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: