi cial reservoirs. Furthermore, because they form very suspended sediment concentrations for the summer fl ood effective sediment traps, these reservoirs form an impor- of 1955 at Gaafra (near Aswan) peak at over 5500 mg l−1 tant component of the suspended sediment budget of the in mid-August and then decline steadily to <500 mg l−1 by Nile Basin. As the Ethiopian Highlands are the main the end of October (Figure 13.12b). Many of the sus- source of suspended sediment in the basin, the river pended sediment sampling programmes have tended to systems draining this region have witnessed some of the focus on the fl ood season and the major fi ndings of this most severe sediment management problems since the work on the main Nile in Egypt have been reported by construction of large dams. Shahin (1985) and may be summarized as follows: The Roseires Dam (1966) on the Blue Nile and the Khashm el Girba Dam (1964) on the Atbara River (Figure • The mean annual pre-impoundment fl ux of suspended 13.1) both encountered major problems with sedimenta- sediment at Aswan was about 125 million tonnes. tion within a decade of their construction (Elsheikh et al., • Approximately 98 % of the annual suspended sediment 1991). The Roseires Dam was built close to the Ethiopian load is transported during the fl ood season from August border to store Blue Nile water for irrigation use in central to November and suspended sediment concentrations Sudan. According to these authors, the storage capacity of may range from about 100 to >5800 mg l−1 during this the Roseires reservoir (3.02 × 109 m 3) had fallen by 18 % period (Figure 13.12b). after 10 years and 21 % after 15 years – while the capacity • The relationship between suspended sediment concen- of the Khasm el Girba reservoir fell by 25.4 % after 7 years tration and water discharge shows a clear tendency for of operation and by 42.3 % after 12 years. Bathymetric clockwise hysteresis with sediment concentration rising survey showed that 540 million m3 of sediment had been rapidly and peaking before the main fl ood peak (Figure deposited in the Roseires reservoir between 1966 and the 13.12b). 1975/1976 hydrological year – giving a mean annual • Typical particle size distributions for samples of the value for sediment deposition of about 54 million m3 for suspended load at Aswan contained 30 % clay, 45 % silt this period. The total volume of deposited sediment had and 25 % sand – and the proportion of sand tended to increased to 640 million m3 by 1980/81 (Elsheikh et al., increase as the fl ood season progressed. Samples of 1991). suspended sediment collected from the delta region of In the Ethiopian headwaters of the Blue Nile and Atbara Lake Nasser displayed a mean particle size distribution Rivers, mean annual rates of soil loss today amount to with 30 % fi ne sand, 40 % silt and 30 % clay (El- about 40 t ha−1 (2 mm year−1) on mountain slopes, but attain Manadely et al., 2002). rates of over 300 t ha−1 (15 mm year−1) during cultivation years, or some 5–10 times more than in non-mountainous Data on bed load fl ux in the Nile Basin are extremely areas (Hurni, 1999). In many areas the increasing demand scarce but Shahin (1985) reports that studies in the pre- for land has meant a reduction in fallow to virtually zero 282 Large Rivers

