Landscapes: Geology, Geomorphology and Hydrology

Abstract

The long history of Angola’s physical environment goes back to its Gondwanan origins, 550 million years ago. To give context to the present landscapes of Angola, a brief geological history of the Earth provides a backdrop to the dramatic evolutionary innovations that occurred in terrestrial ecosystems during the Miocene Epoch (23-5.3 million years ago), which shaped Africa’s savanna biomes as we know them today. The geomorphological patterns of Angola developed in response to periods of uplift (the Mayombe and Bié swells, the Angolan Escarpment), subsidence (the Congo and Kalahari Basins), erosion and deposition (the Planalto, Congo and Zambezian Peneplains). Angola has 12 main landscape groups, with which its ecoregions are closely aligned. The country is drained by six major transnational hydrological basins, providing the ‘water towers’ of southern and central Africa. The ecosystem services which these water resources provide to the region are described in terms of their role in pollution regulation, hydro-electric power generation and biodiversity maintenance.

FormalPara Key Concepts and Questions: This Chapter Explains

• What accounts for the great diversity in age, structure and properties of the rocks on which the country is built.

• What the main geological and rock groups are, and how they are distributed across Angola.

• What the country’s main landscapes are and where they can be found.

• How the landscapes of Angola have been shaped by geological events, erosion by rivers, and the redistribution of loose sediments by wind.

Context: Angola’s Ancient Gondwana Origins

You have to understand the past to understand the present. (Carl Sagan, 1980)

The biodiversity of Angola and the ecological processes that determine the patterns of distribution and the abundance of its biota have evolved over many hundreds of millions of years, most especially since the formation of the supercontinent Gondwana. Familiarity with the deep history of Africa’s geological evolution, even at a general level, helps in the understanding of current features and phenomena. The following outline focuses on some key events of the past 550 million years, and defines relevant geological and geomorphological terminology. A summary of geological history and major timelines (Eons, Eras, Periods and Epochs, and the first appearance of major biological groups) is presented in Fig. 4.1. The most ancient rocks in Angola belong to the Precambrian Eon, predating the Phanerozoic.

Fig. 4.1

The geological timescale, referencing the first appearance of major biological groups. Ma (million years ago) indicates the age of first appearance of major plant and animal groups

Angola’s landscapes are underpinned by dynamic geological processes which originate within the Earth’s lithosphere—the topmost layers of the Earth—the crust and the uppermost mantle. The lithosphere is made up of igneous, sedimentary and metamorphic rocks.

• Igneous rocks, formed deep within the crust, include granites and gabbros, which today can be found in many parts of Angola where they have been exposed by erosion. In Angola, they date from 2700 Ma (Ma = million years ago).

• Sedimentary rocks are formed where eroded materials—gravel, sand and mud—accumulate on the floor of basins (depressions, lakes, oceans) and become cemented over time into conglomerates, sandstones and shales.

• Metamorphic rocks are the product of the transformation by heat and pressure of igneous or sedimentary rocks. For example, under such conditions, limestone transforms into marble, shale into schist, granite into gneiss and sandstone into quartzite.

The global lithosphere is divided into seven major tectonic plates—vast slabs of solid rock, made up of cratons, mobile belts and basins. Globally, the plates have been displaced from their original positions over periods of hundreds of millions of years by the process of plate tectonics. The plates typically conform to each major continent and surrounding oceans. Within the African Plate, extensive and stable foundations of the continental lithosphere are known as cratons—such as the Congo Craton which underpins most of Angola, also known as the Angolan Shield. Precambrian Basement Complex rock exposures—such as the granites that are visible over much of Angola—are common and prominent (Fig. 4.2).

Fig. 4.2

Large granite outcrops are a prominent feature of western Angola’s landscapes, such as these bornhardt domes in Cuanza-Sul. Photo John Mendelsohn

Some 550 million years ago, the supercontinent Gondwana was formed through the accretion (joining together) of several ancient plates and cratons. Between 180 and 140 Ma Gondwana started to break up.

• First, around 180 Ma, East Gondwana (Antarctica, India, Australia, Madagascar) split from West Gondwana (Africa and South America).

• By 130 Ma Africa started to break from South America with the formation of the Atlantic Ocean, which was fully established by the late Cretaceous (80 Ma).

• Much later, by 34 Ma, the separation of Australia from Antarctica and the formation of the Drake passage was completed, events of great significance for Angola, as they resulted in the start of the circumpolar oceanic circulation. During this period, the icing of Antarctica and the aridification of southern Africa was firmly established.

• Around 14 Ma, the Panama Isthmus formed, joining North and South America, and blocking the oceanic circulation from the central Pacific into the Atlantic.

• Between 10 and 7 Ma, the Benguela Current developed along the southwest African coast, bringing cold water northwards and triggering the production of fog along the Namib margin. The patterns of oceans and continents as we know them today had been established.

The tectonic plates are still in motion, moving Africa into Eurasia, and India into Asia, with frequent earthquakes signaling the process.

