Landscapes and Landforms of Botswana

This chapter provides a short overview of geological units that have surface expression along with the dominant landforms and landscapes of Botswana. It introduces the western portion of the country which features young regional arenosols and is home to few outcrops. This is often simply referred to as the Kalahari. The exception is the Ghanzi Ridge, a highly denuded Precambrian Orogen, sitting between two regional, but largely concealed cratons. The lower-lying east is home to Archean hills and the countries major drainage network. The second part of the introduction covers the geomorphic settings associated with the various book chapters with much focus on the landscapes of the Kalahari, northern Botswana and eastern lowveld. Especially the north and northeast are home to a variety of fluvial systems, which include the Okavango, Zambezi Catchments and Makgadikgadi dry lake. In conclusion, this chapter draws attention to the tertiary education of geomorphology in Botswana.

Other than modest amounts of local rainfall, all other surface water in northern Botswana comes from river catchments in Angola, and to a lesser degree from north-western Zambia. The four major river systems that supply water to Botswana are the Cubango and Cuito (which jointly supply the Okavango Delta), the Cuando (which provides the water of the Linyanti Swamps and Savuti Channel) and the Zambezi (which fills the Chobe river). The catchments lie either on shallow soils (much of the Cubango catchment and the eastern upper catchment of the Zambezi), or on deep sands (all of the Cuito and Cuando, part of the Cubango, and much of the Western Zambezi catchment). River discharges following rain from the former are relatively rapid, reaching northern Botswana towards the end of the rains. Those from catchments of deep sand are extremely slow, arriving in Botswana later during the dry months.

The presence of a large (approximately 2000 km²) peatland in a semi-arid climatic setting such as the Kalahari is unusual. Peat forms in permanently flooded areas in the Okavango Delta primarily due to the perennial input of large volumes of water from a distant catchment in the highlands of Angola, into a valley formed by rifting. Peat deposits form in three distinct settings in the Okavango: backswamp settings where open water is converted into homogeneous emergent peatlands, lake and channel margins where the peatland is patchy, and the inlets to lakes that connect to the primary distributary channel, which presently is the Okavango-Nqoga-Maunachira River system. An unusual feature of peat formation in backswamp areas, as well as in lake and channel margin settings, is that frequently mats of fine organic detritus on the bed rise to the water surface and are colonised by emergent plants. Once thus colonised, peat production is accelerated due to the higher productivity and less easily decomposed tissue of emergent plants compared to submerged and floating-leaved plants. In backswamp settings, the floating mats are extensive (hundreds of square metres to hectares) and lead to the formation of homogeneous plant communities that cover large areas. In the case of lake and channel margins, floating mats form small isolated features (up to a few square metres) that are blown to the lake or channel margin by wind. Their accumulation on the leeward sides of lakes and broad streams gives rise to a patchy and heterogeneous plant community. Where lakes are connected to the primary distributary channel, papyrus debris collects as large floating rafts along channel margins, ultimately to be deposited in the lake inlet. Thus, large lakes are converted to papyrus swamp over periods of decades. Channel switching of primary channels leads to radical changes in the flow such that formerly flooded areas dry out and peat deposits are destroyed over periods of decades due to desiccation. However, a new cycle of peat formation takes place in the newly flooded area. Peat deposits in the Okavango are thus not permanent features but have a lifespan of about one or two centuries. Given increasing recognition that peat formation in “dryland” wetlands requires an elevated base level, the hypothesis proposed here is that chemical sedimentation in the lower reaches of the Okavango elevates the base level, and peat formation is an inevitable and passive consequence. This leads to the formation of an alluvial fan with a remarkably uniform slope from the fan apex to the toe of the system.