and an expansion of the area under cultivation. In one 0.02 m s−1 (Shahin, 1985). Thus, the trap effi ciency in the region in Gojjam Province the area cultivated rose from Aswan High Dam is close to 100 % and under normal 40 % in 1957 to 77 % in 1995, while natural forest land conditions only the fi nest fractions of the suspended load decreased from 27 % to 0.3 % (Hurni, 1999). These rates are transported to the downstream portion of the reservoir. of accelerated soil erosion can be compared with long-term The total volume of deposited sediment between 1964 and denudation rates for the catchment. The mean rate of geo- 1998 has been estimated to be 3.5 km3 and 90 % of this logical erosion over the past 23.5 Ma was 0.01 mm year−1 sediment is within the Sudanese side of the reservoir (El- in the Ethiopian headwaters of the Blue Nile (Williams and Manadely et al., 2002). This estimate is based on a series Williams, 1980). The rate of erosion in the Blue Nile of re-surveys of the original Nile river channel bed levels headwaters during the 70 years before completion of the and valley fl oor cross-sections that were conducted before Aswan High Dam was 0.12–0.24 mm year−1. In contrast, in the construction of the dam. The last major sedimentation the 1970s annual soil loss from parts of the Ethiopian survey was carried out in 1998 when 15 cross-sections Plateau was roughly 0.4–1.0 mm year−1. This represents a were surveyed in the reservoir’s delta between 487 and two orders of magnitude increase in the rate of soil loss – a 337.5 km upstream of the dam and the results are shown rate that is much faster than corresponding rates of soil in Figure 13.13a. Comparison of the pre-reservoir bed formation. levels and the 1998 survey shows that net sediment depo- Removal of forest in the seasonally wet tropical uplands sition ranges from ca. 2.4 m (at cross-section 23) to a of Ethiopia has changed the local hydrological balance, maximum of 60.2 m at cross-section 6 (Figure 13.13a) resulting in reduced infi ltration, increased runoff and (El-Manadely et al., 2002). It is important to appreciate increased soil loss. The downstream effects are equally that the width of the cross-sections (CS) increases from signifi cant and in this instance are not confi ned to Ethio- 300 m at CS 23 to 4100 m at CS 26 to 9300 m at CS 22 pia. By 1996, the capacity of the Roseires reservoir had (Figure 13a) and this widening leads to a decrease in fl ow been reduced by almost 60 % by the infl ux of sediment competence. from the Ethiopian uplands and that at Khashm el Girba Suspended sediment concentrations in this part of Lake on the Atbara, by 40 % (Swain, 1997). It thus appears that Nasser/Nubia based on samples collected in July 1978 deforestation in the headwaters of the Blue Nile and and May 1998 are shown in Figure 13.13b and refl ect Atbara Rivers during the past 40 years has triggered a this decrease in fl ow velocity moving northwards along wave of accelerated soil loss from cultivated land and has the delta complex. The 1998 samples contain maximum led to siltation of reservoirs many hundreds of kilometres mean suspended sediment concentrations of 172 mg l−1 downstream. (487 m upstream of the dam) and this value declines Lake Nasser and Lake Nubia (the latter is the Sudanese steadily to less than 50 mg l−1 around 150 km further sector of the reservoir behind the Aswan High Dam) down reservoir (El-Manadely et al., 2002). A general (Figure 13.1) have a maximum surface area of 600 km2 fi ning of the suspended sediment load accompanies the and this equates to a maximum storage capacity of 162 km3. decline in sediment concentration. Mean clay content El-Manadely et al. (2002) have described the nature of the increases from around 30 % at the entrance to Lake three storage zones in the reservoir: Nasser to >70 % over the same distance. The survey work by El-Manadely et al. (2002), based on the calcu- (1) the dead storage zone – used for sediment accumula- lated volume of sediment deposited between 1964 and tion up to 147 m asl – with a maximum capacity of 1998 in the Lake Nasser delta, gives an estimate of 31 km3; 310 years for the lifetime of the dead storage zone (2) the live storage zone – 147 to 175 m asl – with a allocated for sediment accumulation (31 km3) in Lake maximum capacity of 90 km3; Nasser. These researchers also point out that signifi cant (3) the fl ood-buffer storage zone – 175 to 182 m asl – with volumes of sediment may be transported down reservoir a capacity of 41 km3. towards the dam when large fl ood events follow a sus- tained period of low fl ows because this results in net Stanley (1996: 191) has described the sediment deposition erosion of some parts of the Lake Nasser delta complex. in Lake Nasser as ‘the New Nile delta’ and by the mid- A further modifi cation to the sediment budget of this vast 1990s this rapidly growing feature was about 200 km long, reservoir will follow the completion of the new dam at 12 km wide and about 40 m thick – extending northwards Merowe on the fourth cataract as this will result in a to Abu Simbel. When the annual fl ood reaches Lake marked reduction in suspended sediment fl ux to Lake Nubia, the fl ow velocity falls from >1.0 m s−1 to about Nasser/Nubia. The Nile 283

