SOUTHERN AFRICA until the next summer. Today many bushmen work as labourers on tne far11s and ranches owned by both Africans and Europeans. 8.5 WETLANDS OF THE ZAMBESI BASIN Alfl> THB LOWLANDS OF MOZAMBIQUE by R.H & J.S. KEPHAM Extensive wetlands occur along the upper Zambesi River and its tributaries as they cross the Central African Plateau. The most important of these are the seasonal floodplains of the Barotse and Chobe regions, Fig. 8.10. After leaving the high plateau, and traversing the Victoria Falls, where the upper course of the river is deemed to end, it flows comparatively swiftly and is generally confined to its incised channel throughout its middle course. However, extensive wetlands occur in the Kafue Basin, which drains into this part of the Zambesi via its largest tributary, the Kafue River. There are also two major impoundments 011 the middle course of the Zambesi, one at Kariba and another at Cabora Bassa. At this latter place the lower course of the river begins, and again, in Mozambique, the river overtops its banks seasonally. In addition, areas of seasonal and permanent swampland occur on some of its lower tributaries, notably the Sbire River, which drains Lake Malawi. Elsewhere in Mozambique, seasonal wetlands occur along the lower courses of most rivers as they traverse the coastal plain, and also around nwnerous coastal lakes and lagoons. The wetlands of the Kafue are probably better known than the others on the Zambes i system, and are therefore described in more detail. 0 22°E 24 °e 2e e 28 °e 30°E 32 °e ;,.. ~ ~ i: '! m l-· V( \ zarnbes• 0 tc.af u • f lat s 1e s 0 100 200 300 km L-------------------------·------------~ Fig. 8.10 Wetlands of the Upper Zambesi Basin 503 AFRIQUE AUSTRALE THE UPPER ZJJIBESI UGION The Zambesi River bas sources in Angola and Zambia. In its upper coµrse i t crosses the sandy Barotse reg ion of westet"n Zambia where there are tht"ee major wetlands, the northern. centt"al and southern floodplains. Hereafter it entet"s a faulted trough in Kart"oo sediments lying on lavas, between Ngonye Falls and Kulilo Rapids, througb whicb it flows swiftly. After this it again tt"avet"ses a sandy plain at the north eastern extremity of the Caprivi Strip, where its flood waters sometimes mingle with those of a southern tributary, the Chobe. The confluence of these two rivers is at Kazangula, some way above Victoria Falls, at which latter place the upper course of the rivet" is deemed to end. 8.5.a THE CENTRAL BAROTSE FLOODPLAlN 1. Geography The Barotse Flood Plains are situated on the upper course of the Zambesi River on the Central African Plateau in western Zambia, Fig. 8.10. The Central Flood Plain is the largest and is representative of all three. It i s 240 km long and up to 40 km wide, wi th a total area approaching 7700 km2. In traversing the plain the river meanders so much that its length exceeds 590 km. 2. Geology Tbe floodplains lie on Kalahari Sands and the river bottom is predominantly composed of shifting sand. Deposition of alluvium over the flood plain each year is not heavy and the waters of the river are clear at all seasons. 3. Climate This is warm to hot, with average mid-day temperatures reacbing 32°C in August, but often being lower than this in the rainy season due to cloud cover. Tbe mean annual rainfall on the flood plain is about 1050 mm, with 907. of this falling between November and Marcb, and SOT. in January and February. 4. HydrologJ Over 1250 mm of rain falls over the headwater catchments of the Zambesi each year. Rain usually begins in late october and finishes in April. Flooding of the upper Zambesi normally commences in November, but the date varies depending upon the intensi ty of the early rains. Tbe Central Barotse Flood Plain is inundated to a depth of l. S-Jm when the flood peaks in April. The flood recedes over a six month period from May until October, but by September most of the floodplain is dry, and water is restrieted to lagoons connected to the river by backwater channels, and numerous isolated lakes. At this time the main channel is some 25-SO m wide and 3-6 m deep. 504 SOUTHERN A.FRJCA S. Pbysico-cbeaical properties of the water Conductivity of water in the upper Zambesi remains below 65 micro-Siemens through the months February to August, then rises abruptly to peak at a mean value of 126 micro-Siemens in mid October, after which i t declines slowly to the February figure. The lowest conductivities are recorded at the peak of the flood, in April, with a mean value of 57 micro-Siemens. pH and total dissolved solids also peak in October, with a mean average of 52 ppm total dissolved solids. The upper Zambesi drainage area is not productive; burning of the entire watershed oceurs annually and has depleted the nutrient status of the sandy soils. 