| Cambodia has a land area of 181,035 square kilometers
in the southwestern part of the Indochina peninsula, about 20 percent
of which is used for agriculture. It lies completely within the tropics
with its southern most points slightly more than 10? above the Equator.
The country capital city is Phnom Penh. International borders are shared
with Thailand and the Lao People's Democratic Republic on the West and
the North, and the Social Republic of Viet Nam on the East and the Southeast.
The country is bounded on the Southeast by the Gulf of Thailand. In
comparison with neighbors, Cambodia is a geographical
contact country administratively composed of 20 provinces, three of
which have relatively short maritime boundaries, 2 municipalities, 172
districts, and 1,547 communes. The country has a coastline of 435 km
and extensive mangrove stands, some of which are relatively undisturbed.
The dominant features of the Cambodian landscape are the large, almost
generally located, Tonle Sap (Great Lake) and the Bassac River Systems
and the Mekong River, which crosses the country from North to South.
Surrounding the Central Plains which covered three quarters of the country's
area are the more densely forested and sparsely populated highlands,
comprising: the Elephant Mountains and Cardamom Mountain of the southwest
and western regions; the Dangrek Mountains of the North adjoining of
the Korat Plateau of Thailand; and Rattanakiri Plateau and Chhlong highlands
on the east merging with the Central Highlands of Viet Nam.
The Tonle Sap Basin-Mekong Lowlands region consists mainly of plains
with elevations generally of less than 100 meters. As the elevation
increases, the terrain becomes more rolling and dissected. The Cardamom
Mountains in the southwest rise to more than 1,500 meters and is oriented
generally in a northwest-southeast direction. The highest mountain in
Cambodia -Phnom Aural, at 1.771meters - is in the eastern
part of this range.
The Elephant Range, an extension of Cardamom Mountains, runs towards
the south and the southeast and rises to elevations of between 500 and
1,000 meters. These two range are bordered on the west are narrow coastal
plain facing the gulf of Thailand that contains Kampong Som Bay. The
Dangrek Mountains at the northern rim of Tonle Sap Basin, consisting
of a steep escarpment on the southern edge of the Korat Plateau in Thailand,
marks the boundary between Thailand and Cambodia. The
average elevation of about 500 meters with the highest points reaching
more than 700 meters. Between the northern part of the Cardamom ranges
and the western part of the Dangrek, lies and extension of the Tonle
Sap Basin that merges into the plains in Thailand, allowing easy accesses
from the border of Bangkok.
The Mekong River Cambodia's largest river, dominates
the hydrology of the country. The river originates in mainland China,
flows through Myanmar, Laos, Thailand before entering Cambodia.
At Phnom Penh, with alternative arms, the Bassak River from the south,
and the Tonle Sap River linking with the " Great Lake " itself
-Tonle Sap - form northwest. It continues further southeastward to its
lower delta in Viet Nam and to the South China Sea.
The section of Mekong River passing through Cambodia
lies within the topical wet and dry zone. It has a pronounced dry season
during the Northern Hemisphere winter, with about 80 percent of the
annual rainfall occurring during the southwest monsoon in May-October.
The Mekong River average annual flow at Krati? of 441 km3 is estimated
as 93 percent of the total Mekong run-off discharge into the sea. The
discharge at Krati? ranges from a minimum of 1,250m3/s to the maximum
66,700m3/s.
The role of Tonle Sap as a buffer of the Mekong River system floods
and the source of beneficial dry season flows warrants explanation.
The Mekong River swells with waters during the monsoon reaching a flood
discharge of 40,000m3/s at Phnom Penh. By about mid-June, the flow of
Mekong and the Bassak River fed by monsoon rains increases to a point
where its outlets through the delta cannot handle the enormous volume
of water, flooding extensive adjacent floodplains for 4-7 months. At
this point, instead of overflowing its backs, its floodwaters reserve
the flow of the Tonle Sap River (about 120 km in length), which then
has the maximum inflow rate of 1.8m/s and enters the Grate Lake, the
largest natural lake in Southeast Asia, increasing the size of the lake
from about 2,600 km2 to 10,00 km2 and exceptionally to 13,000 km2 and
raising the water level by and average 7m at the height of the flooding.
This specificity of the Tonle Sap makes it the only "river with
return " in the world.
After the Mekong's water crest (when its downstream channels can handle
the volume of water), the flow reverses and water flows out of the engorged
lake. The Great Lake then acts as a natural flood retention basin. When
the floods subside, water starts flowing out of the Great Lake, reaching
a maximum outflow rate of 2.0m/s and, over the dry season, increase
mainstream flows by about 16 percent, thus helping to reduce salinity
intrusion in the lower Mekong Delta in Viet Nam. By the time the lake
water level drops to its minimum surface size, a band 20-30 km wide
of inundate forest is left dry with deposits of a new layer of sediment.
This forest, which is of great significance for fish, is now greatly
reduced in size through salvation and deforestation. The area flood
around Phnom Penh and down to the Vietnamese border is about 7,000 km2.
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