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.

[5] 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.

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.

[9] 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.

[11] 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 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.

Livestock 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.

Historically, the fully floating Nile cabbage (Pistia stratiotes) was an important plant in the Sudd, but it has largely been replaced by the invasive water hyacinth (Eichhornia crassipes).

[14] White-eared kob, tiang and Mongalla gazelle take part in one of the largest mammal migrations on Earth, numbering about 1.2 million individuals in total.

[18] The long-running civil war in Southern Sudan seriously disrupted conservation efforts in the Sudd, especially as the widespread availability of weapons has encouraged wildlife poaching, including of elephants.

[19] The Sudd swamp is sustained by the water from the southwestern tributaries (the Bahr el Ghazal system) and consumes a proportion of the main river through evaporation and transpiration.

Sir William Garstin, Undersecretary of State of Public Works of Egypt, created the first detailed proposal for digging a canal east of the Sudd in 1907.

[20] By bypassing the swamps, evaporation of the Nile's water would vastly decrease, allowing an increase in the area of cultivatable land in Egypt by 8,100 km2 (2,000,000 acres).

The rusting remains of the giant German-built excavation machine—variously nicknamed either "Sarah" or "Lucy"[22]—are visible on a Google Earth image at the south end of the canal, where it has been located since it was destroyed by a missile.

[24] The complex environmental and social issues involved, including the collapse of fisheries, drying of grazing lands,[25] a drop of groundwater levels, and a reduction of rainfall in the region,[26] limits the practicality of the project.