Figure 13.13 (a) Patterns of sediment deposition in Lake Nasser in 1973, 1985 and 1998. (b) Suspended sediment concentrations in Lake Nasser based on sampling programmes in July 1978 and May 1998. (a, b) Reprinted from Lakes and Reservoirs, 7, El- Manadely, M.S. Abdel-Bary, R.M., El-Sammany, M.S. Ahmed, T.A., Characteristics of the delta formation resulting from sediment deposition in Lake Nasser, Egypt, pages 81–86, Copyright (2002), with permission from Blackwell Publishing

13.7.2 Suspended Sediment Dynamics Downstream (<2 µm) is now the major particulate fraction transported of the Aswan High Dam from the reservoir behind the Aswan High Dam to the downstream reaches of the Egyptian Nile. Stanley and The trap effi ciency of Lake Nasser/Nubia is close to 100 % Wingerath (1996) have studied the clay mineralogy of Nile and this has created a dramatic shift in sediment dynamics sediments to assess the impact of dam construction on the and geomorphological processes in the reaches down- composition of the suspended load. The clay fraction of stream of the Aswan High Dam. The sluices of the dam suspended sediments entering Lake Nasser contains a are opened periodically to maintain main channel fl ows for large proportion of smectite (∼70 %) with some kaolinite navigation, irrigation and for hydroelectric power. Between (<25 %) and illite (<10 %) refl ecting the composition of the 1965 and 1973 the mean fl ux of suspended sediment volcanic terrains in the Blue Nile headwaters. However, passing through the Aswan High Dam was estimated to be the proportion of kaolinite increases to ∼70 % in the clay- about 2.5 million tonnes (Shahin, 1985). Stanley and rich suspended load downstream of the dam as changes to Wingerath (1996) state that about 98 % of the sediment the sediment budget increase the signifi cance of other load (mainly silt and fi ne sand) that had previously been sources of kaolinite. This clay mineral is blown into the transported to the delta and the Mediterranean Sea is reservoir from the desert and washed in from wave erosion deposited in Lake Nasser/Nubia and clay-sized material of the reservoir margins. It is also introduced into the main 284 Large Rivers

