In general outlines, the present pattern of the drainage of northern and western Thailand probably originated in late Tertiary time. A widespread peneplain truncating previously folded rocks may have existed. Then tectonic movements transverse to the major streams in late Tertiary developed several structural basins in which streams ponded along their courses. These results were modified by warping. As sedimentation progressed, the present drainage patterns developed. Regional uplift toward the end of the Tertiary rejuvenated the streams of northern Thailand, and major streams cut deeply into old peneplaned surfaces. Streams adjusted their courses along bends of soft rocks to form strike lowlands; in other places, they cut down across resistant rocks to form water gaps or gorges.
Most of northeast Thailand is occupied by the Khorat basin, drained by the Mun River. The sediments of the Khorat series were downwarped into a large structural basin in the late Tertiary. The dendritic pattern of the Mun drainage thus appears to be related to a generally downwarped surface with uniform bedrock. However, in the southeast, the Mun crosses the structural lip of the basin in an antecedent course to enter the Maekhong.
An important structural movement in the late Tertiary brought about the formation of the Čhaophraya River region, starting a cycle of alluviation which has continued to the present. At first, the northern shore of the Gulf of Thailand may have been as far north as Uttaradit. Accumulating sediments have gradually moved the shore southward.
In the Peninsula, the present drainage system is largely inherited from late Tertiary time. As far south as Chumphon, streams draining east to the Gulf generally follow steep, young canyons in headwater reaches, while in the coastal zone they flow in graded courses through open valleys planated in folded rocks. The lower valleys of many streams on the western slope of the Peninsula have been drowned by recent submergence. In their middle and upper reaches these streams follow graded courses through mature valleys, which in many places are transverse or oblique to structural trends. Structural movements in late Tertiary times also led to local development of several small basins.
The heart of continental Thailand is occupied by the Čhaophraya drainage basin. Waters from the Ping, Wang, Yom, and Nan Rivers of the north converge in the Upper Plain of the Central Valley to form the Čhaophraya near Paknampho. Downstream, minor rivers from the west and the Pasak and other rivers from the east enter the main stream and its several distributaries. The Čhaophraya flows through Bangkok and enters the Gulf of Thailand to the south at Samutprakan (Paknam); its main distributary, the Nakhon Chaisi (Suphan or Tachin) enters the Gulf just to the west. Basic data on the Čhaophraya system is given in Table 1, page 52.
Two other rivers assist the Čhaophraya in forming the Bangkok Plain although they flow separately to the Gulf of Thailand. To the west is the Maeklong, whose waters come from the Khwae Yai and Khwae Noi, which flow out of the Western Mountains and meet to form the Maeklong at Kančhanaburi. To the east is the Pračhin, whose waters come from the mountains along the Southeast Coast and the Cambodian frontier, and from the tributary Nakhon Nayok, which drains the heavily precipitous south-facing slope of the Khorat rim.
All of Khorat and the northern part of North Thailand lies in the drainage basin of the Maekhong, the "great river" of Southeast Asia. In Khorat, the Mun (La Moon) and its major tributary, the Chi, flow eastward into the Maekhong on the Thailand-Laos border east of Ubon and north of Pakse ( Laos). All other Khorat rivers independent of the Mun-Chi system also enter the Maekhong along this boundary. Several small rivers of North Thailand also flow into the Maekhong.
In northern and western Thailand a sizeable mountainous area drains westward into the Salween River, which marks the Thailand-Burma boundary for a distance before re-entering Burma proper and emptying into the Andaman Sea.
BASIC DATA ON THE ČHAOPHRAYA
Čhaophraya--Ping 621 miles
Čhaophraya ( Paknampho to sea) 224 miles
Drainage Area (in square miles)
Upper Basin: Ping River 14,965
Nan River 11,733
Yom River 7,900
Wang River 4,622
Lower Basin: Bangkok Plain ( Čhaophraya and
Nakhon Chaisi Rivers) 10,490
Pasak River 7,035
Minor tributaries 5,062
Total Drainage Area 61,807
There are no large rivers or drainage basins along the Southeast Coast or in the Peninsula. Small rivers and streams tap the nearby mountainous areas and enter the sea--the Gulf of Thailand for the Southeast Coast and the Peninsula East Coast; the Andaman Sea and the Strait of Malacca for the Peninsula West Coast. The watershed divide along the Peninsula follows the Thailand-Burma boundary from the Maeklong drainage basin to the Isthmus of Kra. It then follows approximately the center of the Peninsula along the highest peaks to the boundary of Malaya.