Study area: Lower Mekong Basin

The Mekong River is the world's 12^th longest river and the 7^th longest in Asia. Its estimated length is 4,350 km and it drains an area of 795,000 km², discharging 457 km³ of water annually. From the Tibetan Plateau this river runs through China's Yunnan province, Burma (Myanmar), Laos, Thailand, Cambodia and Vietnam. The Mekong river basin is functionally divided into two parts: the Upper Mekong basin (UMB) and the Lower Mekong basin (LMB) [27]. The UMB, located in the temperate and high altitude semi-tropical zone of China (Lancang Jiang), is covered by alpine and mountainous areas with a low population density while the LMB, located in the tropical zone of South-East Asia, drains more than 76% (60 000 km²) of the Mekong basin and is characterized by a low, flat topography with a high population density [20]. The LMB is well known for its rich freshwater biodiversity; particularly fish which provides a livelihood for million people [28]. The hydrology of the LMB is characterized by two monsoons, from May to October and from November to March, with the former bringing most of the annual rain. The water level begins rising in May and peaks in September reaching an average flow of 45 000 m³/s [27].

In Cambodia, Tonle Sap Lake, which is the largest freshwater lake in Southeast Asia [23], is connected to the Mekong through the Tonle Sap River. This creates an exceptional hydrological cycle; during the rainy seasons the excess water from Mekong river enters the Tonle Sap lake, expanding the area it covers from 2 500 km² to 15 000 km² and creating an extensive wetland around the entire lake. When the rain ceases and water levels drop in the Mekong, reverse flow drains the lake which flows into the Mekong delta [29]. The Mekong delta is known for its high density of artificial canals which are for domestic and agricultural utilization, connected to Bassac and Mekong rivers [30]. During the wet season, 35–50% of the total surface area of the delta is flooded [31]. Sea-water intrusion dominates the hydrology along the coastal areas with water level fluctuations of more than 3 m due to the tidal regime [14]. The reverse flow from Tonle Sap is very important to balance the Mekong delta in Vietnam during the dry season ensuring freshwater flow into the Mekong delta and protecting against sea water intrusion.