The University of Manchester Research The River Nile Document Version Accepted author manuscript Link to publication record in Manchester Research Explorer Citation for published version (APA): Woodward, J., Macklin, M. G., Krom, M. D., & Williams, M. A. J. (Accepted/In press). The River Nile: Evolution and Environment. In A. Gupta (Ed.), Large Rivers: Geomorphology and Management (Second ed.). John Wiley & Sons Ltd. Published in: Large Rivers Citing this paper Please note that where the full-text provided on Manchester Research Explorer is the Author Accepted Manuscript or Proof version this may differ from the final Published version. If citing, it is advised that you check and use the publisher's definitive version. General rights Copyright and moral rights for the publications made accessible in the Research Explorer are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Takedown policy If you believe that this document breaches copyright please refer to the University of Manchester’s Takedown Procedures [http://man.ac.uk/04Y6Bo] or contact [email protected] providing relevant details, so we can investigate your claim. Download date:24. Sep. 2021 Chapter 14 The River Nile: Evolution and Environment Jamie C. Woodward1, Mark G. Macklin2, Michael D. Krom3 and Martin A.J. Williams4 1 Department of Geography, The University of Manchester, Manchester M13 9PL, UK 2 School of Geography, University of Lincoln, Lincoln, LN6 7TS, UK 3School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 4 Department of Earth Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia 14.1 INTRODUCTION The Nile Basin drains about one tenth of the African continent and contains the longest river channel system in the world (>6500 km). The evolution of the modern drainage network and its fluvial geomorphology reflect both long-term tectonic and volcanic processes and associated changes in erosion and sedimentation, in addition to sea level changes (Said, 1981) and major shifts in both climate and vegetation during the Quaternary (Williams and Faure, 1980; Talbot and Williams, 2009; Woodward et al. 2015a; Williams 2019). In the Holocene, abrupt climate shifts, human impacts in the form of land use change over the last few millennia, and large dam construction over the last hundred years or so, have had major catchment-wide impacts on the hydrology and sediment budget of the River Nile. The river basin extends over 35° of latitude (4° S to 31° N) incorporating a great diversity of climates, river regimes, biomes and terrains – including the Equatorial lakes plateau of the White Nile headwaters, the semi-arid volcanic uplands of Ethiopia, the Sahara Desert, and the vast delta complex at the Eastern Mediterranean Sea. The Nile is also the world’s longest exotic river – it flows for almost 2700 km without any significant perennial tributary inputs. The true desert Nile begins at Khartoum (15° 37′ N 32° 33′ E) on the Gezira Plain where the Blue and White Nile converge (Figure 14.1). These two systems, and the Atbara to the north, are large rivers in their own right with distinctive fluvial landscapes and flow regimes. The Nile has a total catchment area of around 3.3 million km2 and about one third of this is the desert Nile of Sudan and Egypt (Figure 14.1 and Table 14.1). The Nile Basin supports a vast range of ecosystems (Rzóska, 1976) and has played a central role in the development of a rich diversity of cultures (Wendorf et al., 1976; Welsby, 1998, 2001; Shaw, 2003; Spencer et al. 2017). It has been an important corridor for the movement of people and animals throughout the Quaternary and the desert Nile and delta saw the rise of the Kingdoms of Egypt (Davies and Friedman, 1999) and Sudan (Welsby and Anderson, 2004) and their great irrigation-based civilizations (Hassan, 1988; Macklin et al. 2013; Bunbury, 2019). As of 2019, more than 250 million people live within the basin and about half are almost totally dependent on Nile waters for economic and domestic needs. By 2030 the population in the Nile basin is expected to exceed 320 million people (Nile Basin Initiative 2012). While much of the basin is still sparsely populated, rapidly growing urban centres, most notably Cairo (population >19.5 1 million in 2018) and greater Khartoum (population >6 million) – and extensive tracts of agricultural land on the valley floor and delta – are heavily or totally reliant on Nile flows for irrigation and power generation. Figure 14.1 Map of the Nile Basin showing the drainage network, major tributary basins and extent of the desert Nile (after Woodward et al. 2015b). Lake Victoria in the White Nile headwaters is the largest lake in Africa by area (~60,000 km2). Table 14.1 Drainage basin area of the Nile and major tributary basins. The area of the Desert Nile is the difference between the total area of the Nile basin minus the sum of the White