The alluvial plain of the Čhaophraya River, its tributaries and distributaries, and a surrounding piedmont belt, form the great Central Valley of Thailand. The Central Valley contains the most easily utilized soils, the main concentrations of agriculture and population, and the best transportation network. It is clearly the heart of Thailand.
The Central Valley extends approximately 300 miles from the mountains of North Thailand to the Gulf of Thailand. From east to west it is 100 to 150 miles wide with its greatest width in the south. Physically, and to some degree culturally, the Central Valley may be divided into three subregions: the Bangkok Plain, the Upper Plain, and the Marginal Plains.
The Bangkok Plain begins above Nakhonsawan, where the waters of the Ping and Nan Rivers meet to form the Čhaophraya. Near Ayutthaya the Pasak River which drains the western slopes of the Khorat marginal mountains enters the plain. The Bangkok Plain also receives the drainage of two other rivers, the Maeklong and the Pračhin (Ban Pakong). This plain has an average width of 75 miles and a length somewhat under 200 miles. The total area is 11,950 square miles.
In the vicinity of Wat Sing, about 15° 15′ north latitude, the Čhaophraya River branches and its true delta begins. To the north there are occasional low hills or separate mountains close to the banks of the stream. East of Wat Sing there is a single inselberg-like feature, Khao Baokhlarn, with an altitude of about 800 feet above sea level; south, the hills stand farther and farther from the rivers. Near Wat Sing, two broad streams, the Nakhon Chaisi (Suphan or Tachin) and the Čhaophraya, separate and continue for 125 miles to the sea without again coming together. The Nakhon Chaisi, by far the weaker of the two rivers, continues south without further distributaries, but the Čhaophraya soon has another branch, the Noi (the "small river"), and a little further south the Lopburi branches off towards the east. It is the Lopburi which receives the Pasak from the east. Both the Noi and the Lopburi branches bring their waters back again to the main stream.
The Bangkok Plain is the one true plain of aggradation in Thailand. It has a surface of alluvial material and was itself formed by alluviation. The alluvium consists of a very thick deposit over an erosional surface. Heavy silt deposits are added annually on this mature deltaic plain. The plain of true aggradation extends inland as far as Lopburi, about 85 miles from the coast, and along the coast between the mouth of the Pračhin and the Maeklong, about 60 miles. The plain is continually extending itself southwards as the large rivers deposit their water-borne sediments in the Thai Gulf. Land is growing seaward at an estimated rate of 15 to 2.0 feet a year.
Sedimentation has evidently been going on here for a long time. About a mile out from the present-day shoreline there is a flat sand bar composed of a series of sedimentary reefs, which have been built up by the surf of the southwest (summer) monsoon, an on-shore wind. The material is brought down by the large rivers, the currents of which increase, especially during the ebbing tide. The surf has built successive offshore bars, each with a sheltering lagoon on its shoreward side. Fresh water mixes with the salt water of the gulf, and the finer materials settle out in the lagoon, gradually filling the depression behind the bar. Long sandy ridges and intervening lower strips parallel to the coast have developed south of Chomburi and on the Peninsula East Coast north of Songkhla and east of Chumphon. On the Bangkok Plain itself these parallel ridges and lagoons have been covered by river sediment spread out over the surface, but they have been exposed in excavations.
Inland the plain rises very gradually. The roads of Bangkok (20 miles from the coast) are about six feet above the mean sea level of the Thai Gulf; the level of the plain at Ayutthaya (60 miles from the coast) is approximately 13 feet above sea level; and at Paknampho (150 miles from the coast) it is 77 feet above sea level. It is this very gradual slope of 1:10,000 which allows flooding over the entire plain during high water in the rivers and streams. As far upstream as Ayutthaya the levels of the rivers and their tributaries also rise and fall with the tide.
Borings have shown that the alluvium is made up of alternating silts, sands, and clays. At 400 feet bedrock still is not reached. This plain is purely one of aggradation and has been built up in an apparently synclinal depression extending from the north to the south and onwards into the Thai Gulf.
Around the edges of the plain are low hills and a little farther back are steep mountains. To the east of the plain near Kaeng Khoi the first limestone mountains tower above the foothills along the railway to Khorat. About Phra Phutthabat and Lopburi are extensions of these limestones. To the west of the plain, not far beyond the Nakhon Chaisi River, rise low hills separated from one another by wide erosion plains of young alluvium.
The numerous rivers and small streams are continually aggrading the plain with a deposit of silt from yearly flood waters. At least once in a year, most often in October and November, if the rains continue beyond the heavier rains of September, almost all the Bangkok Plain is under water, forming an extensive fresh water lake out of which rise the tall trunks of sugar palms which in some places are scattered through the fields. Rural settlements, with their homes standing on posts surrounded by coconut and areca palms or by bunches of thorny bamboo, appear as islands in a vast inland sea.