Mapping of Africa’s megafans according to a set of criteria (radii > 80 km, widths > 40 km; high topographic smoothness; result: n = 87), suggests a direct relationship between fluvial megafans and the thirty relatively young tectonic swells of the continent. Although ten are barren of megafans, fully 85% display this relationship and are thus named ‘swell-flank type’. Another control was also identified: almost two thirds of this group was related to swell flanks margined by a rift-related depression. Clustering is significant in this ‘the swell-and-rift’ subtype: 23 in South Sudan (Muglad-Melut troughs), nine in Kenya (Anza Rift), and four in southern Chad (Salamat, Dosseo, and Bongor rifts). The remainder (‘swell-flank only’ subtype) were found to be scattered widely. Only 3%, however, were exclusively related to rifts (e.g., the Okavango megafan of Botswana). Africa’s megafans total at least 1.2 Mkm2, average megafan unit area being 13,200 km2. Flanks of the largest swells (e.g., Congo Basin flanks of the East African swell) are devoid of megafans, perhaps because of enhanced recent uplift. Coasts are similarly devoid of megafans, possibly for the same reason. Cratonic blocks where swell growth is less prominent are also devoid. Africa’s largest rivers are associated with few megafans.

The layered sediments at Sinus Meridiani, Mars, ~ 1 km thick and covering 300,000 km2, have been probed by the rover Opportunity. Numerous observations on these rocks are reevaluated through the poorly-known model of the fluvial megafan. We conclude that at least some sections of the Meridiani stack are vestiges of large fluvial megafans. Our reasons include the following: the southern uplands of Mars are a feasible sediment source; sediment was likely delivered via rivers that cut the extensive valley network that drain the upland toward Meridiani; the units cover large areas commensurate with terrestrial megafan landscapes, and display the same very low slopes; megafan landscapes lie directly adjacent to upland sediment sources, as seen at Meridiani; megafans require neither closed basins nor waterbodies for sedimentation to occur; numerous examples of fluvial channels appear in some units; and morphologies of the widespread raised ridges of the ridge-forming unit (RFU) are suggestive of indurated channel networks seen on megafans in Oman. Features of vast aggradational landscapes as encapsulated in the novel megafan analogue thus provide answers to several key observations, whereas existing fluvial analyses usually apply the classic attributes of erosional landscapes, leading to significant difficulties in interpretation of the Meridiani units.

New findings around fluvial megafans have accrued from the world survey presented in this book, and challenge some commonly accepted generalisations. Among a list of unexpected results are that (i) megafans constitute a landform and sedimentary body of regional significance on Earth, subsidiarily on Mars, despite their relatively small number; (ii) any topographic step, high or low, can provide the anchor point for a megafan apex; (iii) most megafans are associated with tributary drainages, seldom with axial drainages; (iv) megafans form in all climates. Megafan sizes, shapes, nesting patterns, drainage configurations, tectonic settings, and sediment dispersal styles are summarised, classified, and compared to other large fluvial sediment bodies. Finer mosaics of landscape elements and landforms belonging to the rheic zone (belts of fluvial incision, narrow or wide, that cut into fan surfaces) and perirheic zone (extensive land surface beyond the reach of the fan-forming river) are reviewed from modern analogues, and their implications for identifying megafans and other distributary fluvial systems in the rock record are examined. Vocabulary defining megafans and their environments has been sharpened as a result, with some avenues for further investigation laid out in this closing chapter.