6. Algae Phytoplankton are abundant in the surface waters of the isolated lakes and some lagoons during the dry season, but make only an insignificant contribution to the primary production of the whole system judged over a full year. Periphyton is present on virtually all submerged vegetatlon, reeds, aquatic grasses, tree limbs and submerged aquatics and is a very important food source for juvenile fisb. No species lists are available. 7. Kacropbytes Tbe lagoons and laites are generally less than 3 m deep in the dry season and contain beds of submerged aquatic plants, among which ceratophyllum demei:-sum is abundant, and some waterlilies and other floating-leaved speeies occur on permanent pools, but vegetation in the river channels is virtually confined to the banlts. Here, erosion caused by cattle has seriously reduced the density of aquatic vegetation, both on the main river and its tributary streams. Species such as Eehinochloa stagnina, Floscopa glomerata, Ipomoea aguat.ica, Leersia hexandra and Vossia cuspidata oecur in backwaters and wherever the eurrent is not strong. In April the floodplain is covered by an immense shallow lake, largely grass and reed-filled, except in the deep water of depressions formed by lagoons, laites and river channels. Primary pl"oduetion in the system over the year, is predominantly terrestrial and decomposition of vascular plant debris is most important to the ecology of the flood plain. Several commercially important speeies of fisb eat deeaying vegetation; others feed on invertebrates whieh depend upon this material. Wi th the drying of the plain there is much die back of the vegetation, and the entire area i.s burned each year. 8. Fauna Tbere is little infonnation on the invertebrate fauna, but zooplankters are comparatively searce, only ever becoming abundant in permanent pools during the dry season. So heavily grazed is the plain that large wild animals have been largely di splaced from i t. Even antelopes have beeome unco11Don over the past 40 years, and crocodiles are now uncommon in the river. 505 AFRIQUE AUSTRALE 9. Human activlties A fishing industry on the plain pr:'ovides the bulk of the protein in the diet of about 200 000 people. Fishing is carried out using gill nets, seines, fence and weir tr:"aps, as well as baskets and spears. Gill net fishing occurs throughout the year and dugout canoes are widely used. The floodplain is covered by thousands of reed, mud and stone weir:'s, but this method is most successful only when water is flowing on to, or off, the plain. Tall fence traps are successful when the flood is rising. Many species are trapped when they leave the river to spawn in the shallow rising waters over the flood plain. Basket fishing is car:'ried out in all seasons, but is mcst t"ewarding in July and August when the receding waters trap fish in small pools. It is predominantly carried out by women. Small mud and grass walls may be used to lead fish into traps comprlsing grass baskets. These are referred to as •maalelo's'. Spear fishing takes place from the banks of the river and lagoons during the dry season. Sorne 73 species of fish are caught commercially on the flood plain including four species of Clarias, four species of Tilapia, and three species each of Serranochromis and Haplochromis. A carnivorous fish, Hydrocynus vittatus <tiger fish> is also very important, together with Mormyrus lacerda, Schilbe !!!!~· Hepsetus odoe, Labeo lunatus and Synodontis woosn8Jlli. The dry season catch from the flood plain bas been estimated as being in the region of 3 x ic/: kg. Most fi sh are marketed locally as access to the area i s poor. Much of the catch is dried, or smoke dried, especially after the rains start. Accounts of the fisheries are given in two reports by FAO, Rome, 1968 and 1969, 'Fishery Development in the Central Barotse Flood Plain' . Cattle Gra.zing: Sorne 300 000 head of cattle gf"aze the flood plain during the dry season and the adjacent forested area during the wet season. Overgrazing is a problem and the mortality rate due to malnutrition and interna! parasites is great. Drainage schemes to increase the efficiency of cattle ranching promise to be deleterious to the fisbery. THE !UDDLE ZAMBESI REGION The Kafue Basin lies on the Centt"al African Plateau and is confluent witb the Zambesi Valley, providing a sub-catchment to that f"iver system of appt"oximately 155 000 km2. It is entirely tropical being situated between latitudes ll°JO'S and l6°50'S, but because of its elevation ït has a comparatively temperate cllmate.