Lake Ngami and the Mababe Depression form elongated troughs peripheral to the Okavango Delta from which they currently receive inflow. The two basins originated as structural depressions resulting from East African Rift (EAR) propagation along the Kunyere and Thamalakane (Mababe) fault lines and are also embedded in older Okavango Delta fans. Both basins are partially ringed by palaeo-shorelines at heights that vary from 945 to 920 m with a dominant 936 m level. Dates of shoreline formation are currently under debate. (Moore et al. 2012) suggest that the Ngami and Mababe basins were mostly submerged as a result of major inflowing river captures during the Early-Mid Pleistocene. Major shorelines in the basins developed during the Palaeo-Lake Thamalakane (PLT-936 m) period of the Mid Pleistocene at 200–500 ka. This contrasts with interpretations in (Burrough and Thomas 2008), who suggest that palaeo-shorelines at ca. 936 m were formed on numerous occasions within the last 100 ka due to climatic conditions and feedback factors. Hence controversies revolve around possible dates and palaeo-climatic conditions for shoreline formation. Preliminary work on basin sediments suggests that palaeo-lakes in both basins operated as separate mostly closed system alkaline lakes for the last 65 ka, inferring an absence of numerous large high-level palaeo-lakes (at 936 m) during this interval. As further information is needed, a comparative deep drilling programme is recommended for both basins with full sedimentological analysis and sample dating to resolve issues regarding past climates and the possible extent of both early (Mid Pleistocene) and later (Late Glacial to Holocene) palaeo-lakes in the region.

Visible from space, the Makgadikgadi pans are one of the most distinctive geomorphic features of southern Africa. Now seasonally dry, this collection of pans is the sump of what was once one of Africa’s largest inland lakes. Formed by uplift along the Kalahari-Zimbabwe axis in the early Miocene and re-sculpted during the Pleistocene by both climatic events and neo-tectonism, the geomorphology of this basin tells a story of significant hydrological dynamism within northern Botswana, a story that has arguably influenced the evolution and dispersal of our own species (Chan et al. 2019). The boundary of the 37,000 km² basin withholds a suite of landforms including saltpans, dunes, relict shorelines, lacustrine spits and fluvial deltas, each with its own inter-linked geomorphic history. Today, its landscape remains geomorphically active, providing one of the largest sources of dust in southern Africa with knock-on effects for global climate.

The northern part of the Chobe Enclave (an administrative district of northern Botswana) is an agricultural area situated between relatively pristine national parks situated in the Middle Kalahari Basin. It belongs to the Linyanti-Chobe structural basin and constitutes a syntectonic depocenter formed within a large structural depression, known as the Okavango Graben, a tectonic structure of a likely trans-tensional nature. The landscape includes fossil landforms, such as sand dunes, pans, sand ridges, and carbonate islands resulting from palaeo-environmental and palaeo-drainage changes through the Quaternary and associated to (neo)tectonic processes. In addition to river- and wind-reworked Kalahari sands, the sediments include diatomites and carbonate deposits, forming inverted reliefs and originating from palustrine palaeo-environments. The Linyanti-Chobe basin is at the convergence of several ecoregions from tropical and subtropical grasslands to savannas and shrubland biomes. The hydrological cycle in the northern Chobe Enclave is governed by a complex interplay between the Okavango, Kwando, and Upper Zambezi drainage basins, which originate from tropical watersheds of the Angolan highlands. Finally, the widespread development of termite mounds impacts the diversity of soils and sediments of the northern Chobe Enclave, which is also reflected in the vegetation.

In this chapter, the Chobe-Zambezi channel-floodplain system is defined as the fluvially influenced area that is located around and between the Chobe and Zambezi rivers approaching their confluence. This area is located in the ‘Four Corners’ region, the informal term given to the region where the Botswana, Namibia, Zambia and Zimbabwe borders meet. The large-scale structure and medium-term (10²–10⁵ years) development of the channel-floodplain system is related to a combination of tectonic activity and climatically-driven changes to flow and sediment supply. The system has developed in a region of subsidence that is related to the East African Rift System. Upstream of the Mambova Rapids, the modern sinuous, alluvial channels are flanked by extensive floodplain wetlands, with crevasse splays, gullies, oxbows, scroll plains, abandoned channels and backwaters (stagnant or slow-flowing, channel-like depressions) all widespread. Collectively, these fluvial landforms create the physical template for shorter-term water, sediment and ecosystem dynamics. A strong flood and drying season dynamic is evident; river stages typically rise from January and peak around April, before subsequently falling again. The Zambezi provides the largest flow volumes, with flow spreading gradually from north to south through a complex system of active and partially active channels and floodplain wetlands towards the Chobe. Along the two rivers, lateral channel migration and extension of splays, gullies and backwaters has been negligible over at least the last 40–50 years, with few new oxbows forming. To the east, both rivers cross the uplifting Chobe fault, with each river forming complexes of steeper, bedrock anabranching channels in the region of the Mambova Rapids. The two rivers ultimately coalesce ~10 km farther downvalley, and continue as the Zambezi River. A longer term (>10⁶ years) developmental model is outlined, which posits that headward retreat of the Victoria Falls, at present located ~80 km downstream of the Chobe fault, will initiate a phase of erosion that will cross the fault in ~1–2 million years’ time. This phase of erosion will initiate deep channel incision, river network reorganisation and wider landscape denudation.

