THE LOWER MEKONG RIVER PLAIN IN CAMBODIA: LANDFORMS AND DEPOSITS

The Lower Mekong River Plain contains Cambodian Plain and Mekong Delta in Vietnam. The Great Lake or Lake Tonle Sap occurs in Cambodian Plain. The lake is connected with the Mekong via the Tonle Sap River. The Tonle Sap River meets with the Mekong at Phnom Penh, and the Bassac diverts from the Mekong there. Based on the interpretation of aerial photos and satellite images with field reconnaissance, geomorphologic features of the plain are summarized as follows: 1) surrounding uplands, 2) gentle fan in the western area, 3) natural levees and back marshes along the Mekong, 4) relatively flat lowland along the Tonle Sap river, and 5) human-induced feature of Colmatage system developing along the Bassac river. These features are closely related with flood characters. Geomorphologic features of this area were confirmed to cause different flood characters during the major floods in 2000, 2001 and 2002: no inundation occurred in 1) uplands and 2) a gentle fan, 3) almost all back marshes were inundated along the Mekong while natural levees were not, 4) flood water covered along the Tonle Sap river, and 5) relatively light flood damages along the Bassac. Subsurface deposits of the plain are also examined with bore-hole logs and augering. More than 100-m thick Quaternary deposits occur in the central part, while less than 20 m in Phnom Penh city reflecting bedrock topography.