Sudd area as a ramsar site: biophysical features

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Dr. Riak K. M

University of Bahr El Ghazal,

This paper summaries the biophysical features of the Sudd wetland complied in the Key Documents of the Ramsar Convention Information Sheet on Ramsar Wetlands.


Sudd (5,700,000 hectares, 07°34'N 030°39'E) is one of the largest tropical wetlands in the world, located in Southern Sudan in the lower reaches of Bahr el Jebel, the name given to the White Nile as it flows northwards. Ecologically the Sudd wetland is composed of various ecosystems, grading from open water and submerged vegetation, to floating fringe vegetation, seasonally inundated woodland, rain-fed and river-fed grasslands, and floodplain scrubland. It is a wintering ground for birds of international and regional conservation importance, such as Pelecanus onocrotalus, Balearica pavonina, Ciconia ciconia and Chlidonias nigra; and is home to some endemic fish, birds, mammalian and plant species, and to the vulnerable Mongalla gazelle, African elephant (Loxodonta africana) and shoebill stork. Migratory mammals and Nilotic cattle depend on the wetland for their dry season grazing. Because of its uniqueness, Sudd wetland met all eight Ramsar criteria for listing wetland as site of international importance.


The Sudd wetland, situated in the lower reaches of Bahr el Jebel in Southern Sudan, is perhaps the largest tropical wetland in the world. It is located in the administrative region of Southern Sudan Government. The nearest states are: Jonglei State (from South East), Upper Nile State (North East), Lakes State (South West) Unity State (North West) and Central Equatoria State (South).

The Ramsar Site lies within the floodplains of the Sudd Region. The Sudd wetland is inundated by freshwater inflows from the spill of Bahr el Jebel as well as sheet flow from the runoff of the adjacent flat terrain.
The exact area of the Sudd is not known because it has always been subject to the fluctuations of river discharge and rainfall. Its greatest extend occurs in September but shrinks during the dry season. In periods of high flood and rainfall such as in 1917-1918, 1932-1933 and 1961-1964, the floodplain remained flooded well into the dry season. During periods of low flood and rainfall such as in 1921-1923, the floodplain shrinks to the extent that even the permanent swamps dry up.
The area of the Sudd Wetland Designated as Ramsar contains the permanent swamps that extends from Bor to Lake No and is approximately 57x105 ha (5,700,000 ha) in size.
The land is flat with heavy clay soil. Longitudinally, the Sudd wetland falls within three distinct belts. A narrow southern belt extending 146 km long and 10 km wide has a featureless floodplain with a self regulating mechanism that fills and empties to the west (Alliab valley) and east (Mongalla-Jumeiza) depressions. The Flat (gradient of 1.0-1.5 cm/km) central portion of the floodplain extending from Bor to Lake No has a width of 40 km and is the largest. It has a depth of < 10m and water velocity at 1 m/sec. The small northern portion extending from Lake No to the confluence of Bahr Elzeraf with the White Nile is the narrowest and smallest (2 km wide).

The lateral extent of the wetland is greatly influenced by the flooding regime and precipitation. Ecologically the Sudd wetland is composed of various ecosystems, grading from open water and submerged vegetation, floating fringe vegetation, seasonally inundated woodland, rain-fed and river-fed grasslands and finally floodplain scrubland.

Hydrologically the Sudd wetland is regarded as a giant filter that controls and normalizes water quality and a giant sponge that stabilizes water flow. It is the major source of water for domestic, livestock, and wildlife use. The Sudd wetland is a wintering ground for birds of international and regional conservation importance and is home to some endemic fish, birds, mammalian and plant species. Migratory mammals depend on the wetland for their dry season grazing.

The occupants living within and adjacent to the Sudd region are almost exclusively Dinka, Nuer and Shilluk. The socio-economic and cultural activities of these Nilotes are entirely dependent on the Sudd wetland. Pasturalism, fishing, game hunting and agriculture are the major economic activities. They depend on the regular annual floods and rains to regenerate floodplain grasses to feed their cattle. They move from their permanent settlements on the highlands to dry season grazing in the intermediate lands (toich) at the beginning of the dry season and return to the highlands in May-June when the rainy season starts.

