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.
All these valleys, except Morrope and Chao, are watered by rivers which have their sources far in the recesses of the mountains, and which furnish an abundant supply in the season when irrigation is needed.
Chao rises 450 ft.
(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.
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.