The central region consists of two plains and two sets of mountain ranges. The Plateau Central (Central Plateau) extends along both sides of the Guayamouc River, south of the Massif du Nord. It runs from the southeast to the northwest. To the southwest of the Plateau Central are the Montagnes Noires, whose most northwestern part merges with the Massif du Nord. Haiti's most important valley in terms of crops is the Plaine de l'Artibonite, which lies between the Montagnes Noires and the Chaîne des Matheux. This region supports the country's (also Hispaniola's) longest river, the Riviere l'Artibonite, which begins in the western region of the Dominican Republic and continues for most of its length through central Haiti, where it then empties into the Golfe de la Gonâve. Also in this valley lies Haiti's second largest lake, Lac de Péligre, formed as a result of the construction of the Péligre Dam in the mid-1950s.