The Cenozoic Era (66 Ma to present) saw the land surfaces of Africa established, the results of which help in understanding the evolution of Angola’s terrestrial ecosystems. Since the start of the breakup of Gondwana the surfaces of the African continent were intermittently uplifted and subsequently eroded downwards to form a mosaic of extensive denudation plains, known as planation surfaces and called peneplains or pediplains. (These generic terms are used interchangeably by geomorphologists. Here preference is given to the Angolan custom of using peneplain). The oldest and most extensive of these are known as the African and Post-African surfaces. The products of many millions of years of downward (peneplanation) or backward (pediplanation) erosion, these surfaces formed between 85 and 42 Ma (African), 19–15 Ma (Post-African I) and 3–1 Ma (Post-African II), according to the studies of geomorphologists (Knight & Grab, 2018; Partridge & Maud, 1987). Remnants of these surfaces are visible today, most prominently as the Central African Plateau which dominates the Angolan planalto and the extensive peneplains of the subcontinent. Many of these surfaces carry sedimentary layers such as in the Kalahari Basin, which contains thick layers of windblown and waterborne sediments, and in which laterite, silcrete and calcrete have formed during long periods of relative tectonic stability and gradually changing climates.

With the splitting of Africa from the other continents, escarpments were formed along the coastlines of southern Africa by a process of shoulder uplift along the rifted continental margin and regional elevation of the subcontinent. The face of the Great Escarpment of Southern Africa gradually retreated in parallel with the coast. The pace and distance of retreat has varied from place to place, influenced by the prevailing rainfall climate. The escarpments of southern Africa are most prominent in Angola, and along the east coast of South Africa (Fig. 4.3).

Fig. 4.3

The Angolan Escarpment reaches its most dramatic expression at the Serra da Chela. At 2300 m above sea level, the plateau above the quartzitic scarp face is a remnant of the African Planation Surface

Southern Africa experienced a geomorphologically stable period during most of the Cenozoic (from 66 Ma to the present) with only minor and locally enhanced levels of uplift, subsidence and erosion (Fig. 4.4). These erosional periods are believed to be related to uplift of the subcontinent. The driving tectonic forces of uplift are known as mantle swells. Two of these—the Mayombe and Bié swells—uplifted the existing Angolan landscape by about 500 m. The Bié swell accounts for the highlands of Angola, which today serve as the ‘water towers’ of southern Africa (Sect. 4.3). Two subsiding internal draining (endorheic) basins—the older Congo Basin and the more recent Kalahari Basin—developed and were filled by sediments throughout the Cenozoic. The uppermost and most recently deposited sediments are the Kalahari sands. The Kalahari sands, mostly less than 100 m deep but up to 500 m deep, were formed during the past 5 Ma through the deposition of sediments from the water and wind erosion of the Post-African Surface. They cover over 2.5 million km²—the largest continuous body of sand on Earth (Fig. 4.5).

Fig. 4.4

Map of the Africa’s major mantle swells, rifts, escarpments and sedimentary basins. Note the Congo and Kalahari basins and the Mayombe, Bié and Namibian swells. Redrawn and simplified from Knight and Grab (2016) Quaternary environmental change in southern Africa: physical and human dimensions. Cambridge University Press, Cambridge

Fig. 4.5

The sluggish Cuchi River feeds the Cubango drainage of the gently undulating landscape (at 1200–1300 m above sea level) of the Post-African Planation Surface, Cuando Cubango. Photo Antonio Martins

River incision was accelerated from 34 Ma by a drop in global sea-level of 70 m consequent to the formation of the Antarctic ice sheet, the circum-polar circulation, and the regional increase in elevation of southern Africa on newly forming swells. More recently, erosion cycles of landscapes, following uplift in the Pliocene (5.3–2.6 Ma) of from 100 m in the west to 900 m in the east of southern Africa triggered the rejuvenation of river systems. The planation surfaces of this period have been called ‘Post-African’ (i.e. formed after the ‘African’ surfaces) by many researchers following the terminology of King (1962). Erosion stripped away much of the African Surface, but many remnants are preserved to this day where they are capped by resistant duricrusts—laterites, silcretes and calcretes (Knight & Grab, 2016).

The mosaic of African and Post-African planation surfaces, with both ancient and more recent tectonic, erosional and depositional sequences, climatic interactions and feedbacks, have created a confusing imprint that only the trained geomorphologist can readily interpret. But these processes provide explanations for many questions of biogeographic and ecological importance, such as river capture, the creation of the Angolan Escarpment, the Kalahari Basin, the deep gorges of the Cunene River and the extensive floodplains of the Zambezi catchment.

Against this brief overview of Africa’s landscape evolution, the geology, geomorphology and hydrology of Angola will be described before examining more detailed facets of the soil substrate on which plants grow and animals live.

1 Geology: A Brief History of the Earth and of Angola

A necessary first step in learning about the geology of a country such as Angola, and of the planet's evolutionary history, is to become familiar with certain timelines and associated geological layers—the succession of rock formations known as its stratigraphy. This is best understood through reference to a geological map of Angola, and the sequence of geological layers laid down during the long history of the Earth (Fig. 4.1). The geological map of Angola (Fig. 4.6) emphasises the four main geological Eras of planet Earth—Precambrian (541 Ma and older)—where Ma = million years ago, Paleozoic (541–252 Ma), Mesozoic (252–66 Ma) and Cenozoic (66 Ma and younger). A brief synopsis of these Eras, and the geological Periods and Epochs within them, will provide orientation for the many references to geological terms that appear in this volume.

