Physical Geography

1/ A central position in Indochina
Long considered a landlocked country, with no access to the sea and partitioned by the terrain, Laos now enjoys a crossroads position, bordered as it is by China to the north, Viet Nam to the west, Cambodia to the south, Thailand to the south and west, and Burma to the northwest. Large investments in infrastructure over the past few decades have made the “corridors” advocated by the Asian Development Bank a reality.

Laos is located at the junction of the two major geographical groups that structure Southeast Asia: the Yunnan Fan on the one hand, from which large rivers from the Tibetan plateau diverge, the Yangtze Kiang, the Irrawadi, the Slouen and the Mekong, and on the other hand, the great alluvial plains to the south, which overlook the sea. The Mekong is therefore the link between a mountain range in the north and a more open one in the south. Laos has an elongated shape: its relatively modest area (236,000 km²) is equal to that of Great Britain. It extends in latitude for more than 1000 km but it is relatively narrow (500 km at its widest point in the north, 200 km in the south but only 100 km at Pak Kading. This lengthening in latitude induces climatic nuances, increases the length of borders and imposes an effort at political unification.

The Mekong River, which crosses the country from north to south, is the country’s natural link, but it is also a somewhat artificial border, the result of a turbulent history since the eighteenth century. Historically, it was, along with its tributaries such as the Nam Ou and the Nam Tha, a penetration axis for populations of the Tai-Kadai linguistic branch who came to repel or mingle with peoples of Austro-Asiatic origin.

Confronted with neighbours with easily exploitable resources, demographically dynamic and more politically structured, Laos, after a prosperous period in the seventeenth century, suffers from its isolation and its harsh sanitary conditions to be today only a country of just over 7 million inhabitants with the lowest density per km² in the entire region.

2/ A mid-mountain country
The plains extend mostly along the Mekong River southward from Vientiane province and along some tributaries. On the other hand, high mountain regions are few and far between: the highest point in the country, Phou Bia, located in the north of Vientiane province, reaches 2820 m. Most of Laos is between 200 and 1000 m above sea level: it is hills, sometimes steep medium mountains and plateaus: among the latter the most notable are the Xieng Khouang plateau in the northern part and the Bolaven plateau in the south.

We are used to distinguishing between Upper Laos and Lower Laos. The first encompasses the northern half of the country, massive, difficult to penetrate, without a real plain but crisscrossed by numerous rivers. The relatively lower, narrower southern half, where the wider plains and valleys make it easy to travel, is called Lower Laos. Its eastern part, corresponding to the Annamite Cordillera, is high and forms the border with Viet Nam: it is easily crossed only at a number of points.

3/ A relief formed in three stages
The first region, corresponding to the middle and lower Mekong valleys and the Isan area, is the oldest. These are very old mountains from the early primary era and totally worn out. All that remains is a tray called a base. A second region corresponds to primary formations that were revived at the beginning of the secondary era. The third region consists of mountains formed during the Secondary Era. Debris from erosion from the first region accumulated on its edge and caused the next two uplifts

Region 1 now corresponds to low-lying regions. This basement is covered with sandstone formed with the debris of wear, it has a raised rim in the extreme south-east, but in this southern part there have also been superimposed flows of volcanic rocks and basalts raised by fractures at the beginning of the Quaternary era: this is the case in particular of the Bolaven plateau

Region 2, made up of mountains raised at the turn of the Primary and Secondary eras, is home to the highest points of the country created by granite upwellings (Phou Bia, Phou Laï) and reliefs caused by the arrival of volcanic rocks (between Pak Lay and Phou Pha Khao). There are two areas:

To the east, the Annamite Cordillera and to the center the Xieng Khoang plateau. The first is composed of mostly crystalline rocks (granite and crystalline schist) or alternatively calcareous rocks, while the second is a flattened area of the schist and crystalline cordillera.

To the west is the Pak Lay Arc, a folded chain where volcanic rocks, the andesites, usually appear, close to basalts.

Region 3, located to the north and northwest on the one hand and to the northeast on the other, is composed of mountains covered with secondary sandstone in the first case, and in the second case of mountains associated with schist valleys corresponding to hollows invaded by the sea at the beginning of the Secondary Era.

