The Sudd area is one of the largest wetland areas in the world. The shallow and flat inland delta between 5.5 and 9.5 degrees north latitude covers an area of 500km south to north and 200km east to west between Mongalla in the south and Malakal in the north. Its size is highly variable, averages with 30,000 square kilometers and may, during the wet season, be over 130,000 km², depending on the inflowing waters, with the discharge from Lake Victoria being the main control factor of flood levels and area inundation. A main hydrological factor is also that within the Sudd area, consisting of various meandering channels, lagoons, reed- and papyrus fields, half of the inflowing water is lost through evapotranspiration in the permanent and seasonal floodplains (Sutcliffe, 1999). A major feature of the area, even if not completed and not functional, is the Jonglei Canal, planned to bypass waters from the Sudd to avoid evaporation losses and increase the amount of water discharged at the outlet of the Sudd.
The Sudd is the largest freshwater wetland in the Nile basin stretching from Mongalla to near the Sobat confluence with the White Nile just upstream of Malakal as well as westwards along the Bahr el Ghazal. The size of the Sudd varies largely from season to season and year to year. From 1961 to 1963 a great increase in the inundated area occurred when the level of Lake Victoria rose and the outflow increased. The total area is related to the amount of water reaching Bor from Albert Nile and from torrents or seasonal watercourses that can add substantial amounts to the flow in the upstream end of the Sudd. During the 1960th increase in Lake Victoria discharge, where flows at Mongalla have roughly doubled, the flows at Malakal at the northern end of the swamps had increased by 1.5 times the previous average flow. As a consequence of these high flows, the areas of permanent swamp and seasonal floodplains have, together, increased to 2.5 times their former size. The swamps have increased the most but even so the seasonal floodplain is 1.5 times its previous size (Mefit-Babtie, 1983).
From the southern inflow of the Bahr al Jabal ("River of the Mountain") at Mongalla, the defined riverbed successively widens into a floodplain, where the waters flow in meandering river stretches, various channels and lagoons throughout the dry season, expanding over the semi flooded grasslands during the flood season with rising water levels. Slightly downstream of Bor, the Bahr el Zeraf ("River of the Giraffes") river is branching of the Bahr el Jebel to the east, diverting part of the flow, to join the Bahr el Jebel again just before reaching Malakal. During the course of its flow, the Bahr el Jebel passes Lake No, where also the Bahr el Ghazal ("River of the Gazelles") is connected to the Bahr el Jebel, contributing an inflow with seasonal variation. At Malakal, the Sobat River joins into the system. The combined flows then stream to the north as the White Nile in a defined bed, joining with the Blue Nile waters at Khartoum to form the main Nile.
Hydrologically the Sudd plays an important role in storing floodwaters and trapping sediments from the Bahr el Jebel. In average 55 percent of the waters entering the Sudd are lost by evapotranpiration and thus only half of the flow of the Bahr el Jebel at Bor reaches the northern outlet of the Sudd (Baecher, 2000). Waterlevel fluctuations can be found in a range of up to 1.5 meters, depending on the intensity of the floods, typical precipitation values are 800 to 900 mm with evaporation values of 1500mm and evapotranspiration values of up to 2150mm (Mefit-Babtie, 1983).
The morphology of the area is defined by the channel and lagoon system of the permanent Sudd swamps, the adjacent flood plains and the surrounding flat terrain. The Bahr el Jebel runs to the north-northwest and therefore in an angle to the gradient of the floodplain which is sloping down to the north. While north of Juba the river flows in an incised trough. The banks of this trough decrease in height from south to north with the Bahr el Jebel approaching Bor and end into the Sudd floodplain just north of Bor on the eastern bank and towards Shambe on the western bank. In the southern part, the river meanders from side to side in the restraining trough in one or more channels, but further north the swamp is not limited by higher ground and the system of river channels becomes increasingly complex. The characteristics of the river with the network of channels and lagoons are distinguishable from satellite imagery and digital elevation models.
