The most conspicuous features of Thailand's terrain are high mountains, a central plain, and an upland plateau (see fig. 7). Mountains cover much of northern Thailand and extend along the Burmese border down through the Malay Peninsula. The central plain is a lowland area drained by the Chao Phraya and its tributaries, the country's principal river system, which feeds into the delta at the head of the Bight of Bangkok. The Chao Phraya system drains about one-third of the nation's territory. In the northeastern part of the country the Khorat Plateau, a region of gently rolling low hills and shallow lakes, drains into the Mekong River through the Mae Nam Mun. The Mekong system empties into the South China Sea and includes a series of canals and dams.

Together, the Chao Phraya and Mekong systems sustain Thailand's agricultural economy by supporting wet-rice cultivation and providing waterways for the transport of goods and people. In contrast, the distinguishing natural features of peninsular Thailand are long coastlines, offshore islands, and diminishing mangrove swamps.