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.
The Meping and Mewang on the W., rising among the loftiest ranges, are rapid and navigable only for small boats, while the Meyom and Menam, the eastern pair, afford passage for large boats at all seasons and for deep draught river-steamers during the flood-time.
The Menam is the largest, deepest and most sluggish of the four, and in many ways resembles its continuation, the Menam ChaoPhaya lower down.
The Menam Chao Phaya, the principal river of Siam, flows from the point where it is formed by the junction of the rivers of northern Siam almost due S.
Below Paknam Poh, it throws off three branches, the Suphan river and the Menam Noi on the right, and the Lopburi river on the left bank.
At a point a little more than halfway down its course, the Menam Chao Phaya receives the waters of its only tributary, the Nam Sak, a good-sized stream which rises in the east of northern Siam and waters the most easterly part (the Pechabun valley) of that section of the country.
The whole course of the Menam Chao Phaya lies through a perfectly flat country.
In central Siam, after Bangkok and Ayuthia, places of importance on the Menam Chao Phaya are Pak-Nam at the river mouth, the seat of a governor, terminus of a railway and site of modern fortifications; Paklat, the seat of a governor, a town of Mohns, descendants of refugees from Pegu; Nontaburi, a few miles above Bangkok, the seat of a governor and possessing a large market; Pratoomtani, Angtong, Prom, Inburi, Chainat and Saraburi, all administrative centres; and Lopburi, the last capital before Ayuthia and the residence of kings during the Ayuthia period, a city of ruins now gradually reawakening as a centre of railway traffic. To the west of the Menam Chao Phaya lie Suphanburi and Ratburi, ancient cities, now government headquarters; Pechaburi (the Piply of early travellers), the terminus of the western railway; and Phrapatoom, with its huge pagoda on the site of the capital of Sri Wichaiya, a kingdom of 2000 years ago, and now a place of military, agricultural and other schools.
The number of teak logs brought out via the Salween and Menam Chao Phaya rivers average 160,000 annually, Siam being thus the largest teak-producing country of the world.
These negotiations bore important fruit in the Anglo-French convention of 1896, the chief provision of which was the neutralization by the contracting parties of the central portion of Siam, consisting of the basin of the river Menam, with its rich and fertile land, which contains most of the population and the.
The zone between Burma and Siam, established by an agreement between Great Britain and France dated the 15th of January 1896, declared " the portion of Siam which is comprised within the drainage basin of the Menam, and of the coast streams of a corresponding longitude," neutral as between them.