Siam’s shallow gulf kept a phantom Dragon Tail off maps
The Gulf of Thailand—once the Gulf of Siam—is a shallow South China Sea inlet between mainland Southeast Asia and the Malay Peninsula, about 800 kilometres long, up to 560 wide, and roughly 320,000 square kilometres in surface area. The IHO now lists the Thai name as official.
Greek, Roman, Arab and Persian mapmakers knew these waters as the Great Gulf, Magnus Sinus, and the 1486 Ulm printing of Ptolemy labels it that way. Portuguese sailors in the sixteenth century finally erased the imaginary Dragon Tail peninsula that had framed it. Siam, the old name for Thailand, probably comes from Sanskrit śyāma, meaning dark, and the gulf took the new national name after the country changed its own in 1939.
Thailand holds about 70 percent of the shore, Cambodia 16.5, Vietnam 13 and Malaysia under one percent. The Bay of Bangkok forms the northern tip where the Chao Phraya arrives, joined by the Mae Klong, Bang Pakong and Tapi. Average depth is just 58 metres and the deepest point only 85, so exchange with the open sea is sluggish and river water keeps salinity near 3.05–3.25 percent. Saltier South China Sea water creeps into the central trough below 50 metres, where currents have sculpted pockmarks into long mounds and ridges.
During the last glacial peak, 26,000 to 20,000 years ago, seas stood about 125 metres lower and no saltwater reached this basin at all. It was instead a freshwater lake in Sundaland, fed by rivers draining about 1,020,000 square kilometres, the Chao Phraya chief among them.
Today pressures pile up. Some 1,660 kilometres of Thai coast face the gulf, and 670 of them lose over five metres a year, partly because mangroves were cleared for shrimp ponds. Only five percent of its coral is rated fertile, and bleaching hit Ko Thalu in 2016 after water topped 30 °C for weeks. A 10-kilometre raft of plastic drifted off Chumphon in 2017, and a 2022 pipeline leak spread 20 to 50 tonnes of oil toward Rayong. Still, the gulf yields 41 percent of Thailand’s marine catch, Eden’s and Omura’s whales draw watchers, and Malaysia and Thailand jointly develop contested areas around Ko Kra and Ko Losin.
Source: Gulf of Thailand