channel by clear water erosion of valley fl oor sediments features of the coastline are the Rosetta and Damietta and rocks downstream of the dam. Stanley and Wingerath promontories and the broad Burullus headland between (1996) argue that the kaolinite trace can be used to monitor them that was created by sediment delivered from the the impoundment-related alterations to river fl ows and former Sebennitic branch of the Nile about 6500 14C years sediment composition. These impoundment-induced BP (Orlova and Zenkovitch, 1974; Frihy et al., 2003) changes to the regional sediment budget from Lake Nasser (Figure 13.15a). Since the completion of the Aswan High to the delta are summarized in Figure 13.14. Dam, coastal erosion and sediment redistribution have been major concerns and, in very general terms, the 13.8 THE NILE DELTA AND THE actively prograding delta has been replaced by an eroding EASTERN MEDITERRANEAN coastal plain (Stanley and Warne, 1993; Frihy et al., 2003). The eroding promontories are the principal sources 13.8.1 The Delta and Coastal Zone of sediment for the coastal zone. Rates of shoreline change The modern Nile Delta began forming in the early have been estimated from beach surveys and Frihy and Holocene about 7500 14C years BP and is the largest dep- Komar (1993) reported maximum rates of retreat along ocentre in the Mediterranean region (Stanley and Warne, the Rosetta promontory of 106 m year−1 with rates of 1993). For much of this time the delta complex was built 10 m year−1 and 7 m year−1 at the Damietta and Burullus up by a series of large distributaries that conveyed water promontories, respectively. Zones of sediment accretion and sediment across the delta and to the coast. Nine major were identifi ed within the embayments between the prom- channel systems have been identifi ed in the Holocene ontories with a maximum shoreline advance averaging deposits of the northern delta and between two and fi ve about 13 m year−1. Research by Frihy and co-workers over of these were active at any one time (Sestini, 1992; Stanley the last two decades has shown that the sediment budget and Warne, 1993; Stanley, 1996). Today, downstream of at the coast can be viewed as a series of regional littoral Cairo, the Nile divides into two main distributaries – the sedimentation cells that extend from Alexandria to Akko Rosetta and Damietta channels and these fl ow to the north- on the northern margin of Haifa Bay, Israel. Each cell west and north-east, respectively, where they have formed contains a coherent system of sediment sources, fl uxes and marked promontories at the coast (Figure 13.15a). These sinks (Frihy et al., 2003). two distributaries have been the main conduits for water and sediment fl ux to the coast since the end of the nine- 13.8.2 Sediment Supply to the Eastern teenth century. The delta covers an area of around Mediterranean Sea 22 000 km2 and the distance from Cairo to the coast is about 160 km. The rate of alluvial sedimentation on the The Nile has been the major supplier of suspended sedi- delta and the fl oodplain of the lower Nile before 1964 has ment to the Eastern Mediterranean Sea since Late Miocene been estimated to be 60 to 150 mm century−1 with a mean times until the construction of the Aswan High Dam. In rate of 0.8 mm year−1 (Shahin, 1985). the Late Quaternary and Holocene marine sedimentary Since the closure of the Aswan High Dam in 1964, the record across the region, sands of Nile provenance have main sources of suspended sediment downstream of been identifi ed in the Nile Cone and along the margins of Aswan have been fl uvial erosion of the river channel Israel, while fi ner grained sediments (clays) have been margins and direct inputs of aeolian dust. In fact, sus- traced as far north as Turkey (Stanley and Wingerath, pended sediment concentrations greater than 850 mg l−1 1996). While the Nile was the dominant supplier of fi ne- have been recorded in Nile fl ows at Cairo since the closure grained particulate material to the Levantine basin up to of the dam (Stanley, 1996). However, only a very small 1964, aeolian dust from the Sahara has long formed a very proportion of this material actually reaches the coast signifi cant component of the Eastern Mediterranean because the delta’s large barrages and the very high marine sediment budget. Krom et al. (1999) have used density of drainage canals and irrigation channels form a strontium isotopes (87Sr/86Sr) to map the dispersal of Nile- vast and very effi cient network for trapping sediment. derived suspended sediments across the Levantine basin Figure 13.15b provides a graphic illustration of the human to establish those parts of the marine environment that will impact on the drainage network on the delta. Many of the be most affected by the trapping of sediment in the Aswan larger artifi cial channels drain into the deltas wetlands and High Dam. Their results are shown in Figure 13.16. It is lagoons so that sediment delivery to the coast is further clear that Nile-derived sediment is dispersed widely across reduced (Stanley, 1996). the Levantine basin with the highest values (>80 %) to the The coastal zone of the delta extends from east of Alex- east of the Nile Cone and towards the coast of Israel fol- andria to Port Said – a distance of about 240 km. The main lowing transport by easterly-directed surface currents The Nile 285

Figure 13.14 Geomorphological processes and the suspended sediment budget in the desert Nile (between Khartoum and the Medi- terranean Sea) in relation to the sources and fl ux of kaolinite-rich sediments before and after the construction of the Aswan High Dam. See text for explanation. Reprinted from Marine Geology, 133, 1, D.J. Stanley and J.G. Wingerath, Nile sediment dispersal altered . . . , pages 1–9, Copyright (1996), with permission from Elsevier Figure 13.15 (a) The major geomorphological features and canals and pumping stations of the Nile delta and coastline. The arrows show the direction of major coastal currents and associated sediment fl ux. The main barrages on the delta are also shown. (b) Map of the delta showing more than 10 000 km of irrigation canals and drains. This map is based on charts produced by the Defence Mapping Agency (1970–1993). This area represents the most intensively modifi ed portion of the Nile drainage basin. (a, b) Reprinted from Marine Geology, 129, 3–4, Daniel Jean Stanley, Nile delta extreme case of . . . , pages 189–195, Copyright (1996), with permis- sion from Elsevier The Nile 287

