Chapter 2
The Chobe Environment

Christina Skarpe1 and Susan Ringrose2

1 Faculty of Applied Ecology and Agricultural Sciences, Hedmark University College, Norway

2 PO Box HA 65 HAK, Maun, Botswana

Environmental factors set the conditions for living organisms and ecological processes in all spatial and temporal scales. At the largest scales continental drift has determined what genetic material is available for evolution, and is a reason for the largely different floras and faunas of different continents. Variation in geology and climate, topographic relief and hydrology creates environmental heterogeneity which promotes diversity of plant and animal communities and of ecosystems. If these environmental factors are seen as having bottom-up effects on species and communities, others such as fire, herbivory and human activities, for example, forestry, agriculture and livestock grazing, might be seen as having top-down effects. Which factors form the environment for ecological processes and which are interactive components of the ecosystem depends on the scale of observation, and for example fire and herbivory are important factors in savanna ecology, but constitute interactive parts of ecosystems in all but the smallest scales (Skarpe, 1992).

Geomorphology

The Chobe ecosystem is part of the dissected Southern African plateau, formed over time by intermittent uplift of the region following the fragmentation of Gondwana, some 180 million years ago. The Kalahari upland basin, which is inset into the plateau, is one of the largest inland sedimentary basins of Africa. During the Jurassic-Cretaceous periods it received considerable deposition (Karoo deposition) which now form the host sediment for the Kalahari sands. The sands which are generally up to 250 m thick, underlie most of Botswana including the Chobe area. The Kalahari sands however thin out over the Chobe area where basalts are exposed in the uplands south of the river. Early drainage dissected the original Southern African plateau and included the proto-Zambezi, Kwando and Okavango Rivers (Moore and Larkin, 2001).

The Chobe area has developed as a result of palaeoenvironmental shifts which have influenced the courses of the original Okavango, Kwando and Zambezi rivers. The three initial proto-rivers were truncated by epeirogenic flexuring (minor uplift) along the Ovambo-Kalahari-Zimbabwe axis, which includes the area to the south of the Makgadikgadi Pans (Moore et al., 2009). This caused the rivers to drain into an early extensive Makgadikgadi-Okavango-Zambezi depression, producing a large palaeolake which may have covered most of northern Botswana and the adjacent Caprivi about 60 million years ago (McCarthy and Rubidge, 2005). This palaeolake became smaller over time as the East African Rift system began to extend south-westward, leading to a fault controlled drainage diversions which formed the Zambezi and Kwando river courses (Modisi et al., 2000). For instance, the original N-S draining Kwando river was diverted north-eastwards and now drains into and beyond a mini-delta on Kalahari sediments abutting the still active NE–SW trending Linyanti fault. Although some water continues to drain through the Linyanti River and on eastwards as the Itenge River, in most years there is barely enough flow to reach the Chobe area. The Chobe River is flooded mainly by back-flow from the Zambezi River during the annual floods. Nevertheless, the steepness of the river banks in the Chobe National Park and the abundance of calcrete in the river cliffs all testify to earlier, more vigorous stream development and the effect of continuous uplift of the entire Southern African plateau.

During the last 400,000 years, embracing the later Pleistocene and Holocene periods, the climate of the southern hemisphere has shifted as documented by the Vostok ice core in Antarctica (Petit et al., 1999) with extensive cold and dry glacial periods being interspersed by warm, wet interglacials. These alternations of cold-dry and warm-wet intervals have strongly influenced palaeo-climatic change hence landform development, throughout northern Botswana (e.g. Partridge et al., 1999; Thomas and Shaw, 2002; Ringrose et al., 2005; Huntsman-Mapila et al., 2006). Within Chobe National Park, evidence depicting cooler-dry intervals includes extensive systems of fossil sand dunes which are still visible over much of the centre and east of the Park. Evidence of warm-wet periods with higher water levels includes the remnants of several strandlines which stand at least 20 m above the present Mababe depression (Figure 2.1). The Mababe depression is still linked with overflow systems from the Kwando and Okavango, to this day. The higher strandlines indicate former (Late Pleistocene) increased water inflow attributable in part to the past expanded flow down the Zambezi, Okavango and Kwando rivers (Burrough and Thomas, 2008; Cruse et al., 2009). This later diminished due to palaeoclimatic change and due to continued tectonic shifts due through the later south-west propagation of the East African Rift system. This fault controlled tectonic activity is prevalent to this day leading to generally low magnitude earthquakes mostly south of the Kwando-Linyanti area.