1/ Upper Laos, a mountain region
The mountains of this part of the country form a complex ensemble in which, however, several main directions can be distinguished

To the west, reliefs oriented from north-east to south-west in three zones, crossed by major rivers and allowing the creation of north-south and west-east caravan routes that made the ancient kingdom of Lane Xang prosperous.

From Phou Khao Mieng to Phou Pha Khao there is a very clear direction of folds marked in the landscape by horizontal ridges. In detail, the shapes vary according to the nature of the rocks: steep limestone (Phou Pha Khao), volcanic rocks (Phou Koum region) or hard sandstone contrast with the basins and gullies dug in the soft rocks.

The Houa Khong, on the other hand, is a hilly region whose often confusing appearance contrasts with the regular direction of the parallel valleys (Mekong, Nam Tha). The sandstones of the Secondary Era accumulated at the bottom of the folds, the synclines, form raised bands while the tops of the folds, the anticlines, destroyed by erosion, are now hollowed, revealing older terrains.

The Phongsaly region is an extension of the reliefs of the Houa Khong. It is home to the three layers of Laos sandstone. The one in the middle, called red soils because it is mixed with red salt-bearing clays, is fragile and gives hollows. These hollows are dominated by the more resistant layer of upper sandstone that forms reliefs limited by steep slopes. The Lü furrow, which connects the Phongsaly region with those of Muong Xai and Louang Namtha, corresponds to a syncline of sandstone and salt clays.

Since 2022, this compartmentalization has been overcome by the creation, with Chinese help, of a state-of-the-art railway line that connects Luang Phrabang to Kunming via Oudomxay and Louang Namtha.

To the east, reliefs oriented from northwest to southeast.

The Sam Neua region offers a landscape of hills isolated by many but very narrow valleys. The population settled in basins that could be cleared by the main rivers.

The centre is organised around the Xieng Khouang plateau

This plateau forms the northern end of the Annamite Cordillera. This region consists of three parts. In the center is a triangular tray, a piece of which is known as the “plain of jars”. Larger than the Vientiane Plain, it is a peneplain carved into shales and red clays with discontinuous sandstone formations to the southwest. The central part, northwest of the city of Xieng Khouang, better flattened in clays covered with alluvium from the Quaternary era, forms the Plain of Jars. The plateau communicates with the Vientiane plain through the Nam Ngum valley and to the east opens onto the collapsed limestone basin of Ban Ban. In the Tertiary era, this basin was occupied by a lake at the bottom of which clays, gravels and marls that form the soil accumulated.

To the north of the plateau, you can see the Phou Loï massif, which dominates it with its soft granite shapes cut by deep valleys.

To the south, the Phou Bia massif dominates, made up of various rocks, sometimes granite. It is the highest region of Laos with several peaks around Phou Bia exceeding 2500 m. This region comes into contact with Western Upper Laos through the Vang Vieng Basin.

2/ Lower Laos, an open region
Formed by alluvial plains, low plateaus and the narrowest and most divided part of the Annamite Cordillera, Lower Laos, compared to Upper Laos, appears to be a common open region.

To the east the higher regions are formed of various elements

To the north of the Aï Lao pass, the mountain range is narrow and low. This region can be crossed by several passes: Viet Nam Nua, Mu Gia and Aï Lao. A strip of limestone occupies the entire width of the mountain range in the province of Khammouane. These limestones, dissolved by tropical waters, are cut into the shape of peaks and cliffs often pierced by caves. These vertical reliefs delimit more or less extensive flat-bottomed depressions, with rich soil but which communicate with each other with difficulty through canyons or underground river galleries. The Nam Theun flows through the centre of a sandstone syncline preserved in height.