The dunes of the southern Kalahari are amongst the most studied in the world. Occurring in a region embracing arid and semiarid conditions, and dunes that are both marginally dynamic as well as more stabilised, research has focussed both on the processes operating on partially and variably vegetated surfaces, and the palaeoenvironmental histories of dunefield accumulation. Whilst commonly classified as linear dunes, this chapter examines the range and variability of forms in southern Botswana and contiguous areas, their temporally variable dynamics, and the developing chronometric histories of their development during the late Quaternary period.

Dune ridges in northern Botswana and adjacent territories are well beyond the limit of the occurrence of aeolian sand transport in southern Africa today. Striking for their distinct patterning in the landscape evident from the air and in satellite imagery, on the ground these are subdued features that in places can be barely discernible apart from through the patterning of vegetation. Overall, these features have characteristics that attest to their inactivity today and their degradation by non-aeolian processes since their formation. Analysis of ridge ages, orientations and sediments sheds light, and some remaining uncertainty, on the controls on their formation during the Quaternary Period.

Pans, playas and similar morphological structures occur across all continents. They are small, closed sediment sinks that are characteristic of arid and semi-arid regions with low relief. In the Kalahari Basin, small pans are widespread and they function as ephemeral lakes and sedimentary sinks. They are characterised by their flat, salty to clayey surface and slightly elevated surroundings. Some principle sedimentary processes are distinctive for pans. Showers during the short rainy season deliver siliciclastic material by runoff from the local catchment. During the dry season, evaporites precipitate by evaporation of the surface water and supersaturated pore water and groundwater. Additionally, fine-grained sediments are transported on the dry pan surface by strong wind regimes (long-term) or by vortices/eddies from thermal uplifts (short-term). The marginal lunette dunes, a characteristic feature of many small Kalahari pans, resulted from temporary phases of significant aeolian sediment transport. However, there is no hint of longer sedimentation gaps or significant periods of erosion. The prevailing depositional processes contradict the former assumption that deflation is the most prominent process in salt pan formation. The pans contain sediment sequences that are several decimetres to metres in thickness, which reach back until the Late Pleistocene. They contain different kinds of proxy information that can be used for environmental reconstruction and climate modelling throughout the pans’ history.

Conventional rivers are absent from much of Botswana, with only the Okavango, Chobe and Zambezi systems in the extreme north containing perennial flowing water. Ephemeral rivers occur in the eastern hardveld, but the most extensive components of the surface drainage are the networks of fossil or dry valleys (termed mekgacha in Setswana and dum in various San languages) that cross the sandveld. This chapter presents the first holistic review of current knowledge about these enigmatic landforms. It does so using a range of evidence types, from radar remote-sensing to the analysis of historical documents written by missionaries and explorers. The chapter considers dry valley distribution, morphology and contemporary and historical hydrology before discussing valley evolution over longer timescales. It concludes with a synthesis of the main arguments concerning how dry valley systems may have formed, including the balance between conventional fluvial incision and processes such as groundwater seepage erosion.

