1.1 Outline, Purpose, and Scope
Megafans are fluvial sedimentary landforms of very low gradient and fan-shaped planform, with radial lengths of several tens, and up to hundreds of kilometres – i.e., significantly larger than the well-known smaller, mountain-front alluvial fans, but still much smaller than giant submarine fans such as the Indus and Bengal. However, megafans have long been poorly recognised in the literature on subaerial geomorphology and sedimentology. These landforms now require scientific attention, not only for reasons of fundamental Earth science understanding, but because of their importance as landscape elements occupied by major population centres around the world.
Our driving concern in this volume is to derive generalisations from the relatively limited total global population of ~ 270 identified megafans (as documented by remote means (Wilkinson and Currit, Ch. 2) compared with the thousands of examples of small alluvial fans detectable from the air, and many more in the near-surface Neogene geological record. The geomorphic and tectonic settings for megafans on each continent raise different kinds of research problems and approaches to solving them. Thus, in South America, the active Andean orogen has given rise to a large number of currently active megafans. By contrast, the Indo-Gangetic basin displays not only some active megafans but also others that are significantly incised and appear as successions of terraces. Cratonic Africa and Australia show patterns of isolated fans. Five chapters deal with groups of megafans (Africa, Asia, Central Europe, South America), the rest with more detailed studies of one or two megafans. The studies include megafans situated in basins of all major tectonic styles. Antarctica displays no subaerial megafans, and North America is excluded because so few modern examples exist and because of space limitations in this volume.
There seems little doubt about the importance of megafans as landforms on which very large human populations subsist – what Geddes (Reference Geddes1960:253) termed the ‘alluvial plains of profound significance […] that have tended to be almost completely ignored in geomorphology, instead of providing a central theme for physical study…’. Fortunately, Geddes’s (Reference Geddes1960:258–259) comment of six decades ago (‘even a general study of the world’s plains is lacking (…) in spite of their immense environmental significance’) has begun to be reversed. While focusing on northern India, he also suggested that perspectives from the Gangetic Plains might well apply to the study of similar alluvial settings in South America, North America, and parts of Europe, as is attempted in this volume. The vast agricultural potential of such plains, the relative ease of constructing irrigation and transport systems on their remarkably flat surfaces, and their associated vulnerability to extensive flooding – as witnessed in 2008 (and almost annually since 2013) by the Kosi River megafan in northern India (hereafter megafans are named simply after their formative rivers) – all argue for further study, scrutiny, and public awareness. Vast expanses of megafan terrain in African and South America (particularly the Chaco) are coveted for setting up irrigation projects, plantations, or tourism ventures. Water grabs in Mali (Niger megafan), and recent warfare over land in oil- and groundwater-rich South Sudan have also been prominent in current affairs (Pearce Reference Pearce2013). Until recently desolate, untamed and untenured, a number of megafans are among the last frontiers of this planet.
