The Sudd (Arabic: السد, romanized: as-Sudd, Nuer: Baki̱ec, Dinka: Toc) is a vast swamp in South Sudan, formed by the White Nile's Baḥr al-Jabal section. The Arabic word sudd is derived from sadd (سد), meaning "barrier" or "obstruction". The term "the sudd" has come to refer to any large solid floating vegetation island or mat. The area which the swamp covers is one of the world's largest wetlands and the largest freshwater wetland in the Nile Basin. For many centuries the swamp, and especially its thicket of vegetation, proved an impenetrable barrier to navigation along the Nile. The ancient Egyptians failed to penetrate the Sudd and reach the areas south of it. In AD 61, a party of Roman soldiers sent by the Emperor Nero proceeded up the White Nile but were not able to get beyond the Sudd, which marked the limit of Roman penetration into equatorial Africa. For the same reasons in later times, the search for the source of the Nile was particularly difficult; it eventually involved overland expeditions from the central African coast, so as to avoid having to travel through the Sudd. In 2019, a study suggested that increased water flows into the Sudd may be partly causing up to a third of the whole West African rise in atmospheric methane levels over the previous decade.
Location The Sudd stretches from Mongalla to just outside the Sobat River confluence with the White Nile just upstream of Malakal as well as westwards along the Bahr el Ghazal. The shallow and flat inland delta lies between 5.5 and 9.5 degrees latitude north and covers an area of 500 kilometres (310 mi) south to north and 200 kilometres (120 mi) east to west between Mongalla in the south and Malakal in the north. Its size is highly variable, averaging over 30,000 square kilometres (12,000 sq mi). During the wet season it may extend to over 130,000 square kilometres (50,000 sq mi) comprising 21% of the country, depending on the inflowing waters, with the discharge from Lake Victoria being the main control factor of flood levels and area inundation. Since the Sudd area consists of various meandering channels, lagoons, reed and papyrus fields and loses half of its inflowing water through evapotranspiration in the permanent and seasonal floodplains, the complex hydrology has many primary and secondary effects. A major feature of the area is the incomplete Jonglei Canal, which was planned to bypass waters from the Sudd to avoid evaporation losses and increase the amount of water discharged at the outlet of the Sudd. From 1961 to 1963, a great increase in the flooded area occurred when the level of Lake Victoria rose, and the outflow increased. The total area of the Sudd is related to the amount of water reaching Bor from the 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 1960s increase in Lake Victoria discharge, where flows at Mongalla roughly doubled, the flows at Malakal at the northern end of the swamps 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, taken together, increased to 2.5 times their former size. The swamps have increased the most, and the seasonal floodplain is 1.5 times its previous size. From the southern inflow of the Bahr al Jabal ("Sea of the Mountain") at Mongalla, the defined riverbed successively widens into a floodplain, where the waters flow in meandering river stretches and various channels and lagoons throughout the dry season. With rising water levels it expands over the semi-flooded grasslands during the flood season. Slightly downstream of Bor, the Bahr el Zeraf ("Sea of the Giraffes") river branches off the Bahr al Jabal to the east, diverting part of the flow, and again joins the Bahr al Jabal just before reaching Malakal. During the course of its flow, the Bahr al Jabal passes Lake No, where the Bahr el Ghazal ("Sea of the Gazelles") connects to the Bahr al Jabal, 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. Sudd was designated as a Ramsar Wetland of International Importance in 2006. An area of 57,000 square kilometres (22,000 sq mi) was designated.
Climate Hydrologically the Sudd plays an important role in storing floodwaters and trapping sediments from the Bahr al Jabal. Roughly 55% of water entering the area is lost to evaporation. Water levels fluctuate by up to 1.5 metres, depending on the intensity of seasonal flooding. The region receives less rainfall (typically between 55 and 65 centimetres per year) than neighbouring areas at the same latitude. Orographic lifting on the eastern and western sides of the Sudd contribute to that condition.
Geomorphology 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 al Jabal runs to the north-northwest and therefore in an angle to the gradient of the floodplain, which slopes 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 al Jabal approaching Bor and ending in the Sudd flood plain 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 its network of channels and lagoons are distinguishable in 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" vertisol of approximately 500 mm on average. Sandy soils are found only at depths of approximately 30 metres (98 ft) and below, as determined by well drilling profiles. This indicates a very limited groundwater influence on the area's hydrology.
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