Environment

The River Omo at Kurum, in the dry season (David Turton, 1991)

The Mursi live in a rough oblong of territory, bounded to the west and south by the River Omo, to the east by the River Mago and to the north by the River Mara (Map 2). Much of this area consists of a volcanic plain, sloping from east to west, which is being gradually lowered by the action of a large number of seasonal streams flowing westward to the River Omo and northward to the River Mago. The plain is dominated by a range of hills, running diagonally from southwest to northeast, which form a continuation of the Omo-Mago watershed and reach a height of 1,666m. in a dome-like summit, called ‘Dara’ (Map 1). The climate is semi-arid to arid, with mean annual rainfall decreasing from about 800mm to 400mm from north to south. Most of the year’s rainfall is concentrated into two short rainy spells, one between March and April (the primary maximum, or ‘big rains’, called oiyoi in Mursi) and one between October and November (the ‘small rains’, or loru).

The Omo (Warr in Mursi) is one of Ethiopia’s largest rivers. It flows south for over 1000km. from the Blue Nile and Sobat watersheds, to the northern end of Lake Turkana, on the Kenyan border. Since most of its main catchment is at heights of 2000-3000m., its level rises and falls every year in response to the rainfall regime of the Ethiopian highlands. It begins to rise in April or May and reaches its maximum level in August or September, when it overflows its banks in Mursiland and, more extensively, in its lower basin. Having reached its maximum level, the river recedes rapidly during September and October, when people start preparing the recently flooded area for flood-retreat cultivation. It is at its lowest, and easily fordable at several places, by December/January, the height of the dry season (su kiango: literally ‘the stomach of the sun’).

Vegetation in the immediate vicinity of the Omo banks varies between large forest trees, (e.g. Ficus (chamochi), Diospyros (kirithoi), Ziziphus (dangkwe), Salvadora (gegi) and Tamarindus (ragai)) and dwarf shrub grassland (e.g. Tribulus (kathogi), Solanum (turgiyai), Maerua (kamaloi), Sporobolus (keri) and Ricinus (balathi)). Further back from the east bank in the Mursi area, and decreasing in width from north to south, is a dense belt of woody plants or ‘bushland thicket’ (e.g. Euphorbia Tirucali L. (kalangarai), Sarcostemma (kirin kirini), Cissus Quadrangulsaris L (turoi), Sanseviera (kashoi), Acacia Mellifera (radi-a-kora), Adenium Obesum (dorbin) and Plectranthus (lukwe)). The bushbelt gives way abruptly, along the 500m contour, to open wooded grassland, rising gradually to the Omo-Mago watershed. Here are found, scattered or in groups, such trees as Commiphora Africana (olbai), Comiphera penduculata (lareni), Combretum (lalai), Sclerocariya (chobwe), Lannea (kiringeni) and Grewia (kalochi-a-kora).

It has been estimated that the level of Lake Turkana dropped dramatically (for a ‘non-outlet’ African lake) between the late 1890s and the 1970s, due to reduced rainfall over the Omo’s highland catchment area (Butzer, 1971). This resulted in a reduced river flow and, in some cases, the drying out of the Omo’s westward flowing tributaries, leading to the growth of woody vegetation along the Omo banks. Satellite imagery shows that the Omo delta expanded by 500 sq. km during the 1980s and 1990s, suggesting that the lake level and river flows have continued to decrease up to the present (Haack, 1996).

More information

For a rare study of vegetation dynamics in the grassland plain west of the Omo (north of lat. 5 30N), see Michael J. Jacobs and Catherine A. Schloeder, ‘Fire frequency and species associations in perennial grasslands in south-west Ethiopia’, African Journal of Ecology, 40, 2002, pp. 1-9.

For information on the geomorphology and ecology of the lower basin of the Omo see (1) Karl Butzer, Recent History of an Ethiopian Delta: the Omo River and the Level of Lake Rudolf, Research Paper 136, Department of Geography, University of Chicago, 1971; and (2) Claudia Carr, Pastoralism in Crisis: the Dassanetch and their Ethiopian Lands, Research Paper No. 180, Department of Geography, University of Chicago, 1977.

For a discussion of the growth of the Omo River delta, using satellite imagery, see Barry Haack, ‘Monitoring wetland changes with remote sensing: an East African example’, Environmental Management, 20:3, 1996, pp. 411-419.