Fig. 4.6

The major geological formations of Angola, with a stratigraphic log indicating the geological periods in which they were formed. Note the importance of the Precambian crystalline rocks of the western spine of Angola, and the extensive cover of Cenozoic Kalahari sands on the eastern half of the country. From Mateus et al. (2019) Biodiversity of Angola. Science & Conservation: a Modern Synthesis. Springer Nature, pp 53–78

Precambrian Era

• The Precambrian Era is the earliest geological era, dating from the birth of the planet approximately 4600 million years ago to 541 Ma. Very few fossils are known from the Precambrian, of which the first recognizable forms of life—stromatolites—date from 3500 Ma. Later examples (1250 Ma) of these fossilised layers of cyanobacteria can be seen at Humpata in Huíla.

• The extended spine of Angola’s western escarpment, highlands and plateaus is formed by Precambrian crystalline rocks of the Basement Complex and the West Congo System, in addition to scattered outcrops of igneous rocks such as granites, and volcanic rocks such as dolerites. The Basement Complex comprises a lower series of granites, gneisses, and migmatites and an upper series of metamorphosed sediments including schists and quartzites. The terms ‘lower’ and ‘upper’ refer to older and younger geological strata respectively. Lying above the Basement Complex is the West Congo System, a mobile belt composed of metamorphosed sedimentary rocks of the upper Precambrian. These include limestones, schists, slates and quartzites.

Paleozoic Era

• The Precambrian Era was followed by the Paleozoic Era (541–252 Ma). There are few examples in Angola for most of the Paleozoic Era. However, the Carboniferous Period (‘Age of Amphibians’) left its footprint in the glacial valleys and tillites in southwest Angola (Figs. 4.7 and 4.8) and in the Baixa de Cassange. Where they exist in Angola, most of the Paleozoic formations are overlain by Kalahari sands. The Paleozoic Era ended with the Permian Period, at the conclusion of which there was a mass extinction of 95% of Earth's animal and plant species.

  Fig. 4.7

  

  Glacial striations (scratches) on the bed of the lower Cunene at Otjiborombonga. The scratches are cut into bedrock by the abrasion of hard rocks held in the base and margins of glaciers during the Carboniferous (300 Ma) as they cut a deep valley into what is now the Cunene Gorge

  Fig. 4.8

  

  Wall cut by glaciers on conglomerates of the Cunene valley. Photos Eckhart Freyer

Mesozoic Era

• The Mesozoic Era (the ‘Age of Reptiles’, between 251 and 66 Ma), was introduced by the Triassic Period. Dinosaurs were dominant throughout the Mesozoic, and the first birds appeared during the Triassic. Evidence of the Triassic is provided by the continental deposits of sandstones, limestones and shales of the Karoo Supergroup that cover the depression of the Baixa de Cassange, together with Cretaceous marls, conglomerates and shales.

• Following the Triassic was the Jurassic Period—so far not known in Angola—which was in turn followed by the Cretaceous Period (144–66 Ma). The Cretaceous not only saw the appearance of the first flowering plants, but also the end of the dinosaurs at its conclusion, which was marked by mass extinctions at 66 Ma. Cretaceous formations in Angola are found as continental fluvial sediments in the interior and marine sediments along its coastal margin, and as crater-fills in extinct Cretaceous volcanoes.

Cenozoic Era

• The last, (and present) Era is the Cenozoic—the ‘Age of Mammals’. Geologists recognise three Periods and seven epochs within the Cenozoic. The terms Paleogene and Neogene Periods include the five epochs from 66 to 2.6 Ma. The Miocene Epoch (from 23–5.3 Ma) witnessed many major climatic, ecological and evolutionary events of great significance to Africa, as outlined in Box 4.1. The last two epochs (Pleistocene and Holocene 2.6 Ma to present) are referred to as the Quaternary Period.

• Five marine sedimentary basins—(Cabinda, Zaire, Cuanza, Benguela and Namibe-the Orla Sedimentar)—are found along the Angolan coast. The northern basins comprise flat plateaus of red, yellow and grey Pleistocene sands (musseques) which cover earlier Lower Cretaceous to Miocene sediments of clays, limestones, marls, sandstones and conglomerates while older beds dominate the southern (Benguela and Namibe) basins. These latter sediments include many marine fossils.

• The eastern half of Angola is covered by continental deposits of Cenozoic Kalahari sands. Much smaller deposits of Quaternary sand are found in the extreme southwest, forming the mobile dunes of the Namib Desert. On the western high plateau, Quaternary soils have developed through weathering of the Precambrian substrate. The products of erosion are found as alluvial deposits in the Cuvelai Basin, and in the Zambezi headwaters across the vast Bulozi floodplains of Moxico.