Physical Features of the Site:
The topographical features of the area that makes up the Ramsar site designation cannot be described in isolation due in part to its being an integral part of the Sudd Region. The site falls within the mud flats of the Sahelian Region of Africa at the lower reaches of the Bahr el Jebel River (name given to the White Nile as it flows north) sloping gently to the north at an inclination of 1-1.5 cm/km. The Sudd is an alluvial floodplain of recent geological formation and consists of vertic soils which are interspersed by alfisols. The characteristics of the Sudd soil vary along the lateral extent of the floodplain. Soils of the highland areas are made up of heterogeneous loam soils which are generally alkaline, of low organic matter content, high salinity and a clay content ranging from 15 to 40%. The more fertile seasonal flooded alluvial soils occupy the middle portion of the wetland. These soils have a top layer pH that varies with depth from 7.0 to 8.5, and organic content < 5% and low salinity. Soils of the permanent swamps are considered to be of ectomorphic origin through hyacinth plant succession and mineral alluvium deposition by river water. These soils are generally acidic and have an organic matter content that varies from 5 to 45% with variable sand-clay proportions.

The climate of the Sudd wetland varies annually with annual precipitation ranging from 600 mm in the north to 1000 mm in the extreme south of the wetland. Annual maximum temperature is 33°C and a minimum of 18°C. Flooding of the wetland occurs in June-October during an average year and the water rises and falls at 3 m/day with a 7 mm/day net loss through evaporation.

The Hydrology of the Sudd wetland has been described in detail by Howell et al (1985). The Bahr el Jebel River originating from East African Lakes is the main drainage system of the wetland. Other rivers that drain into the wetland have their origin from the Nile-Congo divide. About 50% of the 2.9x109 m3 of water that flows into the Sudd wetland is lost within it through evaporation. The southern limit of the permanent swamp is Bor.

The downstream catchment to the Sudd wetland has a well defined channel with a depth of 10 m lying in a broad valley whose banks are flat on the west and higher to the east. River gradient downstream of the Sudd wetland falls to 1.4 cm/km and water velocity drops to 0.5 m/s. The channel is dotted by small alluvial islands vegetated by dense stands of aquatic macrophytes such as Vossia cuspidata and Phragmites domingensis.

Physical Features of the Catchment’s Area:

The topographical features of the Sudd catchment have been also described in detail by Howell et al (1985). Briefly, the area falls within an even plain sloping gently to the north at a gradient of 1-1.5 cm /km. The depression in which the Bahr el Jebel river runs is bound by scrub vegetation that decrease in height from south to north and virtually disappear north of Bor. Soils of the Sudd wetland area are generally seasonal with very considerable annual variation. Mean annual rainfall stands at 800+ 200 mm per annum with the southern portion of the wetland being wetter than the northern portion. The mean annual temperature in the Sudd area is also seasonal and stands at 33°C during the hot season and 18°C during the cold season. Relative humidity of the area also tracks seasonal changes and stands as 23% during the dry season and 88% during the wet season. Annual insolence at Bor averages 64% per annum.

Hydrological Values:
The rivers that drain into the Sudd wetland bring annually large volumes of water ( 29 billion cubic meters of water) laden with inorganic particles, nutrients and salt ions but leave the wetland without them. As such the Sudd serves as a giant filter that controls and normalizes the water quality. It also regulates flooding by acting as a giant sponge that absorbs excess water during periods of high water yield. The Sudd area loses about 51% of the water that enters it from Bahr el Jebel and almost all the water of Bahr el Ghazal through evaporation.

The socio-economic benefits of the Sudd wetland to the transhumance communities living in its catchment are immense. The wetland is a source of water for domestic, livestock and wildlife use. It provides dry season grazing to the livestock industry, which is the backbone of the Nilotes economy. The concentration of a large mammalian biomass (10,000 kg/km2) in the seasonal floodplain provides excellent opportunities for traditional hunting and tourism if developed.