The Stampriet Transboundary Aquifer System (STAS) covering western Botswana, eastern Namibia and part of the Northern Cape of South Africa is an interesting location to explore landscape evolution and the interaction of a range of surface and subsurface geomorphological processes. This chapter aims to introduce the importance of the STAS. It outlines the geological and hydrogeological framework, the range of surface geomorphological features and considers how these relate to each other and the hydrogeology of this system. The long-term tectonic and sedimentary history of the STAS provided the geological sediments and structures upon which the interactions between surface and subsurface geomorphological processes have occurred. Water–rock interactions control the quantity and quality (natural baseline) of groundwater. Where groundwater reaches the surface zone it influences the persistence of water at the surface and pan sediment chemistry and also reduces the potential for deflation and abrasion of the surface by the wind. The main geomorphological components of the STAS region are the dunes and sand sheet cover of the Kalahari Erg, dry valleys and ephemeral rivers, pans, and calcrete plateaus and other duricrusts. This chapter also explores the possible links between duricrusts and incised valleys, between the incised valleys and dunes of the Kalahari Erg and the geomorphic influence of the valleys on the current wind regime and sediment supply. The role of groundwater in the development of the geomorphology of this system lies particularly within the formation of duricrusts, pans and dry valleys. The exploration of the relationship and interactions between duricrusts, valleys and the dunes of the Kalahari Erg shows stratigraphic evidence for an incision of the valleys that post-dates the formation of duricrusts. It is also possible that valley incision post-dates an early generation of aeolian linear dunes at the surface, although we are not able to offer a timeline for, or confirmation of, the later based on dating sediments because of the upper-age limits of our available chronological techniques.

Calcretes and silcretes are the most widely encountered ‘rocks’ in the Kalahari sandveld that covers much of Botswana. This chapter presents the first holistic overview of current knowledge about these duricrusts at a national scale. It does so by considering the distribution, classification, macromorphology, geochemistry and mineralogy of each duricrust type in turn, alongside various models used to explain their formation. The chapter then reviews our understanding of a variant of duricrust encountered more in the Botswana Kalahari than anywhere else in the world—the silcrete–calcrete intergrade duricrust. The chapter concludes with a summary of knowledge about the age of duricrusts in Botswana before pointing to potential directions for future research.

Caves and rockshelters occur in dolomite, siliciclastic and granite rocks of varying ages in Botswana and this leads to an interesting variety of entrance, passage and cavern types controlled by fracturing, bedding, groundwater levels, springs and slope processes. The dolomitic caves include the Gcwihaba Caves in northwestern Botswana and Lobatse Caves in southeastern Botswana. The siliciclastic caves/rockshelters occur at: Tsodilo Hills in northwestern Botswana; Tswapong Hills and Shoshong in eastern Botswana; Molepolole, Lekgolobotlo (Mmalogage), Manyana, Otse and Lobatse in southeastern Botswana. The granitic caves occur within the Lepokole Hills in eastern Botswana. The formation of the caves/rockshelters and their deposits are overviewed along with description of previous studies and the potential for further research is outlined in terms of landscape evolution and geoarchaeology.

An understanding of the regional geomorphic framework of Botswana played a critical role in the discovery by De Beers of economic kimberlite pipes in the Orapa area in central Botswana. Early prospecting in the eastern Botswana hardveld in the late 1950s to early 1960s resulted in the recovery of a train of diamonds in the east-flowing Motloutse River, extending to the headwaters of this drainage line. De Beers concluded that the Motloutse had previously extended further west, based on an appreciation that the eastern Botswana watershed represented an axis of epeirogenic flexure, which had beheaded the former headwaters. The implication was that the source of the diamonds recovered in this river could be located in the Kalahari sandveld to the west of the drainage divide. Subsequent extension of sampling to the west into the Kalahari was rewarded by the discovery of the Orapa kimberlites in 1967, a year after Botswana achieved independence. We discuss how these sampling programmes in turn provided a feedback loop, making it possible to refine understanding of the regional geomorphic evolution. Because kimberlite pipes typically taper in size with depth, they also provide a gauge for estimating the depth of post-emplacement erosion, providing an important constraint on landscape evolution. Soil sampling in the Kalahari environment demonstrated that bioturbation has translocated kimberlite indicator minerals (KIMs) vertically from sub-Kalahari kimberlite pipes, resulting in their concentration in the Kalahari surface sand directly above the buried kimberlite. A consequence of this vertical mixing of the Kalahari sedimentary section is that luminescence dating of Kalahari quartz grains may produce spurious “mixed” ages. In the south of Botswana, mid-Cretaceous sub-Kalahari kimberlites are overlain by fluvial conglomerates, while in the northeast of the country, pipes of comparable age are overlain by a calcretized silcrete, which has been linked to the development of the African erosion surface. The basal conglomerates and African Surface duricrusts are unconformably overlain by semi-consolidated Kalahari sand. However, both conglomerate clasts and fragments of the duricrust have been identified within craters sediments forming the upper section of the respective kimberlite pipes. This implies that deposition of the conglomerates, and development of the African surface overlapped the episode of kimberlite eruption. This, in turn, provides an important mid-Cretaceous temporal constraint on initiation of Kalahari deposition, and of the African erosion cycle.

