Physiographic Regions

Topography and physiographic zones of the Mekong Basin. Source: MRC (2010)

Tibetan Plateau

The Tibetan Plateau extends over 2.5 million km2, of which about 316 km2 drain into the headwaters of the Mekong River. Located at 5,160 m above sea level, the Plateau is the source of the Mekong and the most densely glaciated region on Earth (MRC 2010).

Three Rivers Area

The Three Rivers Area is aptly named as the Mekong, Salween, and Yangtze all run alongside one another in this rugged mountainous region. Despite its proximity to two major rivers, the Mekong flows for more than 500 km in this region, through a deep ravine plunging to more than 2500 m in some places, with no significant tributaries in this stretch.

Lancang Basin

Located south of the Three Rivers Area, this highland and plateau area is still relatively elevated (2,000-3,000 metres above sea level) but transitions to mid and lowland reaches as the Mekong flows down a steep gradient, broadening as it goes. Small tributary catchments drain into the river from both sides of the mainstream.

Northern Highlands

The Northern Highlands form the upland region covering northeastern Myanmar, northern Thailand, and the northern areas of the Lao PDR. Large tributaries, including the Nam Ta, Nam Ou, Nam Soung, and Nam Khan, enter at the Mekong’s left bank; while the Nam Mae Kok and Nam Mae Ing enter at the right bank.

Khorat Plateau

Lying largely within northeastern Thailand, the Khorat Plateau is a vast, low-lying terrain consisting mainly of sediment and eroded bedrock and surrounded by a rim of highly resistant sandstone. Here the Mekong River is joined by the Songkhram and Mun Rivers on the right bank and the Nam Ca Dinh, Se Bang Fai, and Se Bang Hiang Rivers on the left bank.

Tonle Sap Basin

The Mekong flows through a broad valley to the east of the Khorat Plateau and enters the Tonle Sap Basin just north of Pakse. The Tonle Sap Basin is a large alluvial plain surrounded by hills. At the southern end of the Basin, the mainstream breaks up into a complex network of branching and reconnecting channels. The western and central parts of the Tonle Sap Basin make up the Great Lake.

The Great Lake

The Tonle Sap Lake, also known as the Tonle Sap Great Lake, is located is located in the Cambodian floodplain is the largest body of fresh water in Southeast Asia.

During the dry season the Great Lake drains into the Mekong River via the Tonle Sap River. During the wet season, the high flows in the Mekong River cause the Tonle Sap River to reverse flow and the Great Lake floods. During the peak of the flood season the area of the Great Lake increases six-fold on average, from 2,500 km2 to 15,000 km2, and its volume increases from 1.5 km3 to between 60 and 70 km3. At the end of the wet season, flows in the Tonle Sap River revert to the normal downstream direction, draining excess water off the inundated floodplain surrounding the Great Lake.

This hydrological cycle supports and maintains the high productivity of biodiversity within the lake, particularly of fish, plant communities and wildlife. In Cambodia, 40% of the population depends on the Tonle Sap Great Lake and its flood plains for their livelihoods.

Mekong Delta

Near the mouth of the Mekong, the Bassac River, the largest distributary river channel, splits from the mainstream. The Mekong and Bassac Rivers split into a number of smaller watercourses, and the delta expands to form a wedge-shaped plain that covers an area of 62,520 km2.

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