State of water : Laos
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River Water

Main rivers of the Lao PDR dominantly consist of the first and second tributaries of the Mekong River. There are about 39 main tributaries in the Mekong river basin. Main rivers that have bigger catchmentarea of more than 5000 km², are the following 11 rivers: Nam Ou River Basin is located Northern region, Lao PDR; Nam Suang is located Northern region; Nam Khan is located Lang Prabang Province; Nam Ngum is located Northern-Central part; Nam Nhiep are located Phonesavan of Xiengkhoung province; Nam San is located Bolikhamxay province; Nam Theun/Kading is located Bolikhamxay Province; Sebangfay is located Khammouane Province; Sebanghieng is located Savannaket Plain; Sedone is located Southern part; and Sekong is located South-Eastern part of the country.

Total watershed area of the main tributaries is estimated at about 183,000 km². On the other hand, only 2 main rivers, namely Nam Ma and Nam Ka rivers, are located outside of the Mekong river basin and expanded in the eastern area of Houaphan and Xieng Khuang provinces. Both drain through Viet Nam directly to the South China Sea. Total watershed area of both rivers is approximately 13,000 km²

Generally physical and hydrological characteristics of the main rivers and their catchment areas are as follows:

(1) Nam Ou River is the longest in the northern region of Lao PDR. It originates at Ban Lantoug Gnai Village near the Lao-China border and flow to the south. It has a total length of 390 km to the confluent point with the Mekong River. The total drainage area is 25,000 km² covering Phongsaly province, one third of Uodomxay province, and one haft of Loang Prabang province. The annual discharge is 12,276,964,800 m³

(2) Nam Suang River with a length of 150 km has a source near Ban Sopkok Village at 1,482 meters, flows in a south-west direction for about 50 km and then turns to the west and finally south-west ward to enter the Mekong River. The drainage area is 5,800 km² with 76.4% of the catchment classified as mountainous, and 22.9% as hilly. The annual discharge is 3,654,076,320 m³

(3) Nam Khane River with a length of 935 km has its origin near Phou Nam Pa at an altitude of 1,828 m. It flows down steep slope in the east-west direction to meet the Mekong River at Luang Prabang City at an altitude of 300 m. The drainage area is 6,100. The annual discharge is 29,454,624,000 m³.

(4) Nam Ngum River with a length of 1,403 km and draining as area of 17,000 km² is the largest river of Xiengkhouang and Vientiane provinces and one of the major tributaries of Mekong in Lao PDR. The river source is located in the Plain of Jars about 1,000-1,100 m above mean sea level. After joining with Nam Lik, the Nam Ngum flows down into the Vientiane Plain for about 80 km in southerly direction to Tha Ngone, and then for another 80 km in an easterly direction to join the Mekong River at Pak Ngum District. The annual discharge is 23,021,280,000 m³.

(5) Nam Nhiep River is a local name meaning quiet river. It originates from Phonesavanh Village in Xiengkhuang plateau with an altitude of 1,050 m above mean sea level, flows in a southerly direction to meet an important tributary called Nam Siam near Ban Xiangkhong Village and changes to the southeast direction to meet another tributary, Nam Chian. From this point to outlet near Paksan City, the river flows southerly for a distance of about 95 km. The length of Nam Nhiep is 156 km and catchment area is 4,270 km². The annual discharge is 5,885,248,320 m³.

(6) Nam Sane River, locally known as steep river, originates from diamond of Phou Sam Soum Mountain with an altitude of 2,620 meters above mean sea level, flows to west-northwest through a group of villages around Ban Phouviang Village. It then flows down in the southwest to Ban Thathom Village, and continues to the south-southwest to meet the Mekong at Paksan City. The river is 120 km long with a catchment area of 2,230 km². The annual discharge is 4,271,235,840 m³.