1.2 Expanding Perspectives on Megafans
1.2.1 Earlier Perspectives: Limited Recognition Pre-1990
Before space-based observation of Earth’s landscapes was available, a few megafans had been described. Prime examples are the Kosi in India, which remains probably the most cited example (e.g., Geddes Reference Geddes1960; Parkash et al. Reference Parkash, Awasthi and Gohain1983; Wells and Dorr Reference Wells and Dorr1987a, b; Gohain and Parkash Reference Gohain, Parkash, Rachocki and Church1990; Mohindra et al. Reference Mohindra, Parkash and Prasad1992; Richards et al. Reference Richards, Chandra, Friend, Best and Bristow1993; Singh et al. Reference Singh, Parkash and Gohain1993; Sinha and Friend Reference Sinha and Friend1994; Shukla et al. Reference Shukla, Singh, Sharma and Sharma2001; Goodbred Reference Goodbred2003); the Okavango (for numerous early references, see historical review in McCarthy Reference McCarthy2013); and megafans in the Andean foreland (Cordini Reference Cordini1947, and early references in Iriondo Reference Iriondo1993; Horton and DeCelles Reference Horton and DeCelles2001; Latrubesse et al. Reference Latrubesse, Stevaux and Cremon2012) and in the Pantanal region of SW Brazil (Klammer Reference Klammer1982; Tricart Reference Tricart1982; Souza et al. Reference Souza, Araujo and Mertes2002; Assine Reference Assine2005). In francophone literature, the ‘inland delta’ of the Niger River, in the Sahel region of Mali, was studied by Urvoy (Reference Urvoy1942) and later by Gallais (Reference Gallais1967) prior to awareness of the fluvial megafan idiom sensu hic. This early recognition of deltas and large fans in continental interiors by isolated pioneers displays parallels in the history of science with other very large landforms such as megaflood scars in the Pacific Northwest of the United States (Bretz Reference Bretz1923) – in this case erosional rather than predominantly depositional landforms. At first critiqued by incredulous detractors (see review about the Channeled Scablands of the NW USA in Baker Reference Baker, Baker and Nummedal1978), ‘scablands’ have now not only been validated, but also detected outside their type area by means of satellites and underwater sonar technology – from the very doorstep of modern geology’s European homebase (the English Channel: Gupta et al. Reference Gupta, Collier and Garcia-Moreno2017) to remote regions such as Siberia and other areas in the solar system such as Mars (Burr et al. 2009).
Studies of individual megafans, however, often overlooked wider suites of neighbouring megafans, and thus the broader subregional-scale megafan setting. For example, the Okavango megafan in the Kalahari Basin of southern Africa, visually prominent in aerial imagery, has claimed perhaps even the bulk of attention for the past century. However, it is now known to be only one of a group of at least ten megafans in the region (Wilkinson et al., Ch. 4). Thus, only four multi-fan landscapes benefited from published studies prior to 1990, namely the Indo-Gangetic plains of northern India (Geddes Reference Geddes1960), the Chaco plains of Argentina and Paraguay (Iriondo Reference Iriondo1984, Reference Iriondo1987), the Pantanal (Tricart Reference Tricart1982; Tricart et al. Reference Tricart, caut and Pagney1984) in southwestern Brazil, and the Hungarian Plains (Borsy Reference Borsy, Rachocki and Church1990). These studies were necessarily idiosyncratic to their local basins, with Geddes (Reference Geddes1960:262) noting that some major Himalayan rivers such as the Ghaghara failed to display ‘great alluvial fans or cones’ compared with the continuous set of active megafans generated by major rivers in central South America. Experience from megafan landscapes of one continent was only tenuously transferred to other continents, partly because of language barriers and slow diffusion of the studies, and partly through the assumption that such landscapes were unusual or simply not representative of planetary landforms.
The widely held view of megafans as rare landforms was supported by the small number of known examples and by the scant scientific attention directed to these features, leading to cursory treatment in reference works – mostly as large end members of the spectrum of piedmont alluvial fans. Schumm’s explicit opinion – in his influential 1977 book The Fluvial System – that large ‘wet’ fans [i.e., megafans] must have been widespread during pre-vegetation times probably reinforced the view that few such fans should be expected in modern landscapes. Experienced field geologists have noted that the very low slopes and occasionally immense size have made large fans difficult to recognise in modern landscapes (N. Cameron and R. Miller, pers. comm. to MJW). Lack of recognition was likely reinforced by the age of many megafans, as drainage patterns and fluvial morphology are progressively overprinted in remotely sensed imagery by eolian features, incision and terracing, and vegetation patterns. This has been especially the case within a broad geological mindset that had assumed that vast alluvial landscapes are specifically connected to coastlines in the form of deltas. This view still dogs research into fluvial landscapes and sediments on Mars (Wilkinson et al., Ch. 16).
A cultural component probably also played a part. Megafans are presently almost non-existent or inconsequential in the landscapes of Europe and North America, where Earth science matured as a modern discipline during the twentieth century. This coincidence has probably conditioned the pervasive view that incisional fluvial regimes, so dominant in these continents, are the norm on all continents. Thus, Schumm’s (Reference Schumm1977) classic three-zone model of the drainage system is based on the topographic sequence mountain–valley-confined floodplain–coastal delta, with the Mississippi drainage clearly in mind. This model reinforces the concept of rivers as sediment-bypass systems rather than as potentially aggradational systems in their own right, and implicitly excludes the vast megafan-dominated landscapes that are now attracting growing attention.