Box 4.1: The Miocene: The Springboard of Africa's Modern Ecosystems

The Miocene (ca. 23–5.3 Ma) is considered one of the most pivotal periods for tropical Africa, with several climatic, geological and physiographic changes hypothesised to have led to a complex evolution of African biodiversity. Couvreur et al. (2020)

Geological, climatic and evolutionary events during the Miocene Epoch shaped many of the biodiversity patterns and ecological processes that will be discussed in this volume. The Miocene will be referred to frequently in chapters that follow, and it is thus helpful to understand its characteristics and importance.

The Miocene occurred between 23 and 5.3 Ma (Million Years Ago). It was preceded by the Oligocene and followed by the Pliocene, during a period in which the planet was initially warming but thereafter cooling and drying, due to the reduced ability of the cold atmosphere to absorb moisture and form rain. The continents, following the breakup of Gondwana, had drifted close to their current positions. Africa moved up against Arabia for a brief period, allowing Eurasian biota to migrate onto the continent, before separating once again. Geologists and palaeontologists refer to three stratigraphic sub-epochs: Early Miocene (23–20 Ma), Middle Miocene (20–11 Ma) and Late Miocene (11–5 Ma).

Through the Miocene, important changes in climates occurred.

• During the Middle Miocene, the climate cooled sharply after 14 Ma. The Central African Atlantic Swell uplifted the Mayombe massif and the East African plateau by 1200 m, the latter creating a barrier to the moisture-bearing southeast trade winds, blowing across Africa from the Indian Ocean, and causing the aridification of the Congo Basin and the fragmentation of the rain forest.

• The Antarctic ice sheet, developing since 34 Ma, reached its current extent by 8 Ma in the Late Miocene. The Antarctic Circumpolar Current had already cooled the planet and increased seasonality and aridity. By this time the Namib Desert and the extensive savannas of Africa were well developed.

• The rain forests expanded and the savannas contracted at the end of the Miocene, during a warmer, wetter period in the Early Pliocene (5.3–3.6 Ma) which was soon followed by a cooler, drier period.

• During the Late Pliocene (3.6–2.6 Ma) a further uplift, by 900 m, rejuvenated the Great Escarpment that characterises the geomorphology of western Angola. This had profound influence on the region's biodiversity dynamics during the Pleistocene.

• Hereafter, oscillations of cool dry (glacial) and warmer, wetter periods (interglacials) occurred through the Pleistocene Ice Ages (2.6–0.1 Ma). The impact of these climate changes on the tropical rain forests of Angola will be discussed in Chap. 12.

Of considerable importance to the ecology of Africa was the formation of grasslands during the Miocene. The evolution of silica-rich, fibrous, fire-tolerant grasses, and long-legged herding ungulates (hooved mammals) with high-crowned teeth, was perhaps the key evolutionary/ecological process that shaped modern African ecosystems. Savannas, dominated by water-efficient C₄ grasses (Sect. 10.2), provided the open habitats in which perissodactyls (zebras, rhinos) and artiodactyls (antelope, giraffe) proliferated. Ninety-five percent of modern plants and all modern bird families existed by the end of the Miocene. Apes speciated, and the first bipedal hominins (our ancient ancestors) appeared in Africa at the end of the Epoch. Two key features of Angolan savannas (spiny and thorny trees and shrubs of the arid savannas and the ‘underground forests’ of the mesic savannas (Chap. 14, Box 14.3), evolved through interactions and coevolutionary processes with the herbivore fauna and fire regimes during the Miocene.

In summary, during the Miocene multiple key events occurred:

• The intensification of the Antarctic ice sheet,

• The reinforcement of the Circumpolar Current,

• The establishment of the Benguela Current,

• The cooling and drying of the planet,

• The creation of the Namib desert,

• The expansion of grasslands and savannas and the contraction of forests,

• The evolution of C₄ grasses and the increasing frequency and intensity of fires, and

• The speciation of herding antelope and of spinescent woody plants, reaching their current importance in ecosystem structure and function.

By the end of the Miocene, savanna landscapes and 95% of modern plant, mammal and bird families had appeared. Africa as we know it had been formed.

2 Landscapes, Landscape Ecology and Geomorphology

The general topography of Angola is illustrated in Fig. 4.9. In summary, coastal lowlands lying below 200 m altitude and of 10–150 km breadth occupy 5% of the country’s land surface, leading to a stepped and mountainous escarpment rising to 1000 m (23%), and an extensive interior plateau of 1000–1500 m (65%). Seven percent of the country lies above 1500 m, reaching its highest point at 2620 m above sea level on Mount Moco.

