The distribution of water resources is highly variable during the year owing to unevenly distributed monsoon rainfall. High variations, combined with limited storage and flood control infrastructure, result in devastating floods in the wet season and extreme low flows in the dry season. About 70-75 percent of the annual runoff is generated in three to four months.
Viet Nam has a dense network of 2 360 rivers with a length of more than 10 km each. There are 16 river basins that are larger than 2 000 km2, eight of which have a catchment area larger than 10 000 km2 (Table 2). Other basins are either have a small area, such as Tien Yen and Muc, or have several small coastal rivers grouped together, such as Giang/Huong, Tra Khuc and Cai-Luy. The eight major basins represent 77 percent of the country’s area. The largest basins are the Mekong and the Red river/Thai Binh, covering 45 percent of the territory.