Mun River, main river system of the Khorat Plateau, in eastern Thailand. The Mun rises in the San Kamphaeng Range northeast of Bangkok and flows east for 418 miles (673 km), receiving the Chi River, its main tributary, and entering the Mekong River at the Laotian border. Nakhon Ratchasima and Ubon Ratchathani are the chief towns on its banks. The river is seasonally navigable below Ubon Ratchathani and is used for irrigation.