Chao Phraya River - Chao Phraya Watershed

Chao Phraya Watershed

The expanse of the Chao Phraya River and its tributaries, i.e. the Chao Phraya river system, together with the land upon which falling rain drains into these bodies of water, form the Chao Phraya watershed. The Chao Phraya watershed is the largest watershed in Thailand, covering approximately 35% of the nation's land, and draining an area of 157,924 square kilometres (60,975 sq mi). The watershed is divided into the following basins:

• Pa Sak Basin
• Sakae Krang Basin
• Greater Nan Basin (comprising the Nan Basin and the Yom Basin, and usually divided as such in drainage analyses)
• Greater Ping Basin (comprising the Ping Basin and the Wang Basin, and usually divided as such in drainage analyses)
• Tha Chin Basin (the basin of the Chao Phraya's most significant distributaries)
• Finally the Chao Phraya Basin itself is defined as the portion of the Chao Phraya Watershed drained by the Chao Phraya River itself, and not by its major tributaries or distributaries. As such, the Chao Phraya Basin drains 20,126 square kilometres (7,771 sq mi) of land.

Meanwhile to the west the central plain of Thailand is drained by the Mae Klong and the east by the Bang Pakong Rivers which are not part of the Chao Praya system.

The landscape of the river basins is a very wide, flat, well-watered plain continuously refreshed with soil and sediment brought down by the rivers. The Lower Central plain from the delta north to Ang Thong Province is a flat, low area with an average of 2m above sea level. Further north and into the plains of the Ping and the Nan the elevation is over 20m. Then the mountains that are the natural boundary of the Chao Praya watershed form a divide, which has, to some degree, historically isolated Thailand from other Southeast Asian civilisations.

In fact in northern Thailand the divide roughly corresponds to a long section of the political border of the country today. Southern portions of the divide's boundary correspond less to the nation's political border, because isolation in this area was prevented by the ease of transportation along the lowlands surrounding the Gulf of Thailand, allowing a unified Thai civilisation to extend beyond the watershed without issue. The slightly higher northern plains have been farmed for centuries and saw a major change from the 13th century onwards during the Sukhothai Kingdom in the 13th and 14th centuries and the Ayutthaya Kingdom that succeeded it when rice-growing intensified with the introduction of floating rice, a much faster-growing strain of rice from Bengal. The southern swamps meanwhile changed radically from the 18th century when King Buddha Yodfa Chulaloke moved the capital of Siam to Bangkok, and a process of canalisation and cultivation began, especially as Thailand began to export rice from 1855 onwards.