(7) Nam Theun/Kading River, where the upper part is in Khammouane province while the middle and the lower areas are in Bolikhamxay province in central Lao PDR, has several tributaries: as illustrated on map of stream flow, the upper reaches in the plateau consist of Nam One, Nam Noy, and Nam Theun that have total length of 138 km and drain as area of 2,800 km², the annual discharge is 7,027,166,880 m³. Nam Theun and its tributaries have many promising projects for flood control and hydropower.

(8) Sebangfay River mountainous with some peaks higher than 1,500 meters. Flat land appears around Mahaxay District of Khammoune province, where an important tributary Nam Gong/ kathang is 38.5 km long, drains to Gnommalath District. Downstream of the Sebangfay Bridge, an area of up to 70,000 ha of potential agricultural land exists with wetlands, consisting of freshwater lakes, river, ponds, rice paddy and some freshwater marchers, of around 125 km². It is believed that the constant year round discharge of 13,623,552,000 m³ released from Nam Theun powerhouse will change the river morphology of the Sebangfay from Mahaxay to the confluence with the Mekong.

(9) Sebanghieng River is situated in the southern part of Lao PDR, and adjoins the Sebangfay basin which extends immediately to the north. The two basins ranked as the largest basins of the country. The Sebanghieng originates in the Lao-Viet Nam borders at the elevations of 1,000 – 2,000 meters, flows westward with nine major tributaries, and then flows into the Mekong River at a point about 90 km downstream of Savannaket Province. It has a length of about 3,442 km. The total catchment area at the confluence with the Mekong is 21,516 km², and annual discharge is 15,673,392,000 m³.

(10) Sedone River is a total length of 1,475 km, has its origin in the north-eastern side of Bolaven Plateau near Thateng District at 800 m. The catchments area is 6,170 km², and annual discharge is 5,064,681,600 m³.

(11) Sekong River, the source of the river is near the Lao-Viet Nam borders at an elevation of 1,800 meters. The length of the mainstream to Attapu is about 170 km. The total catchments area is 10,500 km², and annual discharge is 16,146,432,000 m³.

Table 1: River Basin Areas and Annual Runoff of the Major River Basin in Lao PDR
River Basin Name No Watershed area
Annual discharge [m³] Length of main stream [km]
1. Nam Ou 19,700 12,276,964,800 390
2. Nam Suang 5,800   3,654,076,320 150
3. Nam Khane 6,100 29,454,624,000 250
4. Nam Ngum 16,500 23,021,280,000 1,403
5. Nam Nhiep 4,270 5,885,248,320 156
6. Nam Sane 2,230 4,271,235,840 120
7. Nam Theun/Cading 3,370  7,027,166,880 138
8. Nam Sebangfay 8,560 13,623,552,000 190
9. Nam Sebanghieng 19,400 15,673,392,000 370
10. Nam Sedone 6,170 5,064,681,600 1,574
11. Nam Sekong 10,500 16,146,432,000 170
Sources: Department of Hydrology and Methodology, 2004

In general, the water quality of rivers within the Lao PDR, and the Mekong is considered to be good, based on international standards.  The level of oxygen is high and the nutrient concentration is low.  Sediment is the primary pollutant source affecting rivers.  Sedimentation loads in tributaries vary considerably from 41 tonnes/km²/year to 345 tonnes/km²/year.  Tributaries and river reaches with high sedimentation are the Sebanghieng, Sedone, Nam Ou, and the upper and lower stretches of the Mekong (Water Sector Study, ADB, 1998). With the pressure of rapid demographic growth, socio-economic development and urbanization, however, the water quality is increasingly exposed to deterioration. In 1999, it was estimated that 35% of liquid effluent disposal to inland surface waters from all sources was treated, while the quantity was unknown. 

Since the developments in Lao PDR is at the initial stage, generally there were no yet any significant impacts from industrial and other developments to the water environment. Coupling with its abundance on water per capita, the water resources status of the country, both quality and quantity, is being in good conditions.

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