The Red River (Song Hong) is the largest river in northern Vietnam. It rises in the mountains of Yunnan Province in the People's Republic of China and has a total catchment area of 139,000 sq.km. The river flows for over 500 km through Vietnamese territory before entering the sea in the Gulf of Tonkin. The triangular delta has its apex in the region of Hanoi, about 120 km from the coast. Here the river divides into its two main distributaries, the southern Song Hong (Red River) and the northern Song Duong.

The mean annual discharge of the Red River at Sontay is 114 cubic kms, equivalent to an average flow of 3,640 cubic metres per second. Some 74.4% of the river discharge occurs during the rainy season, from June to October. At peak flow during the rainy season, the river discharges 30,000 cubic metres per second, as compared with only 430 cubic metres in the dry season. The river carries a very large silt load amounting to 115 million tons per year. The average silt content of the water at Sontay is 990 gm per cubic metre, and the maximum 6,950 gm. The amount of sediments reaching the sea has increased in recent years because of deforestation, intensive cultivation and severe soil erosion in the catchment area. These high concentration of sediment give the water its red colour.

The maximum tidal range along the coast of the delta is approximately 4m. Salinities increase from about 0.5 p.p.t. in the rivers to 30.0 p.p.t. offshore, and fluctuate widely depending on the flow in the river and state of the tide. The pH varies from 8.0 to 8.4, reaching its highest levels in November and its lowest levels in May and June. Water temperatures vary both seasonally and with distance from the coast; in summer the surface temperature of the rivers is 27-30°C (somewhat higher than the temperature of the adjacent sea), while in winter the temperature is 24-26°C (somewhat lower than that of the sea).

Almost the entire delta has been reclaimed for agricultural land, aquaculture ponds, forestry and urban development. Some parts of the delta have been drained by cutting off river channels, while others have been protected from flooding by dikes averaging 3-4 metres in height. Some of these dikes date back to the eleventh century. Approximately 47% of the delta (820,800 ha) is agricultural land, and there are only some 47,900 ha (5.7%) of permanent lakes and ponds. Almost the entire outer edge of the delta is protected by a high sea wall, beyond which there is only a narrow zone of intertidal sand or mud flats. However, the rate of accretion at the river mouths is about twice that of erosion, and new mudflats and sandy islands are constantly being formed.

The principal land use throughout the delta is the cultivation of rice. The whole region produces about three million tons of rice per year (an average yield of 2,835 kg/ha in 1985). To facilitate rice production, some 1,080 km of embankments, 34,400 km of canals, 1,310 drains, 217 reservoirs and 1,300 pumping stations have been constructed. However, during periods of heavy rainfall, the existing facilities are only able to drain about 70% of the rice-growing areas. The other 55,000 ha remain flooded and are incapable of producing a rice harvest. It has been suggested that the best solution to this problem would be to retain these low-lying areas as wetlands and manage them for fisheries and other wetland products, rather than attempt to drain them for rice production. An agricultural cooperative in Hai Hung Province has already adopted this approach with good results, and it is likely that other cooperatives will follow their example in the future.

The most important site for nature conservation in the delta is at the mouth of the main branch of the Red River in ha Nam Ninh and Thai Binh Provinces (site ha).