1.2.2 Accelerated Scientific Activity since ~ 1990: Global Mapping, Approaches, and Definitions
As mentioned above, a few examples of megafans were known before the 1990s. Blair and McPherson (1994) had specifically excluded large fluvial fans from the alluvial-fan designation. In their view, megafans belonged in the class of typical valley-confined floodplains, to be distinguished from short-radius, higher-gradient piedmont alluvial fans. Blair and McPherson (1994) reasserted the original definition of alluvial fans, namely as coarse-grained features with relatively steep slopes, of the type classically associated with the small desert alluvial fan less than 15–20 km in length, and distinguishable from larger river systems also in terms of sedimentary processes and products. Their view is interesting because they gave little validity to fanlike morphology, which is otherwise the overwhelmingly dominant approach.
Following widely held views, Miall (Reference Miall1996) took instead an inclusive stance of grouping large fans within a more broadly defined alluvial-fan class. However, large known megafans at that time, such as the Kosi megafan of northern India, the Chaco megafans, and the Pantanal, were not included by Miall (Reference Miall1996) (See Wilkinson, Ch. 17, Section 17.3). In a more detailed analysis, Stanistreet and McCarthy (Reference Stanistreet and McCarthy1993) also took a more inclusive view, classifying all sizes of fan-like fluvial landforms as alluvial fans, with categories based on process and included small alluvial fans, braided fans, and the largest, so-called losimean (i.e., low sinuosity and meandering) Okavango type (150 km long).
Simultaneously, the increasing availability of satellite remote sensing products started to open up new potential for the identification of megafans worldwide. For example, starting in 1988 at the Johnson Space Center, astronaut-handheld imagery of continental surfaces revealed what may have been the first global perspective on megafans. It rapidly provided evidence of more than 150 examples, with some components of these inventories presented at conferences or in grey literature (e.g., Wilkinson 2001, 2005, Reference Wilkinson, Marshall and Lundberg2006; Wilkinson et al. Reference Wilkinson, Cameron and Burke2002, Reference Wilkinson, Marshall and Lundberg2006, Reference Wilkinson, Marshall, Lundberg, Kreslavsky, Hoorn and Wesselingh2010; Sounny-Slitine and Latrubesse Reference Sounny-Slitine and Latrubesse2014).
The task of identifying from remote sensing products the global population of all medium and large fans (i.e., > 30 km long) was complemented by Weissmann et al. (Reference Weissmann, Hartley and Nichols2010, 2011), Hartley et al. (Reference Hartley, Weissmann, Nichols and Warwick2010a, b) and Davidson et al. (2013), resulting in an overarching classification of fan-like fluvial deposits based on 415 examples. These authors applied the innovative term distributive fluvial systems (DFS) ‘to encompass fluvial and alluvial distributive landforms at all scales’ (Weissmann et al. Reference Weissmann, Hartley and Scuderi2015:189), in the attempt to circumvent the semantic issues associated with the definition of alluvial fans. Their purpose was first to identify modern DFS and describe what they deemed the important aspects of their morphology and structural setting; and then, given that they saw DFS to be the areally dominant landforms in present-day continental basins, to propose these as the basis for an ‘alternative interpretation for much of the fluvial rock record’ (Weissmann et al. Reference Weissmann, Hartley and Nichols2010, 2011:329). These authors drew a major distinction between ‘tributary’ drainage patterns (Weissmann et al. Reference Weissmann, Hartley, Nichols, Davidson and North2011:329; also ‘tributive’ in Weissmann et al. Reference Weissmann, Hartley and Scuderi2015:214), which they saw as typical of regional degradational landscapes even though such landscapes include some of the largest and most active river floodplains in the world; and ‘distributive’ drainage patterns, which are typical of many landscapes of regional extent that are dominated by fan-like fluvial deposits of all dimensions.
