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 km². 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 distributary)
*Chao Phraya Basin (the land drained by the Chao Phraya River itself, and not by its major tributaries)

The mountainous natural boundary of the watershed forms a divide, which has, to some degree, historically isolated Thailand from other Southeast Asian civilizations. In fact, in northern Thailand, the divide roughly corresponds to a long section of the political border of present-day Thailand. 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 civilization to extend beoynd the watershed without issue.