Figure 2.1 Map of northern Botswana with some features mentioned in text. Drawing by Marit Hjeljord.

Apart from the low undulating topography of fossil dunes and sand ridges, much of the Chobe National Park area is flat with elevations from 1120 m above mean sea level in the north-east and dropping to 920 m in west towards the Mababe depression and the Chobe river (Figure 2.1). Evidence of Holocene or earlier drainage likely lies with the numerous short south-bank tributaries of the Chobe River, which now form dry valleys (e.g. Kalwizikalkanga; Figure 2.2). The Mababe depression, also received relatively recent palaeodrainage from the NE. These drainage lines are now characterised by small pans and dry valleys, for example, the Ngwezumba and Gautumbi valleys and the Nogadsaa and Zweizwe pan areas in Chobe National Park (Figure 2.1; Thomas and Shaw, 1991). Within more recent times, as in the past, cyclic change and the inherent variability of the river related systems are the norm. Recent (2011–2012) high flood levels have been experienced in all the northern rivers (Zambezi, Kwando, Okavango). This has influenced recent inflow events in the Mababe depression, including the Savuti Marsh and its channel, which had been dry for about 30 years. The Savuti channel was infilled via the Linyanti swamps and the Okavango system through the Selinda spillway. Southern Okavango drainage also overflowed into the southern part of the Mababe depression. While to a much lesser extent than the flooding which took place in the Pleistocene-Holocene, these recent flood events are a reflection of the former expansive drainage networks throughout the Chobe area.

Figure 2.2 Map of the core investigated area in northern Chobe National Park. Drawing by Marit Hjeljord.

The water level fluctuations in the Chobe River depend on contributions from different sources, primarily Zambezi and Kwando Rivers, and is out of phase with the local rainy season (October–April). Consequently, there are four distinct seasons in the floodplains along the river: (i) a low water rainy season from October to March; (ii) a high water (floodplains inundated) rainy season from March to April; (iii) a high water dry season from April to June and (iv) low water dry season from June to October. The Chobe River is the only (natural) permanent water in the region, except during periods when the Savuti Marsh, about 130 km to the southeast, contains water. This makes it a vital resource for water-dependant fauna in the ecosystem. The Chobe River gives its name to the northernmost district in Botswana and to the Chobe National Park, which forms much of its southern bank.

Soils

The soils in Chobe National Park are mostly deep to very deep, well to excessively drained arenosols, developed from the Kalahari sands (Figure 2.3; de Wit and Nachtergaele, 1990). The sand consists of quartz with minor feldspars and mica. In the dunes the grains are coated with iron oxide, colouring the sand brown to deep red, while in seasonally wet, reductive environments the soil is grey to almost white (Leistner, 1967; de Wit and Nachtergaele, 1990). The bottoms of the former lake basins, the Mababe depression and the surroundings of Nogadsaa and Zweizwe pans are characterised by calcareous, fine-textured and compact alluvial luvisols and gleysols of lacustrine and riverine origin (Blair Rains and McKay, 1968; de Wit and Nachtergaele, 1990; Chapter 9). These areas have many pans, shallow, poorly drained depressions that hold water for varying periods after rain. Riverine alluvial soils also make up the recent and uplifted fossil floodplains along the Chobe River. Unlike Kalahari sand, which is poor in minerals and plant nutrients, these alluvial soils are calcareous and moderately fertile. The sands have a cation exchange capacity (CEC) of around 2 cmol kg, compared with about 6 cmol kg in fossil alluvial soils (luvisols) along the Chobe River and more than 30 cmol kg in the sodic recent alluvium (gleysols) on the floodplains (Aarrestad et al., 2011; Chapter 9).

Figure 2.3 Distribution of soils in the Chobe area (UNDP-FAO classification system). Drawing by Dr Lin Cassidy.

The material in the eroding vegetated sand dunes and ridges is...