General Ecological Features:
Moving laterally from the centre of the swamps, different successive vegetation zones are recognized grading from:-

  1. Open water vegetation: This ecotype is dominated by free-floating-leaved plants like Eichhornia crassipes, Lemna gibba, Azolla nilotica and Nymphaea lotus as well as submerged vegetation e.g. Potamogeton, Trapa and Ceratophyllum spp in both flowing waters and lakes. Microphytes are prevalent in lakes but less so in the flowing turbid waters of the main channel. The diatom Aulacosiera granulata dominates the phytoplankton during the dry season but is replaced by the cyanobacterium Lyngbya limnetica during the wet season when the river velocity and turbidity are high.

  2. Permanent swamps: These occur on land that is flooded throughout the year. The most prevalent types of plants are the floating and fringe vegetation, dominated along the banks of the river by massive stands of Cyperus papyrus, Vossia cuspidata, Phragmites karka and Typha domingensis. Associates of the Cyperus swamp are climbers and ferns

  3. River flooded grassland or Toich: These grasslands are seasonally flooded to varying extent and depth by the river. During the rainy season the plains of this belt are often swampy and completely covered with luxuriant growth of tall grasses. Species composition and biomass is closely linked to soil type and degree of inundation. The tall grasses are dominated by Phragmites, Sorghum, Hyparrhenia and Setaria spp as well as Oryza and Echinochloa. Two grassland types are recognized. These are wild rice grassland dominated by Oryza longistaminata and Echinochloa grassland dominated by Echinochloa pyramidalis. Sedges, herbs and other grasses such as Sporobolus pyramidalis associate with the Oryza grassland. It is the most productive grassland type in terms of year-round grazing for livestock and wildlife because of the high protein content of dead materials of wild rice grass. The Echinochloa grassland associates with Oryza and legumes like Desmodium hirtum.

Within the toich there are many small seasonally flooded pools that support a variety of plants at different times of the year. They are dominated by grasses and herbs during the dry season, semi-woody herbs (Sesbania rostrata) in the early wet season, and floating and submerged aquatic plants during the remaining part of the rainy season. When the rivers flood, the pools are colonized by green algae (desmids and filamentous) and Cyanobacteria which themselves are replaced by euglenophytes and dinoflagellates following evaporative concentration and nutrient enrichment from cattle and bird feeding. These pools are sources of water for domestic, livestock and wildlife use as well as fish and pasture during the dry season

  1. Rain-flooded grasslands: These are seasonally inundated grassland or rain-fed wetlands. They occur on seasonally waterlogged clay soil which in part is heavily used by livestock. It is made up of a comparatively well drained portion dominated by Echinochloa haploclada, heavily grazed grassland dominated by Sporobolus pyramidalis, Phragmites, Sorghum and a high biomass but nutrient poor Hyparrhenia ruffa grassland in areas that are inundated by rain water and sheet flow. These grasslands act as a source of string and grass for thatching.

  2. Floodplain scrubland (Woodland): It is quite distinct from the grass plains being higher in level. It is the well drained areas around the floodplains. This rain fed belt is an open mixed Acacia forest supporting several species of trees and shrubs and a luxuriant growth of grasses. The tree vegetation is dominated by Acacia seyal, Acacia sieberiana and Balanites aegyptiaca. The tree vegetation is an important source of firewood and building poles. Balanites has additional food and medicinal values. It is the most utilized ecotype for settlement, crop production and wet season grazing

Noteworthy flora:
Plants play important roles in ecosystems as integrators of environmental conditions and as food for herbivores. In the Sudd, the plant communities are comparable to those of other floodplains in Africa (eg Kafue Flats) especially in zonation of grasslands around the floodplain. Usually the deep open water is surrounded by a permanent swamp zone consisting of Cyperus papyrus, Vossia cuspidata and Typha spp. which are important habitats for the endangered Shoebill stork. This in turn is surrounded by seasonally flooded grasslands consisting of Echinochloa stagnina, E. pyramidalis, and Oryza longistaminata as well as Hyparrhenia ruffa at the edge of the wetland.