Fig. 4.9

General topography of Angola. From Huntley (2019) Biodiversity of Angola. Science & Conservation: a Modern Synthesis. Springer Nature

The major landscapes of Angola have been categorised and mapped—based on key features of their topography, geomorphology, geology, climate, soils, phytogeography and agro-ecological potential—into 11 ‘mesological’ or landscape units by agronomist Castanheira Diniz (1973, 1991, 2006). Diniz’s concept of agro-ecological/mesological units closely corresponds with current perceptions of ecoregions. His thinking reflects what is today called landscape ecology, the study of the composition, structure and function of landscapes. Diniz used an interdisciplinary approach, where scales of ecological patterns and processes are integrated with functional utility for human societies and their impacts on the landscape. Important aspects of these 11 broad units (Fig. 4.10) are summarised here, listing the dominant ecoregions found in each landscape type. The economic values of each landscape are illustrated by the agricultural crops that Diniz (1973, 2006) identified as having highest existing or potential productivity within the unit.

Fig. 4.10

The main landscape systems of Angola From Huntley (2019) Biodiversity of Angola. Science & Conservation: a Modern Synthesis. Springer Nature

• 1. Coastal Zone (Faixa litoranea): Angolan Ecoregions 12–14 & 16. This is a mostly continuous coastal peneplain at 10–200 m, broken occasionally by broad river valleys. In places as narrow as 10 km, the coastal belt is mostly of about 40 km width, broadening to 150 km northwards of Sumbe and up the lower Cuanza River. In contrast to the situation on the east coast of Africa at similar latitudes, the Angolan coastline is notable for the absence of coral reefs and coastal dune forests. Long sandbars stretch northwards from rivers such as the Cunene and Cuanza. Mudflats and mangroves occur at most river mouths from Lobito northwards, increasing in dimension and diversity towards the Congo. Much of the coast is uplifted, resulting in sharp sea-cliffs of 10–100 m height. Several inselbergs (isolated mountains) rise above the coastal peneplain, the most important being Serra da Neve, which rises to 2489 m. The coastal plains are composed mostly of fossiliferous marine sediments of the Cabinda, Zaire, Cuanza, Benguela and Namibe sedimentary basins. The northern coastal platforms are covered by deep red Pleistocene sands of former beaches (terras de musseque). Lying below the sands, and exposed over large areas, are Cretaceous to Miocene clays, gypsipherous marls, dolomitic limestones and sandstones. Important beds of Cretaceous fossils occur at Bentiaba and Iembe, the latter including the sauropod dinosaur Angolatitan adamastor. The southernmost segment of the Coastal Belt includes the mobile and mostly vegetationless dunes of the contemporary Namib Desert. The floodplains of coastal rivers provide rich alluvial soils, supporting intensive agriculture. Crops include cotton, bananas, cashew, sugar, manioc, mango and palm oil.

• 2. Escarpment Zone (Faixa subplanaltica): Angolan Ecoregion 6. A broad transition belt lies between the coastal plains and the Marginal Mountain Chain and interior plateaus, and is variable in breadth and gradient. Over much of the escarpment, the transition advances up a complex series of steep steps of between 100 and 600 m height. In the south, between Moçâmedes and Lubango, the escarpment of the Serra da Chela is very sharp, rising 1000 m in relief at Tundavala and Bimbe. The geology of the Escarpment Zone is complex, comprising crystalline rocks of the Precambrian: granites, gneisses, schists, quartzites and amphibolites. The Escarpment Zone (sometimes referred to as the Western Angolan Scarp) includes very hilly country, with mountainous belts in the north, and many inselbergs. The Angolan Escarpment has long been recognised for its biogeographic importance (Chap. 18). Agricultural crops include cotton, pineapples, bananas, robusta coffee, sugar, citrus, beans, sunflower, manioc, mango, millet, palm oil, vegetables, sisal and tobacco.

• 3. Marginal Mountain Chain (Cadeia Marginal de Montanhas): Angolan Ecoregions 4 & 5. These mountain lands, at 1800–2620 m, and underlain mostly by Precambrian rocks such as gneiss, granites and migmatites, lie above the Escarpment along the western margin of the extensive interior plateau, and are known as the Benguela, Huambo and Huíla Highlands. The highest peaks rise to 2620 m above sea level at Mount Moco, 2582 m at Serra Mepo and 2420 m at Mount Namba. The mountains are of biogeographic importance for their montane grasslands, with some elements of the Cape flora, and relict patches of Afromontane forests and endemic bird assemblages. Agricultural crops include potato, vegetables and wheat.

• 4. Ancient Plateau (Planalto Antigo): Angolan Ecoregions 4–7. This extensive plateau (the planalto) drops eastwards from below the Marginal Mountain Chain and encompasses the headwaters of the Cunene, Cubango, Queve and Cutato rivers, comprising rolling landscapes with wetlands and low ridges with scattered granitic inselbergs. Together with the Marginal Mountains Chain, the planalto represents the highlands of Angola, including large areas of Benguela, Huambo, Huíla and Bié provinces. It drops from 1800 m in the west to 1400 m in central Angola. These flat to gently undulating landscapes, together with the Congo and Zambezian peneplains, are known as the African and Post-African planation surfaces (Feio, 1964; Jessen, 1936; King, 1962), and were thought to have been formed during two periods of continental erosion, between 100 and 45 million years ago (late-Cretaceous to mid-Cenozoic) and 23–7 million years ago (Miocene). Agricultural crops include ground nuts, potato, arabica coffee, citrus, beans, maize and wheat.

