2.1.2. Mekong River tributaries and rainfall

The Mekong River is surrounded by many tributaries. In the wet season, the Mekong River receives a large amount of water from the Mekong upstream and local catchments (its tributaries). The major tributaries are as follows.

• The Se Kong River is one of the largest tributaries of the Mekong with a basin area of 28,500 km². It originates in Viet Nam, runs through Lao PDR and joints the Se San River and Mekong River near Stung Treng in Cambodia. Three percent of the basin is located in Viet Nam, 77 percent in Lao PDR, and 20 percent in Cambodia.

The annual rainfall in the catchments area varies from 1,800 mm in the lower basin to 3,000 mm in the upper basin. The average annual discharge at the mouth is estimated at 32.2 billion m³ which is about ten percent of the flow of the Mekong River at Kratie Province.

• The Se San River is the second largest tributary of the Mekong River in terms of drainage area (75,050 km²) and the largest in terms of flow (90 billions m³/year), but these figures include the Se Kong and Sre Pok Rivers, which join the Se San River near its mouth and are such large rivers in themselves that they are treated as independent basins. The Se San Basin, excluding the Sre Pok and Se Kong basins, has a drainage area of 17,100 km² of which 11,000 km² is located in Viet Nam, where the river originates, and 6,100 km² in Cambodia.

The annual rainfall in the catchments area varies from 2,000 mm in the upper basin to 1,500 mm in the lower basin. The average annual discharge at the mouth is estimated at 17.3 billion m³ which is about 3.9 percent of the flow of the Mekong River at Kratie Province.

• The Sre Pok River is one of the large tributaries of the Se San River and joins this river 35 km upstream of Stung Treng Province, the other being the Se Kong River. The drainage area of the Sre Pok River is 29,450 km² of which 8,200 km² is located in Viet Nam and the remaining area in Cambodia. Five other medium size tributaries, the Prek Preah, the Prek Kriang, the Prek Kampi, Prek Te and Prek Chhlong, which amounts to a total catchment area of 15,030 km², joins the Mekong River on its bank in Cambodia.

The rainfall in these basins varies between 1,500 and 2,000 mm/year. The average annual discharge of the Sre Pork River at its mouth is about 29.8 billions m³, which is about seven percent of the flow of the Mekong River at Kratie Province. The five medium tributaries have an annual discharge of 8.0 billions m³, which is only 1.8 percent of the flow of the Mekong River at Kratie Province.

• Stung Prek Preah flows into Mekong River through Sre Pork River, with a basin area of 1,499 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 3,872 m³/s.

• The Stung Prek Krieng flows into Mekong River through Sre Pork River, with a basin area of 2,392 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 4,891 m³/s.

• The Stung Prek Kampi flows into Mekong River through Sre Pork River, with a basin area of 1,133 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 3,366 m³/s.

• The Stung Prek Te flows into Mekong River through Sre Pork River, with a basin area of 4,303 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 6,560 m³/s.

• The Stung Prek Chhlong flows into Mekong River through Sre Pork River, with a basin area of 5,689 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 7,543 m³/s.

• The Prek Thnot River originates from the mountains in the southwest area of Cambodia, and joins the Basac River, a tributary of the Mekong River, in the southern area of the capital city of Phnom Penh. The total length of the river is 280 km and the drainage area is 5,050 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 6,042 m³/s. The longitudinal gradient of the river ranges from 1/2,500 to 1/3,000 from the middle to downstream. The river basin includes most areas of Kampong Speu Province, part of Takeo and Kandal Provinces and Phnom Penh Municipality at the downstream. A large rice production area spreads over the middle and downstream reaches of the basin. However, the rice is cultivated depending on the rainfall (rainfed) and has low productivity and unstable production. At the downstream, floods occur frequently due to backwater from the Mekong River during the wet season.

• The Stung Slakou flows into the Bassac River which is connected to the Mekong River, with a basin area of 1,363 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 3,692 m³/s.

• The Prek Toul Lokok flows into the Bassac River which is connected to the Mekong River, with a basin area of 453 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 2,128 m³/s.

• The Stung Tonle Bati flows into the Bassac River which is connected to the Mekong River, with a basin area of 440 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 2,098 m³/s.

• The Stung in the southern catchments of Cambodia flow into the Bassac River, with a basin area of 3,568 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated to be 5,973 m³/s.

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Table of contents > 2. River Basins > 2.1. The Mekong River System (Basin)

2.1.1. Introduction

2.1.2. Mekong River tributaries and Rainfall

2.1.3. Detailed survey of rainfall and surface water in the Mekong catchment

2.1.4. Water environment in the Mekong River systems