Cambodia is a country in mainland Southeast Asia. It borders Thailand, Laos, Vietnam, the Gulf of Thailand and covers a total area of approximately . The country is situated in its entirety inside the tropical Indomalayan realm and the Indochina Time zone (ICT).
Cambodia's main geographical features are the low lying Central Plain that includes the Tonlé Sap basin, the lower Mekong River flood-plains and the Bassac River plain surrounded by mountain ranges to the north, east, in the south-west and south. The central lowlands extend into Vietnam to the south-east. The south and south-west of the country constitute a long coast at the Gulf of Thailand, characterized by sizable mangrove marshes, peninsulas, sandy beaches and headlands and bays. Cambodia's territorial waters account for over 50 islands. The highest peak is Phnom Aural, sitting above sea level.
The landmass is bisected by the Mekong River, which at is the longest river in Cambodia. After extensive rapids, turbulent sections and cataracts in Laos, the river enters the country at Stung Treng province, is predominantly calm and navigable during the entire year as it widens considerably in the lowlands. The Mekong's waters disperse into the surrounding wetlands of central Cambodia and strongly affect the seasonal nature of the Tonlé Sap lake.
Two third of the country's population live in the lowlands, where the rich sediment deposited during the Mekong's annual flooding makes the agricultural lands highly fertile. As deforestation and over-exploitation affected Cambodia only in recent decades, forests, low mountain ranges and local eco-regions still retain much of their natural potential and although still home to the largest areas of contiguous and intact forests in mainland Southeast Asia, multiple serious environmental issues persist and accumulate, which are closely related to rapid population growth, uncontrolled globalization and inconsequential administration.
The majority of the country lies within the Tropical savanna climate zone, as the coastal areas in the South and West receive noticeably more and steady rain before and during the wet season. These areas constitute the easternmost fringes of the south-west monsoon, determined to be inside the Tropical monsoon climate. Countrywide there are two seasons of relatively equal length, defined by varying precipitation as temperatures and humidity are generally high and steady throughout the entire year.
Geological development
Mainland Southeast Asia consists of allochthonous continental blocks from Gondwanaland. These include the South China, Indochina, Sibumasu, and West Burma blocks, which amalgamated to form the Southeast Asian continent during the Paleozoic and Mesozoic periods.
The current geological structure of South China and South-East Asia is determined to be the response to the "Indo-sinian" collision in South-East Asia during the Carboniferous. The Indo-Sinian orogeny was followed by extension of the Indo-Chinese block, the formation of rift basins and thermal subsidence during the early Triassic.
The Indochina continental block, which is separated from the South China Block by the Jinshajiang-Ailaoshan Suture zone, is an amalgamation of the Viet-Lao, Khorat-Kontum, Uttaradit (UTD), and Chiang Mai-West Kachin terranes, all of which are separated by suture zones or ductile shear zones. The Khorat-Kontum terrane, which includes western Laos, Cambodia and southern Vietnam, consists of the Kontum metamorphic complex, Paleozoic shallow marine deposits, upper Permian arc volcanic rocks and Mesozoic terrigenous sedimentary rocks.
The central plains consist mainly of Quaternary sands, loam and clay, as most of the northern mountain regions and the coastal region are largely composed of Cretaceous granite, Triassic stones and Jurassic sandstone formations.
General topography
Bowl- or saucer-shaped, Cambodia covers in the south-western part of the Indochinese peninsula as its landmass and marine territory is situated entirely within the tropics.
The bowl's bottom represents Cambodia's interior, about 75 percent, consisting of alluvial flood-plains of the Tonlé Sap basin, the lower Mekong River and the Bassac River plain, whose waters feed the large and almost centrally located wetlands. As humans preferably settle in these fertile and easily accessible central lowlands, major transformations and widespread cultivation through wet-rice agriculture have over the centuries shaped the landscape into distinctive regional cultivated lands.
Domestic plants, such as sugar palms, Coconut trees and banana groves almost exclusively skirt extensive rice paddies, as natural vegetation is confined to elevated lands and near waterways. The Mekong traverses the north to south-east portions of the country, where the low-lying plains extend into Vietnam and reach the South China Sea at the Mekong Delta region.
Cambodia's low mountain ranges - representing the walls of the bowl - remain as the result of only rather recent substantial infrastructural development and economic exploitation - in particular in remote areas - formidably forested. The country is fringed to the north by the Dangrek Mountains plateau, bordering Thailand and Laos, to the north-east by the Annamite Range, in the south-west by the Cardamom Mountains and in the South by the Elephant Mountains. Highlands to the north-east and to the east merge into the Central Highlands and the Mekong Delta lowlands of Vietnam.
A heavily indented coastline at the Gulf of Thailand of length and 60 offshore islands, that dot the territorial waters and locally merge with tidal mangrove marshes - the environmental basis for a range of marine and coastal eco-regions.
Soils
"Sandy materials cover a large proportion of the landscape of Cambodia, on account of the siliceous sedimentary formations that underlie much of the Kingdom. Mesozoic sandstone dominates most of the basement geology in Cambodia and hence has a dominating influence on the properties of upland soils. Arenosols (sandy soils featuring very weak or no soil development) are mapped on only 1.6% of the land area."
