The
Mekong River Cambodia’s largest river, dominates the hydrology of
the country. The river originates in mainland China, flows through
Myanmar, Laos and 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 " also called Tonle
Sap
– It continues 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 swells with waters during the monsoon reaching a flood
discharge of 40,000m3/s at Phnom Penh
"River
by Return" The Tonle Sap the
only "river with return "
In
June, the flow of Mekong and the Bassak River fed by the monsoons
cannot handle the enormous volume of water, flooding extensive
floodplains for 4-7 months.
At
this time there is a backflow with these floodwaters going into
the Tonle Sap River (about 120 km in length), which then enters the
Great Lake, the largest natural lake in Southeast Asia, increasing
the size of the lake from about 2,600 square kilometers
to
10,000 square kilometers.
Raising the water level by an average 7m at the height of the
flooding.
The Tonle Sap is the
only "river with return "
(backflow) in the world. - Not a tide. After
the Mekong can once again
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 increasing 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 flood plains around Phnom Penh and down to
the Vietnamese border cover a massive. 7,000 square