The Parnaíba River ( ) is a river in Brazil, which forms the border between the states of Maranhão and Piauí. Its main course is long and the Parnaíba River Basin covers . The Parnaíba River rises in the Chapada das Mangabeiras range, and flows northeastward to empty into the Atlantic Ocean, being the longest river entirely located within Brazil's Northeast Region. The middle and upper regions of this river are separated by the Boa Esperança Hydroelectric Power Plant dam, but is otherwise navigable.
Ecology
The fish species richness in the Parnaíba River Basin has traditionally been considered impoverished, but this has been disproven by recent surveys, which have recorded about 140 native species (including several that remain undescribed) and about 40% of these are endemic. More than 70% of the fish species in the basin are members of either Characiformes or Siluriformes.
The river's delta has large stands of mangrove, as well as dunes, beaches, marshes and other habitats. The delta is considered an Important Bird Area and is home to species such as scarlet ibis (Eudocimus ruber) and rufous crab hawk (Buteogallus aequinoctialis).