Global data surveys supported the notion of a genetic continuum for these landforms, earlier demonstrated by Saito (Reference Saito2003), Saito and Oguchi (Reference Saito and Oguchi2005), and Hashimoto et al. (Reference Hashimoto, Oguchi and Hayakawa2008). The continuum questioned the ‘natural depositional slope gap’ that Blair and McPherson (1994) had argued must exist between alluvial fans and floodplains, and they reclassified megafans as ‘rivers [i.e., floodplains] or river deltas’ (Blair and McPherson 1994:457). Confusion was thus compounded because the slope gap does not exist between debris-flow-dominated ‘torrential’ fans and fluvial megafans, even though a process gap between these features does exist—given that smaller debris cones and alluvial fans are shaped by supercritical flow (and some even by non-Newtonian flow), whereas megafans are dominated by fluvial processes under a critical or subcritical flow regime.
The potential climatic conditions for the development of megafans has been another controversial topic. Earlier claims by Leier et al. (Reference Leier, DeCelles and Pelletier2005) are often quoted to support a climatic explanation for the distribution of megafans. Results from different parts of the world demonstrate that megafans can be generated under a broad spectrum of climatic conditions, which range from periglacial to arid, semiarid and temperate climates. Some writers still invoke aridity or pronounced seasonality as an explanation for the existence of megafans (e.g., Fielding et al. Reference Fielding, Ashworth, Best, Prokocki and Sambrook Smith2012; Rossetti et al. 2014; Plink-Björklund Reference Plink-Björklund2015). This might also include equatorial regions covered today by dense tropical rainforest such as the Amazon.
The chapters in this volume thus address the following four dimensions in the study of megafans: (i) two-dimensional space, and thus the characterisation of present-day sky-view morphologies and other visually detectable patterns; (ii) process, by exploring the spectrum from less well understood local autogenic controls to wider allogenic controls such as tectonic setting, catchment geology, and climate; (iii) time, providing constraints on the age of deposits and landform assemblages; and (iv) stratigraphy, spanning the subsurface from shallow depths to depths of hundreds of metres. Due to disparities in documentation and purpose, it would be impossible for each chapter to address all these dimensions, but the list gives a sense of the approaches used thus far in the study of these large sedimentary bodies.
The present renewed attention to modern fluvial landscapes and their dominant fluvial styles, with potential for preservation in the geological record, has led to more detailed comparisons with large, but nevertheless confined floodplains (Fielding et al. Reference Fielding, Ashworth, Best, Prokocki and Sambrook Smith2012), and with related types of landform such as major avulsive fluvial systems, or MAFS (Latrubesse Reference Latrubesse2015), and large accretionary fluvial systems, or LAFS (R. Nanson, pers. comm. to MJW) (see Wilkinson, Ch. 17, Section 17.6.1).
Based on the universality of larger fans in modern sedimentary basins, Weissmann et al. (Reference Weissmann, Hartley and Nichols2010: 41) emphasised the extensive areal scale and distribution of ‘DFS deposits [that] are probably more common than previously recognised in continental strata, and may form the bulk of the continental fluvial record’. This statement highlighted a critical distinction between rivers in long-term degradational settings (on which most facies and architectural models for fluvial deposits are based), and rivers in aggrading settings, the latter being heavily represented by DFS in modern landscapes. Weissmann et al. (Reference Weissmann, Hartley, Nichols, Davidson and North2011) argued that DFS deposits have a particularly high preservation potential in the rock record. Hartley et al. (Reference Hartley, Weissmann, Nichols and Scuderi2010b) agreed that the many models based on converging river patterns at the channel scale ‘provide a very valuable body of literature’ (Weissmann et al. Reference Weissmann, Hartley and Scuderi2015:214), but citing scale considerations they noted that ‘what we believe is missing in the literature on fluvial systems is an understanding of the larger-than-channel belt and basinal context in which fluvial systems are developed’.
