Geography

Cambodia is a country in Southeastern Asia, bordering the Gulf of Thailand, between Thailand, Vietnam, and Laos. Its approximate geographical coordinates are 13° N 105° E. Its 2,572 km border is split among Vietnam (1,228 km) in the east and southeast, Thailand (803 km) in the north and north west and Laos (541 km) in the northeast, as well as 443 km of coastline to the west along the Gulf of Thailand.

Cambodia covers 181,040 square kilometres in the southwestern part of the Indochina peninsula. It lies completely within the tropics; its southernmost points are only slightly more than 10° above the equator. Roughly square in shape, much of the country's area consists of rolling plains. Dominant features are the large, almost centrally located, Tonle Sap (Great Lake) and the Mekong River, which traverses the country from north to south.

The most distinctive geographical feature is the lacustrine plain, formed by the inundations of the Tonle Sap (Great Lake), measuring about 2,590 square kilometres during the dry season and expanding to about 24,605 square kilometres during the rainy season. This densely populated plain, which is devoted to wet rice cultivation, is the heartland of Cambodia. Most (about 75%) of the country lies at elevations of less than 100 metres (330 ft) above sea level, the exceptions being the Cardamom Mountains (highest elevation 1,813 m) and their southeast extension the Dâmrei Mountains ('Elephant Mountains') (elevation range 500-1,000 m), as well the steep escarpment of the Dângrêk Mountains (average elevation 500) along the border with Thailand's Isan region. The highest elevation of Cambodia is Phnom Aoral, near Pursat in the centre of the country, at 1,813 metres.

Topography

Cambodia falls within several well-defined geographic regions. The largest part of the country - about 75 percent of the total - consists of the Tonle Sap Basin and the Mekong Lowlands. To the southeast of this great basin is the Mekong Delta, which extends through Vietnam to the South China Sea. The basin and delta regions are rimmed with mountain ranges to the southwest (the Cardamom Mountains the Elephant Range) and to the north (Dangrek Mountains). Higher land to the northeast and to the east merges into the Central Highlands of southern Vietnam.

Tonle Sap Basin-Mekong Lowlands

The Tonle Sap Basin-Mekong Lowlands region consists chiefly of plains with elevations generally of less than 100 metres. As the elevation increases, the terrain becomes more rolling and dissected.

The Cardamom Mountains

The Cardamom Mountains in the southwest, oriented generally in a northwest-southeast direction, rise to more than 1,500 metres. The highest mountain in Cambodia - Phnom Aural, at 1,771 metres - is in the eastern part of this range. The Elephant Range, an extension running toward the south and the southeast from the Cardamom Mountains, rises to elevations of between 500 and 1,000 metres. These two ranges are bordered on the west by a narrow coastal plain that contains Kampong Saom Bay, which faces the Gulf of Thailand. This area was largely isolated until the opening of the port of Kampong Saom (formerly called Sihanoukville) and the construction of a road and railroad connecting Kampong Saom, Kampot, Takev, and Phnom Penh in the 1960s.

The Dangrek Mountains

The Dangrek Mountains at the northern rim of the Tonle Sap Basin consist of a steep escarpment with an average elevation of about 500 metres, the highest points of which reach more than 700 meters. The escarpment faces southward and is the southern edge of the Korat Plateau in Thailand. The watershed along the escarpment marks the boundary between Thailand and Cambodia. The main road through a pass in the Dangrek Mountains at O Smach connects northwestern Cambodia with Thailand. Despite this road and those running through a few other passes, in general the escarpment impedes easy communication between the two countries. Between the western part of the Dangrek and the northern part of the Cardamom ranges, however, lies an extension of the Tonle Sap Basin that merges into lowlands in Thailand, which allows easy access from the border to Bangkok.

The Mekong Valley

The Mekong Valley, which offers a communication route between Cambodia and Laos, separates the eastern end of the Dangrek Mountains and the northeastern highlands. To the southeast, the basin joins the Mekong Delta, which, extending into Vietnam, provides both water and land communications between the two countries.

Drainage

Except for the smaller rivers in the southeast, most of the major rivers and river systems in Cambodia drain into the Tonle Sap or into the Mekong River. The Cardamom Mountains and Elephant Range form a separate drainage divide. To the east the rivers flow into the Tonle Sap, while on the west they flow into the Gulf of Thailand. Toward the southern end of the Elephant Mountains, however, because of the topography, some small rivers flow southward on the eastern side of the divide.

The Mekong River in Cambodia flows southward from the Cambodia-Laos border to a point below Kracheh city, where it turns west for about 50 kilometres and then turns southwest to Phnom Penh. Extensive rapids run above Kracheh city. From Kampong Cham the gradient slopes very gently, and inundation of areas along the river occurs at flood stage - June through November - through breaks in the natural levees that have built up along its course. At Phnom Penh, four major water courses meet at a point called the Chattomukh (Four Faces). The Mekong River flows in from the northeast and the Tonle Sab - a river emanating from the Tonle Sap - flows in from the northwest. They divide into two parallel channels, the Mekong River proper and the Basak River, and flow independently through the delta areas of Cambodia and Vietnam to the South China Sea.

The flow of water into the Tonle Sab is seasonal. In September or October, the flow of the Mekong River, fed by monsoon rains, increases to a point where its outlets through the delta cannot handle the enormous volume of water. At this point, the water pushes northward up the Tonle Sab and empties into the Tonle Sap, thereby increasing the size of the lake from about 2,590 square kilometres to about 24,605 square kilometres at the height of the flooding. After the Mekong's waters crest - when its downstream channels can handle the volume of water - the flow reverses, and water flows out of the engorged lake.

As the level of the Tonle Sap retreats, it deposits a new layer of sediment. The annual flooding, combined with poor drainage immediately around the lake, transforms the surrounding area into marshlands unusable for agricultural purposes during the dry season. The sediment deposited into the lake during the Mekong's flood stage appears to be greater than the quantity carried away later by the Tonle Sab River. Gradual silting of the lake would seem to be occurring; during low-water level, it is only about 1.5 metres deep, while at flood stage it is between 10 and 15 metres deep.