some species of cetacean live in rivers and lakes, but are more closely related to oceanic dolphins or porpoises and entered fresh water more recently. such species are considered facultative freshwater cetaceans as they can use both marine and freshwater environments. these include species such as the irrawaddy dolphin, "orcaella brevirostris", found in the mekong, mahakam, the irrawaddy rivers, as well as the yangtze finless porpoise "neophocaena phocaenoides asiaeorientalis".