Discovery of the significance of fluvial megafans came about in the mid to late twentieth century. We suggest reasons why appreciation of their existence came late in the history of Earth science, even after the advent of space-based observation of planetary landscapes. The reasons are partly cultural: megafans are uncommon in the historic cradles of modern geology (Europe, North America). Reasons are also partly theoretical: rivers have been conceptualised chiefly as sediment bypass systems terminating in deltas, rather than as aggradational systems in their own right. Reasons are also perceptual: just as the megaflood origin of channeled scablands was held in disbelief, the inordinate size of megafans has stood in the way of accepting (i) the sheer magnitude of their unit-size and also (ii) their existence as active systems in modern landscapes, rather than just as stratigraphic features in the rock record. Post-1990, scientific activity around megafans accelerated and involved global mapping, classification, and regional investigations into patterns and processes. An overview of this take-off period is provided as a partial introduction to the remaining 17 chapters of this book, which are briefly outlined.

Major morphological characteristics of megafans and the associated drainage networks of multi-megafan landscapes are outlined. Such landscapes differ significantly from well-known erosional landscapes with dendritic drainage in eroded valleys: megafan landscapes display partial cone morphology; longitudinal profiles can be convex, concave or both; interfluves and tributaries are lacking; the fan-forming river behaviour is highly avulsive; fan-margin rivers display three discharge regimes; convergent drainage zones of various types are widespread despite the broad partial-cone morphology; avulsions often occur at subapexes distant from the prime apex; perirheic zones differ significantly from the model developed for valley-confined floodplains. These and other attributes bespeak the complexity that arises as fan-like features increase to megafan proportions. Controls of megafan formation are presented. Megafans are distinguished from small alluvial fans, midsize distributive fluvial systems (DFS) (30–80 km long), valley-confined floodplains, deltas, major avulsive fluvial systems (MAFS), and large accretionary fluvial systems (LAFS) – the latter two relatively new to the geomorphic lexicon. Megafan nesting patterns and several wider continental lowland drainage models that encompass megafans (e.g., central South America) are described. Megafans show striking similarities with attributes of large axial floodplains despite being formed by smaller rivers. Terms with confusing meanings are clarified.

Maps generated from various data sources reveal ten new megafans in the northern Kalahari region where, until now, the Okavango had been the only one recognised. Seven megafans were generated by rivers flowing off the Bié Swell of southern Angola, east to the Zambezi basin and south to the Owambo basin. Only three (Okavango, Cuando, Zambezi) are apexed at shoulders of the Okavango Rift (northern Botswana). Unusually, the Cubango/Okavango River has given rise to two megafans: the upstream Cubango megafan, and the well-known Okavango megafan downstream. Avulsion behaviour of three rivers has also demonstrably shifted discharge between major basins over time: the Cassai has, at times, flowed north into the Congo basin; the Cubango flowed into the Owambo basin (Etosha dry lake), but now discharges into the Makgadikgadi basin (via the Okavango megafan); and the Kunene, which now flows to the Atlantic Ocean, at one time discharged into the Etosha pan. Recognising the existence of so many more megafans than previously appreciated, as well as their autogenic, avulsive dynamics, is an invitation to reconsider the regime of sedimentary sequence deposition in these basins, which may have erroneously been interpreted as resulting from climatic or other external forcing factors.

Using a variety of remotely-sensed data, a worldwide survey of river-generated megafans is presented. Thus far, 272 partial cones reaching minimum lengths and widths of 80 km and 40 km, respectively, have been identified. They all indicate large areas of fluvially-laid sediment distant from present or past shorelines. This more than doubles prior counts of fans of these dimensions, partly as a result of using a different set of criteria. All are visible either as pristine or degraded features, and it is likely that more will be found as older, more eroded individual occurrences are identified. The greatest numbers are found in Asia (n = 87), from Iraq to the clusters of megafans on the south flank of the Himalaya, and also in central Asia. Africa displays a similar number (n = 87), almost all related to relatively low-relief topographic swells. In South America (n = 60), most megafans are found clustered along the east flanks of the Andes Mountains and include the longest known example (704 km). Few occur in Europe (n = 9) or North America (n = 1), in both cases as a result of incisional fluvial regimes operating almost continent wide. Australia hosts 28 megafans. River-generated fans with smaller dimensions are numerous on all continents, however.