Relief and settlement

Northern Laos, the most mountainous region, echoes the pattern of the Yunnan fan some 500 km further south. The tributaries of the Mekong follow a NE-SW direction, recalling that of the upper Irrawadi River, while the Nam Ma follows a NW-SE direction, like the eastern basin of the Red River. Only the Xiengkhuang Plateau, with the Plain of Jars at its center, disrupts this fan pattern. Like an umbrella radiating in all directions, it binds northern Laos to the Annamese Cordillera. The cordillera separates the Mekong Valley from the coastal plains of Central Vietnam and provides the junction with the Bolovens Plateau, an extension of the Vietnamese plateaux.

The Mekong Valley is undergoing abrupt changes of direction in relation to its general meridian orientation, between Huoixai and Luangphrabang in the North, and between Xanakham and Pakxanh in the region of Vientiane. Beginning as a narrow corridor in the north, the valley then broadens into a series of plains between Vientiane and Thakhek, although these are less expansive than those on the Thai bank. The Laotian plains broaden out in the region of Savannakhet, before being hemmed in again in by the southern plateau. The differences in relief between the plains along the Mekong disrupt navigation. Transport is notably restricted by the rapids of Khemmarat, south of Savannakhet, and blocked altogether by the Khone Falls at the Cambodian border.

The  population  density  map  shows  the discontinuity of settlement on the three main plains along the river: Vientiane, Savannakhet and Champassack. Settlement by the Lao ethnic group exceeds 50% almost continuously from Xayabury to the south. It surpasses 87% from Paklai to Vientiane, and from the districts bordering Saravane province to the Cambodian border. The proportion of Lao is just over 18% in the highlands between Xamneua and Vientiane and in Khammouane province in the Center, creating a break between the Austro-Asiatics of the North and South.