The geology of the area is defined by heavy clay soils, highly impermeable with a top layer of black cotton of approximately 500mm in average. Sandy soils are found only in depth of approximately 30m and below, referring to well drilling profiles (Petersen, 2008). This indicates a very limited groundwater influence on the area hydrology.
Vegetation cover of the area can generally be classified in five categories which occur depending on the elevation of the area above river flood level: Swamp, River flooded grasslands (Toic), Rain flooded grasslands, Wooded grasslands, Woodlands, Cultivated area.
The density of the grasslands here is changing depending on the season, being tall grass in the rainy season and short and dry in the dry season, where also frequent burnings occur. The fluvial area is generally overgrown with vegetation, with some main and side channels as well as lagoons of open water. The vegetation distribution is described in further detail in Sutcliffe (1974) and Petersen (2007). The main species are:
Phragmites communis (shallow flooded, buried roots), Echinochloa pyramidalis (shallow flooded, buried roots), Oryza barthii (shallow flooded, buried roots), Echinochloa stagnina (deep flooded, superficial/floating roots), Vossia cuspidate (deep flooded, superficial/floating roots), Cyperus papyrus (deep flooded, superficial/floating roots) and Typha Doniningensis.
The first three species are anchored so their distribution is limited to depth of flooding, for the last species their root system needs to be in water or saturated soil permanently which gives a good indicator on flood patterns. Phragmites communis, Echinochloa pyramidalis and Oryza barthii for example dominate only in areas where the depth of flooding does not exceed 130 cm over a period of ten years or 118 cm for a month in the year. Floating vegetation of Cyperus papyrus had caused blockages in the Sudd swamps in a number of occasions between 1879 and 1900 when, the plants were torn out by increased floods. From this it can be concluded that Cyperus papyrus needs saturated conditions, can tolerate deep flooding but is limited to a certain range of flooding which appears to be 150 cm (Sutcliffe, 1974).
Pastoralists use the Sudd and the surrounding areas extensively. Life stock and rain fed agriculture are the dominant means of support for the largely rural population for which the seasonal flooded grasslands along the Sudd provides valuable grazing lands (Baecher, 2000).
The shallow water is frequented by crocodiles and hippopotami. Sometimes the matted vegetation breaks free of its moorings, building up into floating islands of vegetation up to 30 km in length. Such islands, in varying stages of decomposition, eventually break up. The sluggish waters are host to a large population of mosquitos and parasites that cause waterborne diseases. The Sudd is considered to be nearly impassable either overland or by watercraft. The early explorers searching for the source of the Nile experienced considerable difficulties, sometimes taking months to get through. In The White Nile, Alan Moorehead says of the Sudd, "there is no more formidable swamp in the world."
The Jonglei canal scheme was first studied by the government of Sudan in 1946 and plans were developed in 1954-59. Construction work on the canal began in 1978 but the outbreak of political instability in Sudan has held up work for many years. By 1984 when the Southern Sudanese rebels (SPLA) brought the works to a halt, 240km of the canal of a total of 360km had been excavated.
It is estimated that the Jonglei canal project would produce 4.8 x 109 m³ of water per year (equal to a mean annual discharge of 152 m³/s (5,368 ft³/s)). There are, however, complex environmental and social issues involved, which may limit the scope of the project in practical terms.
Mefit-Babtie Srl. (1983) Development Studies of the Jonglei Canal Area, Range Ecology Survey, Final Report, Volume 2, Background. Khartoum. Sudan
Baecher, G. (2000) The Nile Basin – Environmental transboundary opportunities and constraimt analysis. USAID PCE-I-00-96-00002-00
Sutcliffe, J.V. (1974) A Hydrological Study of the Southern Sudd Region of the Upper Nile. Hydrol. Sci. Bulletin 19, 2 6/1974: 237-255
Sutcliffe, J.V., Parks, Y.P. (1999) The Hydrology of the Nile. IAHS Special Publication No 5. Wallingford. UK