In the Sudd, the river flows through multiple tangled channels in a pattern that changes each year. Papyrus grows in dense thickets in the shallow water, which 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