River systems of Thailand divides into two major branches, the " Tiền Giang" (Front River) and "Hậu Giang" (Back River). These in turn enter the South China Sea through nine estuaries. The following are the principal tributaries of Mekong River which flow through Thailand (tributaries entirely outside the Thai border not shown). The Mekong drains a considerable portion of northeast Thailand. In terms of Thai territory drained, it is second only to the Chao Phraya system. The Salawin River rises in Tibet (Tibetan: རྒྱལ་མོ་རྔུལ་ཆུ།) after which it flows through Yunnan, where it is known as the "Nujiang" River (), although either name can be
Achelous River in the westernmost part of the Trikala regional unit. One of its first tributaries is the Aspropotamos, meaning the white river. The river flows generally southwards, and forms part of the boundary between the regional units of Arta and Trikala, which is also the boundary between Epirus and Thessaly. Further downstream, it forms the boundary of Arta and Karditsa, and further of Aetolia-Acarnania and Evrytania. The river runs into the Kremasta reservoir, which is also fed by the rivers Agrafiotis and Megdovas. On exiting the Kremasta reservoir, the river flows southwest into Aetolia-Acarnania, feeding the Kastraki reservoir, west of the