The Chao Phraya River is the major river in Thailand, with its low alluvial plain forming the centre of the country. It flows through Bangkok and then into the Gulf of Thailand. Written evidence of the river being referred to by the name Chao Phraya dates only to the reign of King Mongkut. It is unknown what name, if any at all, was used for the river in older timeā¦The Chao Phraya River is the major river in Thailand, with its low alluvial plain forming the centre of the country. It flows through Bangkok and then into the Gulf of Thailand. Written evidence of the river being referred to by the name Chao Phraya dates only to the reign of King Mongkut. It is unknown what name, if any at all, was used for the river in older times. The river was likely known simply by Thai word for 'river', ą¹ąø”ą¹ąøą¹ąø³, and foreign documents and maps, especially by Europeans visiting during the Ayutthaya period, usually named the river as the Menam.