The Limpopo River system is a major drainage pathway in southern Africa but very little is known about its catchment-scale geomorphology and dynamics. This study maps its geomorphology using the River Styles Framework as an interpretive tool. This analysis highlights that river geomorphology varies spatially, in particular between bedrock-controlled and overbank floodout reaches which are found throughout the river system. In addition, much of the present reach-scale geomorphology does not correspond to present climate forcing—even though flood events are important drivers of geomorphic change, flood effects are spatially variable. This is due in part to different substrate types and valley width, but also to flow constrictions that compartmentalize the integrity of the river sediment system into different morphodynamic zones. The ephemeral tributaries drawing water from Botswanan territory into the Limpopo are strongly affected by human activity, including river damming and groundwater abstraction. This significantly impacts on river discharge and thus sediment dynamics. The net outcome is that there is limited streamwise sediment connectivity through these tributaries and thus through the Limpopo River system as a whole.

Botswana’s dams and the sand rivers that run through them appear set for unprecedented levels of threat and uncertainty due to climate change this Century. Spatially specific impacts such as severe soil erosion, sand mining, industrial and agricultural pollution, invasive and alien species, dumping of rubbish and the disposal of sewage water, will be accentuated further by the predicted hotter and drier conditions and more extreme weather events. Harmful algal blooms will become more frequent and further threaten an already challenging water supply and management situation. High distribution losses, limiting reservoir safe yields, due in large part to their shallow depths and fluctuating water levels, seem unlikely to be offset by increased conjunctive use of ground and surface water within the existing constraints of supply networks. Transformative change within the water sector is required, that places greater emphasis on water use efficiency and that of green water, along with inter-regional water transfer programs. The arid conditions of the past seem set to return to Botswana, requiring Policy makers to look both inwards at how water is used and valued, and outwards, far beyond their borders, if sustainable water supply solutions to be found.

Gorges in the hardveld of southeastern and eastern Botswana occur within fractured siliciclastic rocks likely influenced by uplift associated with the Ovambo‒Kalahari‒Zimbabwe (OKZ) Axis in the end-Cretaceous to early Tertiary. Nine (Peleng East Gorge, Lobatse Rock Paintings Gorge, Athlone Gorge, Segorong Gorge, Mmalogage Gorge, Mmamotshwane Gorge, Phataletshaba Gorge, Shoshong Gorge, and Goo-Moremi Gorge) of the eleven gorges discussed in this paper originate in a bedrock high with a mean length of 1839 m. Two (Pharing Gorge and Kobokwe Gorge, mean length 1454 m) of the eleven gorges appear to originate in small upstream catchments, and all gorges presented in this paper are wider than the streams they currently hold, implying inherited forms from past humid climatic phases (pluvials), in addition to the influences of modern aquifer flow and wet season runoff.