The claim for a potentially dominant representation of DFS in the geological record was considered controversial or even rejected (Sambrook Smith et al. Reference Sambrook Smith, Best and Ashworth2010; Fielding et al. Reference Fielding, Ashworth, Best, Prokocki and Sambrook Smith2012; Ashworth and Lewin Reference Ashworth and Lewin2012; Latrubesse Reference Latrubesse2015). The ensuing conversation refocused attention on the dimensions of very wide floodplains and their distinctiveness compared with DFS, especially megafans – a discussion ultimately aimed at the larger question of fluvial sedimentation styles and their preservation potential in the subsurface. Miall (Reference Miall2014), in particular, considered the debate on tributary vs. radial drainage patterns to be important because of its bearing on ‘the mappability and predictability of fluvial systems in the subsurface’ (p. 281). Such patterns are investigated in some detail in chapters in this book. Echoing the critique of Fielding et al. (Reference Fielding, Ashworth, Best, Prokocki and Sambrook Smith2012), however, Miall (Reference Miall2014:281) stated that ‘the most important counter argument to the importance of DFS [in dominating depositional patterns in active continental sedimentary basins] is the abundant documentation of the deposits of large rivers in the rock record’. This important and complex consideration, that of ultimate burial and preservation of fluvial sediment bodies (see especially Miall Reference Miall2014, his chapters 2 and 6; and Miall et al. Reference Miall, Holbrook and Bhattacharya2021), is a topic beyond the scope of this volume. Citing the Amazon, Paraná, and Magdalena rivers, Latrubesse (Reference Latrubesse2015) has given evidence that very large axial rivers all display larger areas of active sedimentation than the largest megafans in central South America – illustrating the capacity of large rivers, even in erosional settings, to give rise to very significant zones of deposition. Latrubesse (Reference Latrubesse2015) argued that some sub-environments of foreland tectonic depressions are inimical to preservation of DFS because they promote the erosional destruction of sediment bodies such as megafans due to the effects of tectonics-driven erosion.
Contrary to claims by Weissmann et al. (Reference Weissmann, Hartley, Nichols, Davidson and North2011), Miall (Reference Miall2014) argued that bedforms and macroforms of facies models cannot serve as a basis for differentiating between degradational vs. aggradational (i.e., DFS-type) geomorphic systems because the processes that apply to these features operate in all rivers – whether valley-confined floodplains or unconfined megafan rivers. Miall (Reference Miall2014:280) reasoned that the difference in the setting was ‘irrelevant’ because bedforms and macroforms develop over time periods and scales small enough to operate in rivers of similar discharge range.
Over the last several years, a growing number of studies have nonetheless documented DFS successions in stratigraphic records from various ages and on all continents (Sáez et al. Reference Sáez, Anadón, Herrero and Moscariello2007; Latrubesse et al. Reference Latrubesse, Cozzuol and Rigsby2010; Trendell et al. Reference Trendell, Atchley and Nordt2013; Gulliford et al. Reference Gulliford, Flint and Hodgson2014; Klausen et al. Reference Klausen, Ryseth, Helland-Hansen, Gawthorpe and Laursen2014; Owen et al. Reference Owen, Nichols and Weissmann2015; Astini et al. Reference Astini, Martini, Oviedo and Álvarez2018), and one chapter of this volume addresses these issues (Ventra and Moscariello, Ch. 14). We note that it is extremely difficult to differentiate in the geologic record between megafans and other large avulsive fluvial systems that are not DFS (Latrubesse et al. Reference Latrubesse, Cozzuol and Rigsby2010; Valente and Latrubesse Reference Valente and Latrubesse2012, Reference Latrubesse2015). The existence of DFS in the rock record is not, however, the main focus of this volume, which is directed primarily at modern and submodern fans at the large end of the fan continuum. Nevertheless, because of the importance of the topic, four chapters are devoted partly or mainly to the deeper stratigraphy of surface megafan deposits (Ch. 8, Ch. 9, Ch. 11, and Ch. 15).