• 5. Lower Cunene (Baixo Cunene): Angolan Ecoregions 12 & 13. This landscape unit leads imperceptibly down from 1400 m on the ‘Ancient Plateau’ to the frontier with Namibia at 1000 m. The gentle gradient of the eastern half forms the very clearly defined Cuvelai Basin, which drains as an ephemeral catchment into the Etosha Pan. West of the Cunene the landscape is more broken, with pockets of Kalahari sands between low rocky hills. Agricultural crops include cotton, sorghum, millet, maize and manioc.

• 6. Upper Cuanza (Alto Cuanza): Angolan Ecoregion 7. The upper catchments of the Cuanza and its tributary the Luando, at altitudes between 1200 and 1500 m, form a distinct basin of slow drainage feeding extensive wetlands during the summer rains. Agricultural crops include maize, manioc and sugar.

• 7. Malange Plateau (Planalto de Malange): Angolan Ecoregion 7. A gently undulating plateau at 1000–1250 m which drops abruptly on its northeastern margin by some several hundred metres to the Baixa de Cassange and the Cuango drainage. The escarpment ravines hold important moist forest outliers (such as at Tala Mungongo and Quela). To the west, the plateau is drained by rivers flowing to the Atlantic, most spectacularly by a tributary of the Cuanza (the Lucala) that drops over 100 m at the famous Calandula Falls. Agricultural crops include sugar, sunflower, manioc and maize.

• 8. Congo Peneplain (Peneplanície do Zaire) Angolan Ecoregions 2 & 3. This is a vast sandy peneplain, drained by the northward flowing tributaries of the Cassai/Congo Basin. The penelain stretches eastwards from the margins of the mountainous northern end of the Escarpment Zone in Uíge, to the extensive Chanas da Borracha of the Lundas. These gently dipping plains, mostly at 800–1100 m, are being dissected by the many northward flowing, parallel tributaries of the Congo Basin. The Cuango River, draining the Baixa de Cassange, drops to 500 m at the frontier with the Democratic Republic of Congo. The southern boundary of the Congo peneplain is defined imperceptibly by the watershed between the Zambezi and Congo basins, lying at ca. 1200 m. Agricultural crops include ground nuts, sunflower and manioc. The Cretaceous sediments of many rivers of the Lundas are the source of Angola’s rich diamond industry.

• 9. Cassange Basin (Baixa de Cassange): Angolan Ecoregion 2. A wide depression, several hundred metres below the surrounding plateaus, is demarcated by abrupt escarpments to the west and the densely dendritic catchment of the Cuango to the northeast. The underlying geology comprises Triassic Karoo Supergroup sediments of limestone, sandstone and conglomerates. Within the Basin, several large tablelands (mesas)—remnants of the old erosional (planation) surface—rise above the depression as extensive plateaus. These are flanked by sheer 300 m high escarpments, exemplified by Serra Mbango, which awaits biological survey. Agricultural crops include cotton and manioc.

• 10. Zambezi-Cubango Peneplain (Peneplanície do Zambeze-Cubango): Angolan Ecoregions 7–11. This is the vast peneplain draining deep Kalahari sands, with slow-flowing rivers that meander across the gently dipping plateau from 1200 m at the watershed with the Congo Basin, to 1000 m at the frontier with Namibia. The climatic gradient (moist high ground in the north, drier in the south) across the Kalahari sands is reflected in several ecoregions in this geomorphological unit. Within this extensive peneplain, the Bulozi Floodplain occupies an area in excess of 150,000 km² in Angola and Zambia. Agricultural crops include manioc, maize and sorghum.

• 11. Upper Zambezi Massif (Maciço do Alto Zambeze): Angolan Ecoregion 10. The Calunda Mountains of eastern Moxico, composed of Precambrian schists and norites, dolorites, sandstones and limestones, rise to 1628 m above the Zambezi peneplain which lies at 1150 m. The mountains form a striking contrast to the almost featureless landscape that stretches some 800 km eastwards from Huambo to Calunda. Agricultural crops include maize.

3 Hydrology: Wetlands, Water Towers, Rivers, Floodplains and Estuaries

The scientific study of the movement, distribution and management of water is called hydrology. Understanding the role of water in ecosystems is paramount, and here one needs to commence at the moment water enters the terrestrial ecosystem from the atmosphere as various forms of precipitation, most commonly as rainfall. Consider the life of a single drop of rain falling onto the high peaks of the Marginal Mountain Chain. The raindrop might simply evaporate off plant and soil surfaces, or join many other drops entering the soil vertically by percolating down to the water table, and seeping laterally into a wetland on the mountain slopes and adjoining plains. A wetland is a seasonally or permanently waterlogged area, usually dominated by grasses, and often forming a deep organic layer (peat), which acts as a sponge, gradually releasing water into streams that become rivers as they descend from the highlands. Much of the Angolan planalto is a great water sponge, providing ecological services (water storage, filtering and regulating, plus supporting great biological diversity). These planalto sponges are found in the ecoregions described by Diniz (1991), Fig. 4.10—the Ancient Plateau, Malanga Plateau, Upper Cuanza, and the upper reaches of the Congo and Zambezi Peneplains. Here we will first focus on the rivers and their catchment dynamics. Box 4.1 discusses the ecological services provided by the water towers of Angola, and the human–environment interactions that lead to the loss of these services.

