Landscape and physical features
The entire area of central-southern Laos (i.e. south of Vientiane) lies within the Mekong River
catchment area. Three main physiographic units have been identified in this sector of the basin
(Figure 1): the first, the Khorat Plateau to the west, covers most of north-eastern Thailand and
also includes the Vientiane and Savannaketh plains. The second unit is formed by the Annamite
chain range, which lies to the east along the border with Vietnam. The third unit is the Great
Mekong Plain, which covers most of Cambodia and the river delta in Viet Nam. The northern tip
of the Great Mekong Plain, the plain of Champassak, lies between the first and second units. The
Champassak plain is a triangular-shaped river valley sandwiched between the Phoum Dangrak
hill chain to the west and, to the north-east, the southern-most range of the Boulevan plateau.
The border with Thailand runs along the Phoum Dangrak hill chain, which separates the
Champassak plain from the Khorat Plateau.