This volume is also an attempt to present the variety of research aims and ensuing methodologies that have been employed in the study of megafans. For example, significantly different results are derived from morphological mapping as opposed to geological mapping. In the former case, distal convergent drainage patterns have been excluded from the computation of area, either explicitly (Hartley et al. Reference Hartley, Weissmann, Nichols and Warwick2010a) or implicitly (Horton and DeCelles Reference Horton and DeCelles1997; Barnes and Heins 2009); whereas geological mapping includes the entire unconfined zone occupied by fluvial landforms and sediments of the feeder river (e.g., Assine et al. Reference Assine, Corradini, Pupim and McGlue2014; Latrubesse et al. Reference Latrubesse, Stevaux and Cremon2012, and chapters in this volume).
Scientific study of megafans has involved a variety of entry points. The most prominent has been their morphological similarity to alluvial fans, perhaps because the planform view had become so familiar in the voluminous literature on alluvial fans, with overviews presented by many authorities (e.g., Lecce Reference Lecce, Rachoki and Church1990; Stanistreet and McCarthy Reference Stanistreet and McCarthy1993; McCarthy and Cadle Reference McCarthy and Cadle1995; Cooke et al. Reference Cooke, Warren and Goudie2006). Stanistreet and McCarthy (Reference Stanistreet and McCarthy1993) classified fans primarily by planform with a ternary subdivision by process, namely the elementary alpine debris cone (small range-front ‘alluvial fan’), the braided fluvial fan, and a low-sinuosity/meandering (Okavango) type, a subdivision broadly followed by Miall (Reference Miall1996). The literature nonetheless reveals other approaches. Applying a more strictly sedimentological approach, as noted earlier, Blair and McPherson (1994) simply grouped megafans as a type of landform constructed by fluvial aggradation, in contrast to the processes dominant on piedmont alluvial fans. By retaining the morphological and facies approaches to different degrees, classifications with many nuances and even contradictions have arisen.
Despite the attention paid to features of fan-like planform, the recognition of the full dimensions of many megafans was not immediately obvious. With the long tradition of geomorphic and geological research directed at small alluvial fans, and the relatively small Kosi and Okavango as examples of the few well-known megafans (both ~ 150 km long), simple dimensional attributes were often thought to be smaller than they are now known to be. For example, Horton and DeCelles (Reference Horton and DeCelles2001) gave significantly smaller dimensions for what they termed megafans in the northern Chaco Plains (which included the largest-known on the planet), compared with dimensions ascertained by Iriondo (Reference Iriondo1993), Weissmann et al. (Reference Weissmann, Hartley, Nichols, Davidson and North2011) or Latrubesse et al. (Ch. 5). Under present climatic conditions, most Chaco Plains fan-forming rivers cease to flow hundreds of kilometres upstream of the megafan toe at the trunk Paraná River (Cafaro et al. Reference Cafaro, Latrubesse, Ramonell, Montagnini, Garcia, Latrubesse and Perillo2010; Latrubesse et al. Reference Latrubesse, Stevaux and Cremon2012). This led Horton and DeCelles (Reference Horton and DeCelles2001) to consider that river end points mark the distal margins of the megafans. Consequently, the areas they obtained for the Río Grande, Parapetí, and Pilcomayo megafans were much smaller than those now considered to be representative: ~ 12,600 km2, ~ 5,800 km2, and ~ 22,600 km2, respectively, compared with 58,140 km2, 59,656 km2, and 216,210 km2 measured for the full extent of the cones (Latrubesse, Ch. 5).
As commentary by Latrubesse (Reference Latrubesse2015) reveals, overemphasis on planform as a unifying criterion has also diverted attention from the different sets of processes active on fans of different sizes.
1.3 Chapter Outlines
In the continuation of Part I, Introduction, Wilkinson and Currit (Ch. 2) provide a new map showing the distribution of 272 megafans (defined as fans with lengths greater than 80 km) worldwide, a total that more than doubles the number of features of similar dimension in a previously published distribution (Hartley et al. Reference Hartley, Weissmann, Nichols and Warwick2010a). The extreme variability by continent is apparent (one in North America, 87 in Africa), and the different tectonic styles are briefly mentioned. Building the map provided the raw material for the broad discussion in Chapter 17, Megafans in World Landscapes, which also concludes with an overview of possible future research directions.