Coastal Rivers

Angolan river systems (Fig. 4.11) fall into two categories. First, coastal rivers drain the central and western highlands and flow westwards, where they penetrate the steep Angolan Escarpment to the Atlantic Ocean. Most of these coastal rivers are perennial, relatively short and swiftly flowing. They are highly erosive and carry high sediment loads, as in the northwest and westcentral basins. Backward (headward) erosion by some of these coastal rivers has produced minor basins, such as the amphitheatres of the upper Queve and Catumbela rivers. By contrast, the coastal rivers south of Benguela are ephemeral, only flowing after high rainfall events in their upper catchments.

Fig. 4.11

Main hydrographic basins of Angola. From Marques et al. (2018) Diversity and Distribution of the Amphibians and Reptiles of Angola. California Academy of Sciences, San Francisco

The coastal rivers mostly end in broad floodplains in their lower reaches, such as those of the Cuanza, Longa and Queve rivers. These floodplains deposit heavy silt loads that form the substrate to densely vegetated and permanently waterlogged swamps dominated by papyrus Cyperus papyrus and reeds Phragmites mauritianus. As the rivers approach the sea, estuaries form where the interaction of tidal marine water mixes with the freshwater of the rivers. From Lobito northwards, these estuaries and mudflats are occupied by mangroves and seagrasses, forming a distinctive biome and ecoregion (Chap. 17).

Rivers Draining the Planalto and Eastern Peneplains

The second major category of river systems is that of the Angolan planalto and the extensive peneplains of eastern Angola. Drained by seven large hydrological basins, six of which are transnational, Angola serves as the ‘water tower’ for much of southern and central Africa. These water towers comprise the extensive wetlands that occupy the gently rolling plains (anharas do alto) and shallow valleys (mulolas) of the vast Angolan plateau, characterised by the Montane Grassland (Ecoregion 5) and the Wet Miombo (Ecoregion 7). The grassy sponges and swamps, and deep sands of the plateau, store water through the year, gradually releasing this water into the major river basins. Many of these rivers arise in close proximity on either side of the gently undulating watershed between the Cuanza, Cassai (Congo), Lungue-Bungo (Zambezi), Cunene, and the inland-draining (endorheic) Cubango (Okavango) basins (Mendelsohn, 2022). The rivers that flow across Kalahari sands are slow moving and, due to the filtering action of the sands, are crystal clear and nutrient poor. Faster moving rivers, such as the Cunene and Cubango, are more erosive and carry higher sediment and silt loads, and richer invertebrate, amphibian and fish biomass due to the higher nutrient content of their water. Between the Cunene and Cubango catchments, a separate ephemeral, endorheic system, the Cuvelai Basin, drains southwards into the Etosha Pan.

The biodiversity conservation value of the Angolan river systems is of great significance, feeding as they do two great wetlands of global importance (Okavango and Etosha), and the still under-researched Bulozi Floodplain of Moxico. The Bulozi is possibly the largest ephemeral floodplain in Africa—800 km from north to south and 200 km from east to west—straddling the Angola/Zambia frontier. Other, smaller floodplains, such as those of the Congo tributaries, are important for the swamp forests and gallery forests that line their margins, providing biogeographically significant corridors for Guineo-Congolian species penetrating the Zambezian savannas of the miombo. In strong contrast to the moist closed-canopy gallery forests of the perennially fast-flowing Congo tributaries, are the linear oases of the Namib. These Acacia erioloba, Faidherbia albida and Ficus sycomorus woodlands follow the deep sands of ephemeral rivers such as the Curoca, Bero and Bentiaba from the escarpment to the coast. The nutritious foliage and seedpods of these great trees support the penetration of species such as Savanna Elephant, Giraffe and Greater Kudu deep into the Namib Desert.

River Capture

The current landscapes and geomorphology of Angola are the result of hundreds of millions of years of geological dynamics, accompanied by the erosive action of rain, rivers and wind. Major tectonic movements of the earth’s crust, through faulting and rising or sinking, can result in river capture, where one river cuts into the stream of another, rapidly increasing its erosive power. Such tectonic events shaped the course of the Congo, Cuanza, Cunene and Zambezi rivers many millions of years ago, and more recent tectonic events are currently changing the dynamics of the Cassai and Congo basins. The Palaeo-Congo originally drained into a massive lake in the centre of today’s DRC, while the Palaeo-Cunene flowed due south to the Etosha Pan in Namibia, which for many millions of years was a major lake. About 30 million years ago the Congo was captured by the incision of a small river cutting eastwards from the present mouth of the Congo. The Cunene was captured by a stream that cut backwards from the coast through an existing, but ancient, Carboniferous (300 Ma) glacial valley, which redirected the river to its current mouth at the Foz do Cunene (Figs. 4.7 and 4.8). Less dramatic examples of river capture, but with great biological consequences, are where tributaries of the Congo and Zambezi systems have cut back into one another, changing the direction and volume of water carried by these river systems and mixing their aquatic faunas. The river captures associated with the Congo, Cuanza, Cubango, Cunene and Zambezi have all been of great biogeographic importance.