Nile, Blue Nile, and Atbara basins (after Woodward et al. 2015b). River basin Area (km2) % of basin Nile at the Mediterranean 3,310,000 100 White Nile at Khartoum 1,730,000 52.3 Blue Nile at Khartoum 330,000 10 Atbara at Nile confluence 180,000 5.4 Desert Nile 1,070,000 32.3 This chapter reviews the evolution of the river – with a particular focus on the Quaternary Period including the present-day hydrology and fluvial geomorphology. Since the first edition of this chapter was published (Woodward et al. 2007), there has been much new research on the response of the River Nile to Late Pleistocene and Holocene climatic changes and a renewed focus on the implications of a changing fluvial environment for our understanding of the archaeological record. All of this work provides important context for the contemporary fluvial hydrology and geomorphology of the basin. The suspended sediment budgets of the basin and offshore zone are also discussed and the impacts of the major water resource management developments (mainly large dam construction) on water and sediment fluxes are assessed. 14.2 NILE BASIN RIVER ENVIRONMENTS According to Said (1981) the Nile Basin encompasses five major regions that differ from one another in structure and geological history (Figures 14.1 and 14.2). These are listed below and the characteristic fluvial environments of each region are briefly described: (1) The Equatorial lake plateau in the White Nile headwaters upstream of Juba with densely forested, tropical catchments with perennial flow regimes. (2) The Sudd region swamplands of South Sudan and the low gradient floodplains, broad channel belts and low suspended sediment concentrations of the reaches between Malakal and Khartoum. (3) The Ethiopian Highlands forming the headwaters of the Blue Nile and the Atbara with deep ravines, unstable slopes, entrenched steep gradient gravel-bed streams with highly seasonal flows and high sediment loads. (4) The great bends, rocky cataracts and vast alluvial reaches of the desert Nile with highly ephemeral tributary wadi systems and alluvial fans in the arid lands of Sudan and Egypt and with a strongly modified flow and sediment regime downstream of the Aswan High Dam. (5) The Egyptian region including the low gradient delta complex with its large distributaries, promontories and lagoons, heavily modified drainage and dense network of irrigation canals. Figure 14.2 Long profile of the Nile from the White Nile headwaters to the Mediterranean Sea (after Said, 1994). Note that the Blue Nile and Atbara are not shown Marked contrasts are evident in the hydrology and fluvial geomorphology of each of the five regions. Distinctive large river valley-floor environments are present in each sector and these reflect the strong regional variety in topography, tectonic setting, climate, hydrology, vegetation, sediment supply and other factors. The legacy of Quaternary sediments and landforms on the valley floor is also apparent in many parts of the basin and these may exert an important influence on contemporary fluvial processes and on the chemical and physical properties of alluvial soils, especially soil permeability and soil salinity (Williams et al., 2000). These contrasts in river basin dynamics pose a wide range of catchment management issues and the nature and extent of the human impact and use of river-related resources varies markedly across the basin. 14.3 EARLY ORIGINS AND THE NEOGENE NILE Several detailed reviews and edited collections exploring the evolution of the Nile drainage basin have been published in recent decades (Adamson and Williams, 1980; Williams and Williams, 1980; Said, 1981, 1993; Adamson et al., 1993; Talbot and Williams, 2009; Macgregor, 2012; Woodward et al 2015a; Fielding et al., 2017, 2018; Abdelsalam, 2018; Williams, 2019). These document the development of the modern drainage configuration following the influence of major tectonic phenomena – including the rifting of East Africa – climatic changes and other factors. Adamson and Williams (1980), Adamson et al. (1993), Talbot and Williams (2009) and Abdelsalam (2018) have examined the structure and tectonic history of the Nile Basin and adjacent regions and have related these geological phenomena to the organization of the drainage network, to the location of major sedimentary basins, and to major changes in the former extent of the catchment. They argue that tectonic events of great antiquity – probably dating to the East-African orogenic event (ca. 850-550 Ma) or earlier – have been influential in shaping the Nile Basin and the courses of the large river channel systems that form the modern catchment. The watershed of the present Nile and its relationship to the major rift zones and other large river systems to the east and west of the upper and middle Nile has changed periodically over the past thirty million years (30 Ma).