Bac Bo Gulf
Before 1975 Bac Bo Gulf (Gulf of Tonkin), a saltwater gulf, was called Bac Phan or Bac Viet Gulf, located between Viet Nam and China. It is one of the largest gulfs of the world with an area of about 126,250 km2. The largest width is about 310 kilometers (176 nautical miles); the smallest width is 207 kilometers (112 nautical miles). The coastline of Bac Bo Gulf belongs to ten provinces and cities of Viet Nam, with a total length of about 763 kilometers, and two Chinese provinces, with a total length of about 695 kilometers.
Bac Bo Gulf is quite shallow (less than 60 meters in depth). The main river flowing into the gulf is the Red River. Hai Phong, Vinh (of Nghe An province, Viet Nam) and Beihai (Guangxi province, China) are the main ports of the gulf. Bac Bo Gulf has two passages. The first passage is the Qiongzhou Strait located between the Leizhou Peninsula and Hainan Island, with a width of about 35.2 kilometers (19 nautical miles); the second one (the principal passage) is located between Con Co Island of Viet Nam and Hainan Island of China, with a width of about 207.4 kilometers (112 nautical miles). There are about 2,300 islands in the part of the gulf part located on the Vietnamese side. In particular, there is Bach Long Vi Island located about 110 kilometers from the mainland and about 130 kilometers from Hainan Island. On the Chinese side there are a few small islands in the northeast of the gulf, such as Weizhou Island and Xieyang Island.
Bac Bo Gulf plays an important strategic role for Viet Nam in economy, national defense and security. It contains lots of natural resources and many large fisheries providing important sea products to the people of the two countries of Viet Nam and China. According to forecasts, the seabed and underground of the seabed of the gulf have potential in oil and gas. Bac Bo Gulf is a centuries-old gateway for communication and exchange of Viet Nam with the world and it has special significance for the development of the economy, international trade, national defense and security of Viet Nam.
Bai Tu Long Bay
Bai Tu Long Bay is a concave area in Bac Bo Gulf in the northeast of Viet Nam. It includes some sea areas of Ha Long city, Cam Pha town and Van Don Island district. It is adjacent to the sea in the east; to Ha Long Bay in the southwest; to the mainland in the west (where there is Cam Pha town); and to Co To Island district in the northeast. There are hundreds of small and large islands in Bai Tu Long Bay and many of the large islands are inhabited.
The climate of Bai Tu Long Bay is divided into dry and rainy seasons. The summer is hot and humid, while the winter is dry and cold. The annual average temperature is 15-25°C. Rainfall is 2000-2500 millimeters per year. The tidal scale is 3.5-4 m per day. The salinity is about 31 to 34.5 MT in the dry season and is less in the summer.
The terrain of the seabed here is rather complicated and was formed as the result of geological erosion, intrusion and underground sediment.
The islands of Bai Td Long Bay are located in the geological coastal strip of North Viet Nam running from northeast to southwest in parallel to the coastline on the mainland. The large Tra Ngo Island has a quite special geological structure, i.e. the body of this island is composed of two geological structural grounds of different original formation. The northern part of this island (occupying over 1/3 of the island area) is composed of earthen mountains resting on terrigenous rocks (sandstone, conglomerated pebbles and sand). The karst southern part of the island is composed of limestone mountains with many caves, grottos, small valleys and holes located amid mountains, creating a charming landscape.
On the terrain of Bai Td Long Bay are low hills and mountains with peaks less than 300 meters above sea level. The highest peak is Cao Lo (314 meters in height) on Ba Mun Island. The width of the islands generally is short and islands run in the northeast-southwest direction. At the margins of the islands there are lots of swamps, tidal muddy grounds, rock grounds and narrow sand grounds whose width is from 30 to 70 meters. These swamps and grounds are periodically flooded. Some zones are as large as hundreds of hectares, where there are muddy, sandy and stone grounds, beautiful landscapes and deep-water harbors convenient for boats and ships to anchor: for example, the natural harbors of Cai Quit, O Lon and Cai De. The Cong channel is located between the large Tra Ngo Island and small Tra Ngo Island. The Chuong Nep and Nhang Ria land grounds of the Minh Chau commune and the Son Hao land ground of the Quan Lan commune are the most unique land grounds. The length of the sand grounds of Minh Chau commune may be one kilometer. The grounds are very flat and the sand is very white, thin and smooth. Waves are calm. The sand grounds of the Quan Lan commune are also lengthy but the sand here is yellow and coarser. Waves are stronger.

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