Box 4.2: Human–Environment Interactions: Ecosystem Services Provided by Angola’s Water Towers

Angola is unusually blessed with abundant freshwater ecosystems: the wetlands, streams, rivers, floodplains and estuaries that fall within the country’s seven hydrological basins. The benefits of freshwater systems, essential for human health and wellbeing, and environmental productivity and sustainability, include the four categories of ecosystem services recognized by the Millennium Ecosystem Assessment (MEA, 2003):

• Provisioning Services: the provision of genetic resources, food, fresh water, fish, fuel, fibre, hydropower and other goods;

• Regulating Services: carbon sequestration, flood control, water purification, disease regulation, pollination and the maintenance of biodiversity;

• Supporting Services: soil formation, nutrient cycling and primary production;

• Cultural Services: spiritual, educational, aesthetic, and cultural heritage values as well as recreation and tourism.

The importance of the highland catchments of Angola, the ‘Water Towers’ of much of central and southern Africa, can be illustrated by three key national and international roles. First, they provide drinking water for Angola’s human population. Second, they provide the latent energy that is captured by hydroelectric power projects for use by Angola and Namibia. Third, they sustain the vast natural ecosystems of Botswana’s Okavango Delta, Angola and Zambia’s Bulozi Floodplain and Angola and Namibia’s Cuvelai Basin. However, the sustainability of these roles is threatened by human activities.

First, the quality of water is being severely eroded by pollution from poorly managed urban wastes, from open cast mining, and from agricultural and industrial chemicals (Mendelsohn, 2019, 2022). Despite the then president’s ‘Water for All’ project, initiated in 2007 to meet Millennium Development Goal 6 for water supply (potable water for 80% of peri-urban and rural communities), the official government assessment indicated that by 2015 only 50% of the target population had been reached. For example, the problem of supplying safe drinking water to Luanda’s four million residents of its musseque settlements remains acute. Water selling has developed into the largest sub-sector of the city’s extensive informal economy, involving extractors, transporters and retailers (Cain & Baptista, 2020). Further, Angola is one of the countries with a high rate of waterborne diseases, due to the scarcity and poor quality of water for human consumption. The watercourses are receptors of many effluents, mainly domestic sewage, due to a precarious or inexistent sanitation system and the small number of water treatment plants (Paca et al., 2019). The consequent pollution accounts for high levels of eutrophication of sections of the main bulk water sources (Bengo, Dande and Cuanza rivers) with growth of invasive aquatic plants such as Eichhornia crassipes. Remote rural communities are largely dependent on standpipes fed by boreholes, and pit-toilet sanitation.

Second, the vast river basins and flows of water from the highlands and planalto result in Angola’s hydropower potential being among the highest in Africa, estimated at 18,200 MW. The government’s stated aim is to substantially grow its hydropower generation capacity from its current levels of around 3590–9000 MW by 2025. Hydropower provides 68% of Angola’s electricity needs. Angola’s hydropower development has been mainly located on the Cuanza River, with installed capacity from dams at Laúca (2070 MW), Cambambe (960 MW) and Capanda (520 MW). Within Angola, the Cunene River has a small hydropower plant at Matala (40 MW). However, the Cunene, via the storage facilities at the Gove and Calueque dams, feeds the Ruacana hydropower plant (347 MW), situated just south of the Namibian border, and which provides 50% of Namibia’s electricity needs. The maintenance of reliable river flows to these hydropower plants through responsible land use in the catchments is thus critical to Angola’s socio-economic future. Deforestation of much of the planalto and the combined impacts of shifting cultivation, wild fires and soil erosion are causes of concern.

Third, the water towers of Angola sustain one of the world’s most iconic centres of biodiversity—the Okavango Delta, recognised by UNESCO as a World Heritage Site. The whole ecosystem is dependent on the seasonal flows of water from two Angolan rivers—the Cubango and Cuito. The Cubango sends pulses of water downstream each summer, while the Cuito provides a steady flow throughout the year. It is the pulses of water and nutrients that bring abundant life to the dry lands beyond the Delta’s permanent swamps. As Mendelsohn et al. (2021) describe, if the pulses of the Cubango are reduced or polluted by upstream developments, the rich biodiversity and related lucrative tourism industry of Botswana will be threatened. Of serious concern is the rapid expansion of commercial agricultural projects along the Cubango, increasing from 300 ha in 2011 to 3100 ha in 2021, with plans to expand the area under irrigation to 221,000 ha in the next decade. Such huge irrigation schemes will drain the Cubango during the dry months, and prevent the pulses of water and nutrients from reaching the Okavango, with potentially catastrophic ecological consequences.

These few examples illustrate the vulnerable interconnections that exist between human wellbeing and environmental sustainability, not only in Angola, but across the region, where water quantity and quality are key determinants of livelihoods and biodiversity.