"Sandy surface textures are more prevalent than the deep sandy soils that fit the definition for Arenosols. Sandy textured profiles are common amongst the most prevalent soil groups, including Acrisols and Leptosols. The Acrisols are the most prevalent soil group occupying the lowlands - nearly half of the land area of Cambodia. Low fertility and toxic amounts of aluminium pose limitations to its agricultural use, crops that can be successfully cultivated include rubber tree, oil palm, coffee and sugar cane. The main subgroups are: Gleyic Acrisols (20.5%, Haplic Acrisols (13.3%), Plinthic Acrisol (8.7%) and Ferric Acrisol (6.3%)."
Geographical extremes Northernmost point: Ta Veaeng District, Rattanakiri Province () Southernmost point: Koh Poulo Wai, Kampot Province () Easternmost point: Ou Ya Dav District, Rattanakiri Province () Westernmost point: Malai District, Banteay Meanchey Province ()
Regions
Central plain
The vast alluvial and lacustrine interconnected Cambodian flood-plain is a geologically relatively recent depression where the sediments of the Mekong and its tributaries accumulate as waters are subject to frequent course changes. The area covers . The Tonlé Sap lake and - river system occupies the lowest area. The Tonlé Sap River is a waterway that branches off the Mekong near Phnom Penh in the north-westerly direction and meets the Tonle Sap lake after around . Its waters' flow reverses direction every year, caused by greatly varying amounts of water carried by the Mekong over the course of a year and the impact of monsoonal rains, that coincides with the river's maximum.
The plains of the Mekong and Tonle Sap basin are confined in the North by the Dangrek and Central Annamite Mountains, and to the South by the Cardamom Mountains and Elephant Mountains. The plains completely surround the Tonle Sap Lake in the western half of the country and wind their way through the middle of the country following the course of the Mekong River. The two basins actually form a single body of water, the whole of which effects about 75% of Cambodia’s land cover.
Flow reversal
The Mekong river and its tributaries increase water volumes in spring (May) on the northern hemisphere, mainly caused by melting snows. As the Mekong enters Cambodia (over 95% of its waters have already joined the river) it widens and inundates large areas.
The plain's deepest point - the Tonle Sap - flooded area varies from a low of around with a depth of around 1 meter at the end of the dry season (April) to and a depth of up to 9 meters in October/November. This figure rose to during 2000 when some of the worst flood conditions recorded caused over 800 deaths in Cambodia and Vietnam.
Inflow starts in May/June with maximum rates of flow of around 10,000 m3/s by late August and ends in October/November, amplified by precipitation of the annual monsoon. In November the lake reaches its maximum size. The annual monsoon coincides to cease around this time of the year. As the Mekong river begins its minimum around this time of the year and its water level falls deeper than the inundated Tonle Sap lake, Tonle Sap river and surrounding wetlands, waters of the lake's basin now drains via the Tonle Sap river into the Mekong.
As a result the Tonle Sap River (length around ) flows 6 months a year from South-East (Mekong) to North-West (lake) and 6 month a year in the opposite direction. The mean annual reverse flow volume in the Tonle Sap is , or about half of the maximum lake volume. A further 10% is estimated to enter the system by overland flow from the Mekong. The Mekong branches off into several arms near Phnom Penh and reaches Vietnamese territory south of Koh Thom and Loek Daek districts of Kandal Province.
Southern Mountains
This region represents the eastern parts of the original extent of the wet evergreen forests that cover the Cardamom - and Elephant Mountains in South-West Cambodia and along the mountains east of Bangkok in Thailand.
The densely wooded hills receive rainfall of annually on their western slopes (which are subject to the South-West monsoons) but only on their eastern - rain shadow - slopes.
The Cardamom/Krâvanh Mountains
Occupying Koh Kong Province and Kampong Speu Province, running in a north-western to south-eastern direction and rising to more than . The highest mountain of Cambodia, Phnom Aural, at is located in Aoral District in Kampong Speu Province. The Cardamom Mountains form - including the north-western part of Chanthaburi Province, Thailand, the 'Soi Dao Mountains' - the Cardamom Mountains Moist Forests Ecoregion, that is considered to be one of the most species-rich and intact natural habitats in the region. The climate, size inaccessibility and seclusion of the mountains have allowed a rich variety of wildlife to thrive. The Cardamom and Elephant Mountains remain to be fully researched and documented.
The Elephant Mountains
Chuŏr Phnum Dâmrei - A north-south-trending range of high hills, an extension of the Cardamom/Krâvanh Mountains, in south-eastern Cambodia, rising to elevations of between 500 and 1,000 meters. Extending north from the Gulf of Thailand, they reach a high point in the Bok Koŭ ridge at Mount Bokor near the sea.
To the south-west of the Southern mountain ranges extends a narrow coastal plain that contains the Kampong Saom Bay area and the Sihanoukville peninsula, facing the Gulf of Thailand.
Northern Mountains
The Dangrek Mountains