The river is the border between the Dominican Republic and Haiti for several kilometres , from the town of Pedro Santana to the point where it is joined by the River MacasÃ as , and then turns west into Haiti . It is the longest as well as the most important river in Haiti and the longest on the island of Hispaniola . Forming part of the international border between Haiti and the Dominican Republic , the river 's sources are in the Cordillera Central in the Dominican Republic , however most of its length lies in Haiti .