Thailand may be divided into five major physical regions: the central valley, the continental highlands of the north and northwest, the northeast, the southeast coast, and the peninsula. The heartland of the nation is the central valley, fronting the Gulf of Thailand and enclosed on three sides by hills and mountains. This valley, the alluvial plain of the Chao Phraya River and of its many tributaries and distributaries, is 365 km (227 mi) from north to south and has an average width of 160–240 km (100– 150 mi). On this plain, and most especially on its flat deltaland bordering the Gulf, are found Thailand's main agricultural wealth and population centers.
The continental highlands lie north and west of the central valley. They include North Thailand, surrounded on three sides by Myanmar (Burma until June 1989) and Laos, which is a region of roughly parallel mountain ranges between which the Nan, Yom, Wang, Ping, and other rivers flow southward to join and create the Chao Phraya in the central valley. In the northernmost tip, drainage is northward to the Mekong River; on the western side, drainage runs westward to the Salween in Myanmar. Most of the people of North Thailand live in small intermontane plains and basins that are generally widenings in the major river valleys. Doi Inthanon (2,576 m/8,451 ft) is the highest point in Thailand. Along the Myanmar border from North Thailand to the peninsula is a sparsely inhabited strip of rugged mountains, deep canyons, and restricted valleys. One of the few natural gaps through this wild mountain country is Three Pagodas Pass along the Thailand-Myanmar boundary, used by the Japanese during World War II for their "death railway" (now dismantled) between Thailand and Myanmar.
The northeast, much of it often called the Khorat, is a low, undulating platform roughly 120 to 210 m (400–700 ft) above sea level in the north and west, gradually declining to about 60 m (200 ft) in the southeast. Hill and mountain ranges and scarps separate the northeast from the central valley on the west and from Cambodia on the south; its northern and much of its eastern boundaries are marked by the Mekong River. Most of the northeast is drained by the Mun River and its major tributary, the Chi, which flow eastward into the Mekong. The northeast, in the rain shadow of the Indochina Cordillera, suffers from shortage of water and from generally thin and poor soils.
The small southeast coast region faces the Gulf of Thailand and is separated from the central valley and Cambodia by hills and mountains that rise in places to over 1,500 m (5,000 ft). This is a well-watered area, and the vegetation is, for the most part, lush and tropical. Most of the people live along the narrow coastal plain and the restricted river valleys that drain southward to the Gulf.
Peninsular Thailand extends almost 960 km (600 mi) from the central valley in the north to the boundary of Malaysia in the south and is anywhere from 16 to 217 km (10–135 mi) wide between the Gulf of Thailand on the east and the Andaman Sea (Indian Ocean) and Myanmar on the west. At the Isthmus of Kra, the Peninsula itself is only 24 km (15 mi) wide. A series of north-south ridges, roughly parallel, divide the Peninsula into distinct west and east coast sections. The west coastal plain is narrow— nonexistent in many places—and the coast itself is much indented and often very swampy. The east coastal plain is much wider, up to 32 km (20 mi) in sections, and the coast is smooth, with long beach stretches and few bays. Well-watered (especially the west coast), hot, and densely forested, the Peninsula, unlike most of Thailand, lies within the humid tropical forest zone.