Figure 13.16 The pattern of Nile sediment dispersal in the Eastern Mediterranean Sea based on strontium isotope source tracing. The map shows the percentage of Nile-derived particulate matter in each area. Reprinted from Marine Geology. 155, 2–4, Krom et al., The characterization of Saharan . . . , pages 319–330, Copyright (1999), with permission from Elsevier

(Krom et al., 1999). The pattern of Nile-derived sediment historical context of Pleistocene and Holocene environ- distribution mapped by Krom and his co-workers showed mental change. Profound changes in river behaviour took that Nile sediment was more widely distributed than pre- place during these periods as fl uctuations in Equatorial viously thought and the Aswan High Dam has changed rainfall and monsoon intensity resulted in marked changes the sedimentation dynamics across much of the Levantine in fl ow regimes and fl ow volumes, sediment supply and basin. The bulk of the rest of the fi ne-sediment fraction river channel planform and fl oodplain sedimentation style. on the sea fl oor was made up of aeolian dust from the For example, in the central and lower reaches of the White Sahara. The long-term impacts of this change in sediment Nile valley the large palaeochannels of the Early Holocene quality on the geochemistry and biology of the marine Nile lie adjacent to extensive aeolian dune systems pro- benthic environment are not known. Krom et al. (1999) viding a graphic illustration of the impact of shifts in point out that because Nile particulate matter may play an Quaternary climate from wetter to drier modes. The Blue important role in the removal of phosphate by adsorption Nile has also seen dramatic changes in water and sediment from the deep waters of the marine environment – another fl ux during the Late Pleistocene and Holocene. Further- consequence of the Aswan High Dam may be a phos- more, recent research on Holocene sequences in the desert phate-driven increase in productivity in the Eastern Medi- Nile of northern Sudan (Woodward et al., 2001; Welsby terranean Sea (Krom et al., 1991). et al., 2002) and in the delta (Krom et al., 2002; Stanley et al., 2003) has shown that Nile fl ows and riparian envi- ronments can change abruptly and with potentially cata- 13.9 RIVER BASIN MANAGEMENT AND strophic consequences for local populations. GLOBAL CHANGE The scale of the Nile Basin and the marked basin-wide It is clear from this review that any discussion of the contrasts in river environments discussed in this chapter contemporary fl uvial geomorphology and hydrology of present a wide range of resource management issues and the modern Nile Basin must be set within an appropriate it is not possible to discuss them in detail here. The primary 288 Large Rivers

concerns of the ten riparian states are water storage and allocation for irrigation and hydropower generation. The essence of the political confl ict surrounds the confl ict between the desires of the ‘upstream’ basin states to fully exploit the potential offered by Nile fl ows in their own territories, with the needs of the downstream users – par- ticularly Egypt. The Nile Waters Treaty of 1959 allows the Sudan to abstract up to 18.5 km3 year−1 – this is about 22 % of the estimated mean Nile fl ow of 84 km3 year−1 that formed the basis of the agreement. The rest of the fl ow is accounted for by the allocation to Egypt and losses to seepage and evaporation (Sene et al., 2001). Egypt has the lowest concentration of arable land per head of population of any African state and this has led to more intensive farming methods and to the expansion of the cultivated area through year-round irrigation both on the delta and in the Nile Valley. Political tensions over water allocations for the basin states resurfaced in early 2004 with a meeting of the Nile Basin Initiative in Uganda. The riparian states upstream of Egypt and Sudan aim to develop their hydro- electric potential and the drought-prone lands of Ethiopia and Eritrea have plans for large-scale irrigation schemes of their own. The Nile Basin Treaty of 1929 states:

‘Without the consent of the Egyptian government, no irrigation or hydroelectric works can be established on the tributaries of the Nile or their lakes, if such works can cause a drop in water level harmful to Egypt.’

