Snowy to about 27° N., flow the great rivers of the Indo-Chinese peninsula, the Mekong, the Menam, the Salween, and the Irrawaddy, the valleys of which form the main portions of the states of CochinChina (including Tongking and Cambodia), of Siam (including Laos) and of Burma.
As early as the 12th century B.C., Chinese chronicles, which are almost the only source for the history of Cambodia till the 5th century A.D., mention a region called Fou-nan, in later times appearing under the name of Tchin-la; embracing the basin of the Menam, it extended eastwards to the Mekong and may be considered approximately coextensive with the Khmer kingdom.
Closely allied with the Lao are a number of tribes found throughout the hill regions of the upper Mekong, between Yunnan and Kwangsi in China and the upper waters of the Menam in Siam.
(For map, see Indo-China.) The country may be best considered geographically in four parts: the northern, including the drainage area of the four rivers which unite near Pak-Nam Po to form the Menam Chao Phaya; the eastern, including the drainage area of the Nam Mun river and its tributaries; the central, including the drainage area of the Meklong, the Menam Chao Phaya and the Bang Pakong rivers; and the southern, including that part of the country which is situated in the Malay Peninsula.
Between these ranges flow the rivers Meping, Mewang, Meyom and Menam, turbulent shallow streams in their upper reaches, but slow-moving and deep where they near the points of junction.