Part II, Regional Studies, deals with the continents. Chapter 3 begins with mapping the megafans of the African continent and placing them in tectonic context; eighty-seven megafans are shown to be connected directly to the swells of Africa’s unique basin-and-swell geomorphology. In Chapter 4, Wilkinson et al. identify ten megafans in the northern Kalahari Basin where until now only one was thought to exist, namely the well-known Okavango ‘inland delta’. They show that six of these megafans sit astride basin divides such that the discharges of the six feeder rivers have flowed at times into two different basins. Three chapters are devoted to South America, where megafans are most widely developed and cover the largest contiguous area on the planet. Latrubesse et al. (Ch. 5) give a regional study of the Chaco megafans that stretch from central Bolivia to central Argentina through Paraguay, in which the discharge of the different fan-forming rivers is analysed and the several contributing allogenic controls are examined. In a similar study of somewhat smaller megafans of the Pantanal in southwest Brazil, Santos et al. (Ch. 6) map the intricately nested pattern of megafans and examine the relationship between catchment basin geology and megafan size. Avulsions are a key process on megafans, but their occurrence is sufficiently infrequent that little is known of their periodicity. On that topic, May et al. (Ch. 7) document a detailed chronology of recent avulsions on the Rio Grande megafan of central Bolivia and discuss its possible connections with the Amazon.
In Europe, Fontana and Mozzi (Ch. 8) describe in detail the evolution of two major groups of fans, namely the largest five on the southern piedmont of the Alps in Italy, and those that have developed on the Pannonian Basin. The tributaries of the Danube River, feeding in from the Carpathian Mountains, reveal the effects of glaciation in the case of the Po Basin fans and the lack of glaciation effects in the Pannonian Basin. Gunnell (Ch. 9) gives a full history of the large Loire megafan in central France, from evolution of the shallow receiving basin, to the deposition of its major units, to the subsequent regional incision by major and minor rivers. Furthermore, the Loire River has acted as a ‘divide megafan’, flowing at different times westwards to the Atlantic and northwards through the Paris Basin towards the English Channel.
In southern Asia, Sinha et al. (Ch. 10) explore the major geomorphic difference between the incised western Gangetic Plains and the aggradational megafan country of the eastern Gangetic Plains. Sinha et al. (Ch. 11) update many aspects of the geomorphology of this well-known fan and map the detail of the modern course. In Australia, Lane et al. (Ch. 12) give a brief overview of the distribution of megafans on that continent, then illustrate the behaviour of a megafan on the coast of the Gulf of Carpentaria that enters the shallow marine realm. They map the greater number of avulsions near the present-day coastline and suggest the term megafan-delta for such features. Kapteinis et al. (Ch. 13) use radiometric satellite imaging to identify three megafans for the first time in Australia’s state of Victoria. The flatness of the landscape has led to an apparent coalescence of the two larger megafans in the distal reaches, a comparatively unusual geomorphic occurrence.
In Part III Applications in Other Sciences, Ventra and Moscariello (Ch. 14) and Miller et al. (Ch. 15) report on subsurface fluvial sediments and stratigraphy, the former in a wide-ranging review of continental basins, the latter on the Cubango megafan in northern Namibia. The recent drilling of this megafan came about as a direct result of the identification of the megafan from a mapping study reported in Part I, Chapter 2. Wilkinson et al. (Ch. 16) for the first time apply patterns seen in megafan landscapes to the kilometre-thick layered rocks in the Sinus Meridiani part of planet Mars. This new approach is based on a growing understanding of aggradational landscapes encapsulated in the ‘megafan analogue’.
In Part IV, Megafans in World Landscapes, Wilkinson (Ch. 17) attempts a summary of the major attributes of continental megafans, especially of the drainage networks and large aggradational landscapes, which are so different from those of the more familiar ‘dendritic’ drainage patterns and valley-dominated morphologies of erosional landscapes. In the final chapter (Wilkinson and Gunnell, Ch. 18) broader conclusions are drawn from what proves to be a rich haul of future research topics, such as the still blurred divide between autogenic and allogenic controls over megafan evolution.