Unlike other wetlands, the Sudd wetland has not been shown to be a hotspot of plant species endemism. So far out of the 350 plant species identified in the Sudd region only one species is endemic. Denny's (1984) record was the first description of a previously unknown swamp species named Suddia sagitifolia. Suddia, a rare genus belonging to the Poaceae, is only known to exist in the Sudd Region. The Cyperus papyrus which is threatened elsewhere by pollution and flood control, flourishes in the pristine Sudd wetlands.

Noteworthy fauna:
Animal life in the Sudd wetlands ranges from microscopic to the largest land mammal (elephant – Loxodonta africana). It has poor benthic fauna composed mainly of insect larvae, oligocheates, leaches and mollusks, probably due to high current velocity, large quantities of detritus and low oxygen content. It has however a rich zooplankton community of which there are 17 species of Rhizopoda, 39 Rotifera, 27 Cladocera, 7 Copepoda, 2 Decapoda, 1 Chaoborus and 1 Conchostraca. Insect diversity in the Sudd region is very high with mosquitoes, Chironomids and Tabanids forming the aquatic and semi-aquatic communities while Orthoptera, Hemiptera, Lepidoptera and Coleoptera making up the terrestrial communities. A total of 120 insect species of economic importance have been identified in the Sudd Area of which 63 are mosquito species. In addition, it is a habitat for many species of freshwater mammals and reptiles such as hippopotamus (Hippopotamus amphibius) and crocodile (Crocodylus niloticus) and a wide range of mammals, and invertebrate communities. It is home to endemic wildlife such as sitatunga, Tragelaphus spekei and Nile lechwe, Kobus megaceros which migrate between the swamps and toich, ie follow changing water levels and vegetation. The tallest land mammal, the giraffe (Giraffa camelopardalis) also uses the wetland during the dry season, together with waterbuck (Kobus ellipsiprymnus defassa) and reedbuck (Redunca redunca).
Birdlife in the Sudd region is diverse. Notable birds of the wetland are white pelican (Pelecanus onocrotalus), black crowned crane (Balearica pavonina), white stork (Ciconia ciconia), white-winged black tern (Chlidonias nigra), saddle-billed stork (Ephippiorhynchus senegalensis), white-backed duck (Thalassornis leuconotus), African skimmer (Rynchops flavirostris) and African darter (Anhinga rufa) among others.

Fish diversity in the Sudd is impressive and appears to be a response to the favourable environmental conditions for recruitment and survival offered by the mosaic of habitat types. Some species of Clarias, Polypterus and Protopterus are amphibious because of their ability to aestivate during the dry season. Another unique feature of the Sudd ichtyofauna is the occurrence of the annual-fish Nothobranchus in the rain-filled pools at the edge of the wetland.

Although little is known about the reptilian and amphibian diversity and abundance of the Sudd Region, it is safe to say that it has the largest wild crocodile population in the world because of its large size and remoteness.
Justification for inclusion as a Ramsar Sit:
The sudd is inland wetland that met all eight Ramsar criterions for listing wetland as a site of international importance. The criteria are summarized below:-
Criterion (1) A wetland should be considered internationally important if it contains a representative, rare or unique example of a natural or near-natural wetland type found the appropriate biogeographical region.

The Sudd wetland is probably the best known natural wetland in Africa because of its size and of its location on the longest river in the world (the Nile). The wetland is an important source of water for domestic use and wildlife as well as livestock production especially for the Nilotic cattle that play a significant role in the cultural history of the Nilotes in its catchment.

Criterion (2) A wetland should be considered internationally important if it supports vulnerable endangered or critically endangered species or threatened ecological communities.

