SBR- Content.pmd - INBO

State of the Basin Report - 2003lowland areas. According to MRC estimates, 80 percent of the rural flooding and 20 percent of theurban flooding is caused by tributaries. 2 The four major flood prone areas in Lao PDR are situatedalong the mainstream near large tributaries: i) Vientiane Plain, ii) Khammoune Province (Thakhektown), iii) Savannakhet Province and iv) Champasak Province (Pakse town). 3In Thailand, the principal flood prone areas of the LMB in Thailand include low-lying areas alongthe Mekong and tributaries in Nong Khai, Mukdahan, Nakhon Phanom and Ubon Ratchataneeprovinces.In Cambodia, the Mekong enters thelow-lying delta and becomes a wide,slow-flowing river. The area is very flatand there is extensive lateral flow offlood waters from Kratie to the borderwith Viet Nam. Bank overflows alsooccur along the entire length of theTonle Sap River. Following heavyrainfalls, surface runoff often flowsparallel to the rivers, filling landdepressions lateral to and between thetwo rivers. Up to 4 million ha oflowland areas in Cambodia areinundated annually. 4During the wet season, the dischargecapacity of the Mekong and BassacThe surface area of the Great Lake expands from250,000-300,000 ha in the dry season to 1-1.4 million hain the wet season.Rivers south of Phnom Penh is inadequate to handle flood flows. This results in backflow up theTonle Sap River and into the Great Lake until the water levels in the Mekong and Great Lake areequal (usually in late September). The total natural storage capacity of the Great Lake and theTonle Sap is estimated to be 150 billion m 3 . During the flood season, the surface area of the lakeexpands from 250,000-300,000 ha (dry season area) to 1-1.4 million ha. 5In Viet Nam, flooding frequently inundates the entire floodplain of the delta – up to 1.8 million haIn Viet Nam, the Mekong River formsa highly fertile and productive delta.Flooding in the delta is influenced bytidal effects from the South China Sea,combined with high discharge from theMekong River and heavy rain in thedelta itself. Flooding frequentlyinundates the entire flood plain in theVietnamese delta, that is, up to 1.8million ha. 6 In the Central Highlandsof Viet Nam, steep mountain slopes andinsufficient storage capacity for rainsduring the wet season often result inflash flooding.270