To the south of the Aï Lao pass, the reliefs are wider and higher. It is no longer really the Cordillera but the raised edge of the old plinth that forms the reliefs. There is a series of plateaus that increase in elevation from north to south, separated by large valleys that facilitate penetration: the plateaus of Kha Leung, Ta Oy, Boloven, Kasseng and Lové; the valleys of the Xé Lanong, the Xé Done, the Xé Kong, the Xé Kamane and the Xé Sou. The lowest plateaus are formed of sandstone, the highest of granite. Basalt flows covered the Bolvens and Kasseng plateaus in the early Quaternary period, resulting in fertile soils.

To the west, the plains vary in extent

The Vientiane Plain is the first plain encountered by the Mekong River. It has a triangular shape of 80 km east-west base and 80 km north-south high. It corresponds to the northern end of the ancient basement covered with sandstone that rises at the edges, forming steep slopes (Phou Khao Khouay). These sandstones are themselves covered by alluvium brought by the Mekong and the Nam Ngum, which describes large meanders with a changing course.

We can distinguish the ancient alluvium that appears on the highest parts of the Vientiane Plain and gives poor soils; their tops are often hardened by concentrations of iron and laterite; Their colour is reddish-brown. The recent alluvium that occupies the lower parts is richer, grey in colour and favourable to rice fields.

Downstream of the Nam Ngum, the plain narrows to Thakhek; it widens a little only towards Paksane at the arrival of the Nam Ngiep and Nam Sane.

The plain of Savannakhet is four times larger than that of Vientiane. It is actually a low plateau of gently undulating sandstone. The anticlines reaching the Mekong give rapids: Kèng Kabao, Khemmarat. To the north, the upper sandstones form an east-west steep parallel to the Xé Noy. The soils rich in recent alluvium occupy only the bank of the Mekong at the mouth of the Xé Bangfay in the vicinity of Savannakhet as well as the valley of the Xé Champhone towards Muong Champhone (Kèng Kok).

The Mekong plain to the south widens from Pakse: it forms a triangle 150 km high from north to south and 100 km from base east-west to Không. In the north, in the province of Champasak, sandstones cover large areas (Phou Pasak). To the south, the Sithandone Plain is a large alluvial region where the Mekong divides into numerous arms separating islands (the 4000 islands). The Khône falls on the border are said to be due to a collapse.

Two rich alluvial plains of exceptional extent in Laos extend the Mekong plain along the Xé Dône to the Saravane basin and towards the Xé Kong to beyond Attapu.

The unity of the Lao country is organized around the Mekong and its main tributaries, whose annual variations punctuate the life of the rural world.

In Laos, only the Houa Phan region is located on the eastern slope of the Continental Divide. It is drained by the Nam Sam, Nam Neune and Nam Khiane rivers, whose waters end in the Gulf of Tonkin. In their Lao course, there are mountain rivers with deep valleys with some small alluvial basins such as that of Xam Neua. Most of the country (90% of the area) is in the Mekong River basin.

The Mekong, with its 4200 km in length, is one of the largest rivers in the world. It is the 7th in length and the 5th in flow. Its basin covers 795,000 km² shared by six countries. Laos holds 26%. Leaving Yunnan, he traveled 1800 km in Lao territory where the constraints of history made him a border (with Burma and then Thailand) for great distances. Along its Laotian course, it is fed by important tributaries. Laos can thus be considered as the water tower of the region and it will benefit greatly from it in the production of hydroelectricity.

From the Chinese border to the Vientiane Plain for a length of 800 km, the Mekong flows in a relatively narrow valley. The rapids separate small alluvial basins such as those of Houei Xay and Louang Phrabang: it is a mountain river. The most famous of these rapids, the Tang Au rapids facing Burma, which stopped the famous exploration expedition of Doudart de Lagrée and Garnier in 1867, was dynamited at the beginning of the twenty-first century to allow navigation.

After Paklay when it leaves the mountains, the Mekong is only 165 m above sea level. The river and the valley widen into small plains that follow one another until they reach the Cambodian border. But from the Vientiane plain to the latter, the Mekong is divided into numerous reaches, limited by rapids due to the crossing of rocky bars: the most famous rapids are those of Keng Kabao and Khemmarat and of course the majestic Khône falls (Phapheng and Liphi) at what is today the southern limit of the country.

The tributaries on the left bank originate from the mountain ranges.