Variations in the nature and properties of soils (pedodiversity) on the Late Neogene hilly dryland and Quaternary erosional and depositional surfaces of the Hardveld, spanning the south-eastern to south-central parts of Botswana are well established. Following the updated IUSS Working Group World Reference Base (WRB) soil classification system, this chapter discusses the major soils found on the eastern Hardveld of Botswana. Ten Reference Soil Groups develop on the Hardveld: Regosols, Arenosols, Luvisols, Lixisols, Cambisols, Calcisols, Vertisols, Leptosols, Planosols, and Acrisols. Dominant pedogenic processes in the area include salinization, calcification and decalcification, illuviation, eluviation and erosion. Interbedded fossil soils (palaeosols) within alluvial deposits found in the Hardveld are indicators of environmental and climate fluctuations in the region. The complex interplay of parent material and climate as key active factors, and organisms, topography and time as passive factors, through material fluxes such as addition, removal, transfer and transformation are fundamentally responsible for the formation of different soils found on the Hardveld.

The Tsodilo Hills are a cluster of three inselbergs rising up over 400 m above the Kalahari Desert in the Ngamiland, NW Botswana, to the west of the Okavango swamps. The exposed succession consists of metamorphosed siliciclastic sedimentary rocks deposited on a marine shelf at a margin of the Congo Craton between ca. 1.90 and 1.1 Ga. The sediments formed a Gilbert-type delta grading towards the open shelf covered with large underwater dunes and influenced by tidal currents. The rocks contain specularite, which was mined and traded throughout Southern Africa from the Late Stone Age up to the nineteenth century. The creativity and culture of the local communities is reflected by over 4000 rock paintings and engravings. A permanent lake, up to 7 m deep, existed between 27,000 and 12,000 years ago adjacent to the hills. The geomorphic features of the Tsodilo Hills document processes of both the physical and chemical modifications of the rocks. Two geological-timescale erosional cycles sculpted the area: (i) continental Dwyka glaciation (the Carboniferous-Permian) when Tsodilo Hills formed nunataks, and (ii) post-Karoo formation of African Surface (the late Cretaceous). Steep slopes of Tsodilo Hills show stepped morphology and some cliffs have flared sides. Silica solution resulted in localised arenisation and karst-like features including phreatic zone-related subartesian well, and vadose zone vertical shaft, horizontal tube and karren.

Geoconservation is action taken with the intent of conserving and enhancing geological, geomorphological and soil features, processes, sites and specimens, including associated promotional/awareness-raising activities and the recording and rescue of data or specimens from features and sites threatened with loss or damage. In Botswana, the geoconservation strategies are gatehouses, fencing, guides, information boards and construction of footpaths above nearby streams. Local communities are typically involved with geoheritage sites that occur close to their settlements and Community Trusts have been set up in several instances, that contribute to sustainable rural livelihoods. Geoconservation methods that have been implemented at geoheritage sites in Botswana are reviewed here, and all of the sites are protected by the Monuments and Relics Act, 2001 (Cap 56:02) that describes and contextualizes the preservation and conservation of an ancient monument and relic and ancient working which is known or believed to have constructed, erected or used in Botswana before 1st June 1902. Suggestions for the future improvements of geoconservation measures in the sites are made and include adequate planning for the peri-urban areas that incorporate geoheritage sites, preservation methods of rock art and more geoscientific information to be included in information boards and tours.

Zoogeomorphology is a relatively new discipline and is set for changes this century in Botswana that have not been seen for millennia. The co-evolution of landscapes and wildlife amidst a changing climate means that the precise role of each is difficult to determine. As southern Africa becomes hotter and drier due to anthropogenic climate change this century, past adaptation strategies such as wildlife movements along (Balinsky’s 1962) ‘drought corridor’ will no longer be possible due to land use/land cover change and agriculture and human-related expansion. As the Sixth Great Extinction unfolds, it offers a unique opportunity to study just how significant different biota are in determining geomorphology, albeit as our climate changes. Fauna that has remained intact since the Miocene will largely disappear from African shores and the palaeo-dune fields of the Kalahari may well become reactivated. The real significance of micro-organisms and invertebrates will then be realised as the bioturbators remain to shape the landscapes around them without the distraction posed by humans and the unique mega-fauna that surrounded them.