Chao Phraya River, the principal river in Thailand. It is formed by the junction of the Ping, Yom, and Nan rivers in central Thailand and flows southward for 150 miles (240 km) to the Gulf of Thailand. The Chao Phraya is navigable by river craft for most of its length. Oceangoing vessels travel upstream as far as Bangkok, the Thai capital, 15 miles (24 km) from the mouth. The Chao Phraya valley, which floods during the annual rainy season, is one of the great rice-producing areas of Asia and the most populated part of Thailand.