Cambodia occupies 181,035 square kilometers in mainland Southeast Asia. The country shares a 2,530-kilometer combined land border with Thailand to the west and northwest, Laos to the north, and Vietnam to the east and southeast. Cambodia's southern boundary meets the Gulf of Thailand along a 443-kilometer coastline. The nation sits almost entirely within the lower Mekong Basin, creating a hydrologically unified territory where water shapes terrain, ecology, and settlement patterns in measurable ways.
The Central Plains dominate Cambodia's geography. This low-lying alluvial basin extends across roughly 75 percent of the country, with elevations ranging from 5 to 30 meters above sea level. The plains formed through millennia of sediment deposition by the Mekong River and its tributaries. Annual monsoon flooding spreads nutrient-rich silt across this expanse, creating conditions that historically supported dense rice cultivation. The flatness is extreme in places. Aerial surveys show horizontal variance of less than 2 meters across stretches extending 50 kilometers. This uniformity creates a landscape where river channels, not topography, define the character of different zones within the plains.
The Mekong River enters Cambodia from Laos near Stung Treng in the northeast. From entry to exit, the river traverses approximately 500 kilometers of Cambodian territory before crossing into Vietnam and continuing toward the South China Sea. The Mekong measures between 1 and 3 kilometers wide during the dry season in most Cambodian sections. During peak monsoon months, typically August through October, width can exceed 10 kilometers in certain flood-prone stretches. Flow volume varies dramatically between seasons. Dry season discharge at Kratie averages 2,000 cubic meters per second. Wet season discharge at the same measurement point reaches 38,000 cubic meters per second in years of heavy monsoon. This represents a nineteenfold increase, creating annual cycles of advance and retreat that govern agricultural patterns throughout the basin.
South of Phnom Penh, the Mekong splits into two major channels. The main stem continues as the Mekong proper. The western branch forms the Bassac River, which flows roughly parallel to the main channel before both enter Vietnam. This bifurcation creates a distinctive riverine landscape around the capital. Phnom Penh sits at the confluence where the Tonle Sap River meets the Mekong and Bassac, forming a four-armed junction that historically made the city a strategic inland port. The Bassac carries approximately 30 percent of combined flow during average conditions, though this ratio shifts with seasonal variations.
Tonle Sap Lake dominates the hydrology of central Cambodia. The lake sits in a tectonic depression west of the Mekong, connected to the main river by the 120-kilometer Tonle Sap River. During the dry season, from November through May, the lake covers approximately 2,500 square kilometers with an average depth of 1 meter. When monsoon rains swell the Mekong, backpressure reverses the flow of the Tonle Sap River. Water floods northwest into the lake rather than draining southeast into the Mekong. This reversal, one of the few large-scale examples globally, expands the lake to approximately 16,000 square kilometers with depths reaching 9 meters in the deepest zones. The expansion represents a sixfold increase in surface area and a forty-fold increase in volume. Tonle Sap becomes the largest freshwater lake in Southeast Asia during the flood season.
The annual flood pulse creates distinct ecological zones around Tonle Sap. Flooded forests fringe much of the lake perimeter. These forests spend half the year underwater and half the year on dry or damp ground. Trees have adapted to prolonged submersion. Barringtonia acutangula and certain Syzygium species grow in dense stands that remain rooted while water levels rise 7 to 8 meters around their trunks. Fish migrate into these forests during floods to spawn and feed on the abundance of insects, fruits, and organic matter. When water recedes, fish retreat to the shrinking lake. Fisheries dependent on this migration yield an estimated 300,000 to 500,000 tons annually, according to surveys conducted by the Mekong River Commission between 2000 and 2015. This harvest makes Tonle Sap one of the most productive inland fisheries in the world measured by tons per square kilometer.
Mountains rise along Cambodia's borders, framing the central plains. The Cardamom Mountains stretch across southwestern Cambodia, forming a rugged highland that separates the plains from the Gulf of Thailand coast. The range runs roughly 160 kilometers northwest to southeast, with widths varying from 40 to 100 kilometers. Phnom Aural, Cambodia's highest peak, rises to 1,813 meters within the Cardamoms. The mountains consist primarily of Mesozoic and Paleozoic sedimentary rocks overlain in places with granitic intrusions. Steep terrain and heavy rainfall create dense forest cover. The Cardamoms receive between 3,800 and 5,000 millimeters of annual precipitation on western slopes, making this one of the wettest zones in mainland Southeast Asia. Rivers draining the range carve deep valleys. The Tatai River, Koh Kong River, and others flow west and south toward the Gulf of Thailand through gorges that in places reach 300 meters deep.
The Elephant Mountains form a parallel but lower range south of the Cardamoms, running along the coast in Kampot and Kep provinces. Peaks in the Elephant range reach 500 to 1,000 meters. The Bokor Plateau, part of this system, sits at approximately 1,000 meters elevation and features cooler temperatures than surrounding lowlands. French colonial authorities established a hill station at Bokor in the 1920s to escape lowland heat. Average temperatures on the plateau remain 5 to 7 degrees Celsius below those at sea level year-round. The Elephant Mountains receive slightly less rainfall than the Cardamoms, averaging 2,500 to 3,500 millimeters annually. Rivers draining south from these highlands are shorter and steeper than those from the Cardamoms.
The Dângrêk Mountains form Cambodia's northern boundary with Thailand. This escarpment runs approximately 350 kilometers east to west along the border. The Dângrêk range differs geologically from the Cardamoms and Elephant Mountains. It consists of a tilted sandstone plateau that slopes gradually north into Thailand and drops abruptly south into Cambodia. The scarp face rises 200 to 400 meters above the Cambodian plains. Preah Vihear Temple sits atop this escarpment at approximately 525 meters elevation, commanding views across the plains that extend 100 kilometers south on clear days. The Dângrêk range receives 1,200 to 1,800 millimeters of rainfall annually, considerably less than the southwestern mountains. Deciduous dipterocarp forest dominates the ecology, with trees shedding leaves during the dry season in contrast to the evergreen forests of the wetter Cardamoms.
Cambodia's eastern provinces feature rolling hills and transitional forests. Mondulkiri and Ratanakiri provinces sit at higher elevations than the central plains, with terrain rising from 200 to 800 meters. These provinces represent the western edge of the Central Highlands that extend into Vietnam. Geology consists of basaltic plateaus in Ratanakiri and granitic hills in Mondulkiri. Soils are generally red laterite, less fertile than the alluvial soils of the floodplains but supporting different forest types. Ratanakiri holds volcanic crater lakes, including Yeak Laom, a nearly circular lake formed in an extinct volcanic crater. The lake measures 800 meters in diameter and reaches depths of 48 meters. Water in Yeak Laom is exceptionally clear because the closed basin limits sediment input and the depth prevents mixing that could cloud the water column.