The swamps and floodplains of the Sudd support a rich biota and are an important habitat for endangered and or vulnerable wildlife species. Among the mega-herbivores found in the wetland, the elephant (Loxodonta africana) is vulnerable (VU). Some antelope species that use the wetland are listed by the IUCN as vulnerable, notable among which are Mongalla gazelle (Gazella rufifrons albonotata). Other species include the tiang (Damaliscus lunatus tiang, CITES App. III) and Nile sitatunga (Tragelaphus spekii, CITES App. III). The vulnerable shoebill stork (Balaeniceps rex) can be found at the site too.

Criterion (3) A wetland should be considered internationally important if it supports population of plant and/or animal species important for maintaining the biological diversity of a particular biogeographic region.

The Area supports plant species, which are important for the biodiversity of the Sudd region. The Sudd is the home of Suddia, a plant genus which is only known to exist in the Sudd region. The Nile lechwe (Kobus megaceros) is threatened and is endemic to the Sudd. This antelope species is of potential socio-cultural value to the Shilluk tribe and is utilized on a very limited scale as a source of animal protein and skins for the royal household. Over 350 plant species have been identified in the Sudd Area. It is also home to over 100 fish species, 100 mammalian species and 470 bird species and an unknown number of reptilian and amphibian species. Over 120 insect species of economic importance have been identified in the Sudd area of which 63 are mosquitoes.

Criterion (4) A wetland should be considered internationally important if it supports plant and/or animal species at a critical stage in their life cycles, or provides refuge during adverse conditions.

The floodplains provide an important breeding habitat and support the largest population of resident species of shoebill stork (Balaeniceps rex) in Africa. It also acts as stopover and wintering ground for many migrant water birds such as white pelican (Pelecanus onocrotalus), black crowned crane (Balearica pavonina), white stork (Ciconia ciconia) and white-winged black tern (Chlidonias nigra) among others. Migratory mammals depend on the wetland for their dry season grazing. Among these are white eared kob (Kobus kob leucotis), tiang (Damaliscus lunatus tiang), elephant (Loxodonta africana), Mongalla gazelle (Gazella rufifrons albonotata) and zebra (Equus bruchelli).

The Sudd is also a habitat for many species of freshwater mammals, reptiles and amphibians such as hippopotamus (Hippopotamus amphibius) and crocodile (Crocodylus niloticus) which depend entirely on the wetland for their livelihood. The diverse invertebrate communities support the large fish populations of the wetland and freshwater crabs of the genus Potamonautes occur in the wetland. Also, rare freshwater shrimps such as Caridina nilotica, Palaemon nilotica and Cyclestheria hislopi are found in the wetland.
Criterion (5) A wetland should be considered internationally important if it regularly supports 20,000 or more water birds.

The Sudd swamp is an important site/stopover area for water birds including significant numbers of migratory species. It is of great ornithological importance as it has over 20,000 water birds throughout the year. Estimates of the Range Ecology Survey Team (RES, 1983 cited in Howel et al, 1985) show that at any one time, the Sudd wetland supports among others about 300,000 open bill stork (Anastomus lamelligerus); 100,000 cattle egret (Bubulcus ibis); 100,000 spur-wing goose (Plectropterus gambensis) and more than 20,000 black crowned crane (Balearica pavonina). About 1.7 million glossy ibis (Plegadis falcinellus) spend the dry season in the Sudd wetland (Howell et al, 1985; Postel, 1992).

Criterion (6) A wetland should be considered internationally important if it regularly supports 1% of the individuals in a population of one species or subspecies of water bird.

According to Wetlands International's Waterbird Population Estimates (third edition, 2002), the world population of shoebill stork is between 5,000 and 8,000 birds. The Range Ecology Survey Team estimated the number of shoebill stork (Balaeniceps rex) in the Sudd to be over 5,000 which is within the global estimate. The increase in shoebill stork in the wetland is said to be positively correlated with the increase in the wetland area that was caused by the high floods of the early 1960s.

Criterion (7) A wetland should be considered internationally important if it supports a significant proportion of indigenous fish subspecies , species or families, life-history stages, species interactions and/or populations that are representative of wetland benefits and/or values and thereby contributes to global biological diversity.

