The Sudd is a vast expanse of swampy lowland region in central South Sudan, formed by the river White Nile. The area which the swamp covers is one of the world's largest wetlands in the Nile basin. Its size is highly variable, averaging over 30,000 square kilometers, but during the rainy season depending on the inflowing waters, the Sudd can extend to over 130,000 square km or an area the size of England.