Gulf of Thailand

The Gulf of Thailand is a large, shallow inlet of the South China Sea.

It sits between the southwestern shores of Mainland Southeast Asia and the northern Malay Peninsula. The gulf is bordered by Thailand to the north, west, and southwest. Cambodia lies to the northeast, and the Mekong Delta region of Vietnam is also nearby. The water opens into the South China Sea in the southeast. This body of water covers a seabed area of 304,000 square kilometers. It is a vital part of the regional geography and marine environment.

The physical nature of the gulf is defined by its shallow depth. The mean depth is only 45 meters. The maximum depth reaches only 85 meters. This shallowness causes water exchange to be quite slow. Large rivers flow into the gulf and change its chemistry. The Chao Phraya and Tha Chin rivers feed the Bay of Bangkok. The Mae Klong and Bang Pakong rivers also empty into this bay. The Tapi River flows into Bandon Bay in the southwest. These rivers bring massive amounts of fresh water into the sea. This inflow reduces the salinity, or salt level, to between 3.05 and 3.25 percent. Only the deeper water receives saltier water at 3.4 percent from the South China Sea.

Beneath the surface, the seabed has a unique shape called morphology. In the central depression, the floor is not flat. It features elongated mounds and ridges that run parallel to the basin. These shapes exist in water deeper than 50 meters. They cover tens of thousands of square kilometers. This pattern comes from a process called sediment dewatering. This occurs when fluid seeps out of the mud. This seepage creates small pits and pockmarks on the floor. Over time, bottom currents erode these pockmarks. This erosion turns the pits into long runnels and depressions. Eventually, these processes create the large fields of ridges seen today.

The history of the gulf involves major changes in both maps and names. It was originally known as the Gulf of Siam. This name came from the historical kingdom of Siam. The term "Siam" likely comes from the Sanskrit word "Syama," meaning dark or brown. Western maps used this name for centuries. In 1939, the country officially changed its name to Thailand. Following this, the name Gulf of Thailand became the official term. The International Hydrographic Organization now recognizes this name. Older maps from the Renaissance period sometimes called it the "Great Gulf." This was based on the Latin name "Sinus Magnus."

Geological history shows that the gulf looked very different in the past. During the Last Glacial Maximum, between 26,000 and 20,000 years ago, the world was much colder. Massive ice sheets in the Northern hemisphere lowered global sea levels by about 120 meters. Because the maximum depth of the gulf is only 85 meters, it was once dry land. There was no seawater in the gulf during this time. Instead, a great lake filled with fresh water from rain and rivers. This landmass was part of a larger area called Sundaland. Sundaland connected Mainland Southeast Asia to the islands of Indonesia. This connection disappeared when sea levels rose about 15,000 years ago.

The gulf is a home to many diverse marine species. Five types of sea turtles live here, including the green turtle and leatherback turtle. Many people visit for whale watching to see Eden's whales and dugongs.

However, the environment faces serious challenges. Coral reefs cover 75,590 rai, but only five percent are in fertile condition. In 2010, severe coral bleaching occurred across many sites. Bleaching happens when water temperatures stay above 30 degrees Celsius for over three weeks. Plastic pollution is also a major problem for marine life. At least 300 sea animals die each year from eating plastic trash.

Human activity and tourism also shape the gulf. Many tourists visit islands like Ko Samui and Ko Pha Ngan. Divers travel to find the Samaesan Hole near Samae San Island.

This is the deepest dive site in the gulf. It is known as an extreme site due to unpredictable currents and unexploded ordnance. The gulf is also important for fishing. Thailand catches 41 percent of its total marine catch in these waters. However, the coastline faces issues like severe erosion. Some areas lose more than five meters of coastline every year. This is partly due to the clearing of mangrove forests for shrimp farms.