The Chao Phraya is the major river in Thailand, with its low alluvial plain forming the centre of the country. It flows through Bangkok and then into the Gulf of Thailand.
The Chao Phraya begins at the confluence of the Ping and Nan rivers at Nakhon Sawan (also called Pak Nam Pho) in Nakhon Sawan Province. After this it flows south for 372 kilometres (231 mi) from the central plains to Bangkok and the Gulf of Thailand. In Chai Nat, the river then splits into the main course and the Tha Chin River, which then flows parallel to the main river and exits in the Gulf of Thailand about 35 kilometres (22 mi) west of Bangkok in Samut Sakhon. In the low alluvial plain which begins below the Chainat Dam, there are many small canals (khlong) which split off from the main river. The khlongs are used for the irrigation of the region's rice paddies.
The rough coordinates of the river are 13 N, 100 E. This area has a wet monsoon climate, with over 1,400 millimetres (55 in) of rainfall per year. Temperatures range from 24 to 33 °C (75 to 91 °F) in Bangkok.
Cities along the Chao Phraya include, from north to south, Nakhon Sawan Province, Uthai Thani Province, Chai Nat Province, Sing Buri Province, Ang Thong Province, Ayutthaya Province, Pathum Thani Province, Nonthaburi Province, Bangkok, and Samut Prakan Province. These cities are among the most historically significant and densely populated settlements of Thailand due to their access to the waterway.
The lowland areas of the Chao Phraya watershed in central Thailand have been designated as the Chao Phraya freshwater swamp forests, a tropical and subtropical moist broadleaf forests ecoregion, an area about 400 km (249 mi) north to south and 180 km (112 mi) wide.