Sutcliffe and Lazenby (1994) have reviewed the hydrologi- cal data requirements for planning the management of water resources in the Nile Basin. They emphasize the importance of accurate and consistent long-term hydro- logical records for the development of an effective basin- wide resource management plan. They also highlight the need for more effi cient use of irrigation waters in Egypt and Sudan. Over the last century or so the dominant basin- wide strategy for the management of water resources in the Nile Basin has been the construction of large dams and Figure 13.17 Major water resource development projects in the Nile Basin (after Newson, 1992). The Merowe Dam project on reservoirs. The major water resource development projects the main Nile in Sudan is under construction (Askouri, 2004). are shown in Figure 13.17. The major reservoirs provide Reprinted from Dams, People and Development, Hussein M. essential storage to serve the ever-growing and year-round Fahim, Copyright (1981) and Land, Water and Development, irrigation demands in Egypt and Sudan. Storage losses to M.D. Newson, Copyright (1992). London, Routledge sedimentation are a signifi cant management issue in the Atbara and Blue Nile Basins and a basin-wide sediment ing major wetlands. For example, Williams and Adamson monitoring and management strategy is needed (Sutcliffe (1973) have argued that some of the potential consequences and Lazenby, 1994). The Jonglei Canal project in the Sudd of draining part of the Sudd include exposure of saline (Figure 13.17) remains unfi nished due to ongoing confl icts soils, an increase in channel bank erosion downstream, and and political problems in Sudan (Howell et al., 1988). This a change in water chemistry. scheme was intended to reduce evaporation losses in the Sutcliffe and Lazenby (1994) have pointed out that the swamplands and to increase the net water yield of the issue of climate change and its effect on water discharge White Nile Basin. However, caution is needed when drain- is especially important in the Nile Basin because the low The Nile 289

runoff coeffi cients associated with the rainfall regime basin has so recently experienced. An understanding suggest that even a modest change in precipitation will of the past behaviour of such a river system as the have a disproportionate effect on runoff. They cite the Nile, and of its depositional legacy in the Sudan and average long-term runoff value of just 55 mm at the Egypt, is therefore of both local and world-wide Dongola gauging station in northern Sudan (19° N) to interest.’ illustrate this point. The Nile comprises highly contrasting tributary basins but runoff from each is sensitive to ACKNOWLEDGEMENTS changes in rainfall and this is why marked fl uctuations in river fl ows on the main Nile have been recorded in the We would like to thank Nick Scarle (School of Environ- past (Sutcliffe and Lazenby, 1994). ment and Development, The University of Manchester) The long historical discharge records for the Nile for expertly drawing the maps and diagrams. We also (Hassan, 1981) and the richness of the archaeological and thank Avi Gupta and the two referees for their helpful Quaternary sedimentary records mean that this basin pro- reviews. vides a unique context for the study of global climate change and its impact upon the hydrology and sediment REFERENCES dynamics of a large river and its population. Recent work Abu El-Ata, A. (1978) Egypt and the Nile after the High Aswan in the Nile Basin has suggested that the river system can Dam. Ministry of Irrigation and Land Reclamation, Egypt. change rapidly and fl ow volumes from both the Blue Nile Adamson, D.A. (1982) The integrated Nile. In: A Land Between and the White Nile can decrease abruptly (Krom et al., Two Niles: Quaternary Geology and Biology in Central Sudan 2002). The long fl ood records have been interrogated to (M.A.J. Williams and D.A. Adamson, Eds.). A.A. 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