Fish diversity in the Sudd is impressive and appears to be a response to the favourable environmental conditions for recruitment and survival offered by the mosaic of habitat types. The fish communities in the wetland are comprised of 31 Siluroids, 16 Characoids, 14 Cyprinoids, 11 Momyrids, 8 Cichlids, and 7 Cyprinodonotids. Fish species whose life-cycles start and end in the wetland belong to the following genera: Polypterus, Heterotis, Hyrocymus, Alestes, Distichordus, Citharinus, Labeo, Sarotherodon, Synodontis, Auchenoglaris, Oreochromis, Ctenopoma, Clarias and Protopterus. Eight dwarf fish species of the Nile are endemic to the Sudd wetland and are Cromeria nilotica, Nannaethiops unitaeniatus, Barbus stigmatopygus, Chelaethiops bibie, Andersonia leptura, Aplocheilichthys loati, Epiplatys marnoi and Electris nanus. Nilotes of the Sudd who have no cattle or lost them during the floods of the 1960s depend on fishing for their livelihood. Although fishing in the wetland is mostly of subsistence nature, commercial fishing picked up in the 1970s with arrival of fishermen from West Africa and northern Sudan.

Criterion (8) A wetland should be considered internationally important if it is an important source of food for fishes, spawning grounds, nursery and/or migration path on which fish stocks, either within the wetland or elsewhere, depend.

The permanent and seasonal aquatic systems of the Sudd wetland play important roles in the life-cycles of many fish species identified in the wetland. The different habitat types ranging from open water, riverine, lacustrine to palustrine offer ideal spawning, rearing, growing, feeding and survival grounds for over hundred species of fish identified in the Sudd area. Clarias, Polypterus and Protopterus Spp, for example, are amphibious because they aestivate in the mud during the dry season. Also found in the drier reaches of the Sudd is the annual fish Nothobranchus that spends the dry season in the egg stage. The different species found in the Sudd have a rich supply of food that appeared to have evolved with them. The diverse populations of microphytes, benthos, invertebrates, mollusks and detritus feeders probably created the diversity in the herbivore, omnivore and carnivore fish consumers. Recruitment, growth and survival of most fish species take place within the various habitat types of the wetland though lateral and longitudinal migration may take place.

Since the Sudd wetland is given status of wetlands of international importance and since it is Geographically located in Southern Sudan and falls under the jurisdiction of the Government of Southern Sudan (GOSS), in terms of policy and its management as indicated in the RIS, it is recommended that GOSS in general and the Minister of Environment, Wildlife Conservation and Tourism (GOSS) in particular do the followings

  1. Create awareness and sense of responsibility among major stakeholders in the various levels of government in Southern Sudan.

  2. Form an independent technical commission composed of specialized personals from various wetlands related areas to do:-

    1. Wetland inventory:

    2. Wetland assessment:

    3. Wetland monitoring:

    4. Research and development


Denny, P. 1984. Permanent swamp vegetation of the Upper Nile. Hydrobiologia 110: 79-90.

Equatorial Nile Project 1954 The Equatorial Nile Project. Being the report of the Jonglei Investigation Team. Sudan Government. London.

Harrison, M.N. and J. K. Jackson 1958. Ecological classification of the vegetation of the Sudan. Bull. Min. Agric., Sudan (New Series) No.2.

Howell, P., Lock, M. and Cobb, S. 1985 The Jonglei Canal. Impact and Opportunities. Combridge University Press. Cambridge.

Moghraby, A.I.El and Sammani, M.O.El. 1985 On the Environment and Socio-Economic Impact of the Jonglei Canal Project, Southern Sudan. Environmental Conservation 12: 41-47.

Moghraby, A.I.El. 1982 The Jonglei Canal – Needed Development or Ecodisaster? Environmental Conservation 9: 141-148.

Postel, S. (1992) The Last Oasis. The World Watch Environment Alert Series. Norton Company, New York, London.

Wetlands International. (2002). Waterbird Population Estimates – Third Edition. Wetlands International Global Series No. 12, Wageningen, The Netherlands.

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