SBR- Content.pmd - INBO

State of the Basin Report - 20032. Northern HighlandsThe Northern Highlands include the regionfrom southern Yunnan through Myanmar, LaoPDR and northern Thailand, eastwards intothe northern end of the Annamite Cordillerain Viet Nam. Elevations reach 2,800 m andtopography is generally steep. Geology iscomplex with extensive areas of Permianlimestone, which form the spectacular karsttopography seen across Lao PDR and northernand central Viet Nam, but also with granites,sandstones and gneisses, as well as somevolcanic basaltic overlays.3. Korat PlateauSpectacular limestone hills are seen across Lao PDRand in northern and central Viet NamThe Korat Plateau is an elevated area of triassic sandstone sloping gently to the east, with scatteredlava flows distributed mainly along the southern boundary. The Mekong River cuts deeply into theeastern rim of the plateau, forming sheer cliffs above the river in places and underwater canyons upto 100 m deep in others. Most of the plateau lies within Thailand and has been extensively clearedfor agriculture and is now relatively densely populated. Parts of the plateau are underlain by salineevaporates which are mined for salt. During the dry season, saline water seeps through the topsoilresulting in soil salinity, which may be exacerbated by irrigation. The climate of the Korat Plateauis relatively dry (see Chapter 3), so although tributaries like the Mun and Chi Rivers, which drainthis region, have large catchment areas, they contribute a relatively small proportion of the riverflow. The area has a wide variety of soils, but mostly of the solonetz type, with subsurface clayaccumulations that are high in sodium and generally of low fertility.4. Eastern HighlandsThe Eastern Highlands form a southern extension of the Northern Highlands. The mountains extendabout 700 km from Lao PDR through Viet Nam, reaching altitudes of up to 2,800 m. Granites in theregion of the Kontum Massif are thought to be some of the earliest in Southeast Asia, dating to thePrecambrian, 2.3 billion years ago. A number of the larger tributaries arise in this part of the basin,including the Se Kong, Se San and Sre Pok Rivers.5. LowlandsThe lowlands include two subcomponents: the Cambodian floodplains and the delta. Both areformed from the deposition of recent sediments over older bedrock. The area is essentially flatwith occasional older rock outcrops projecting above the floodplain. The floodplain shows evidenceof numerous recent changes in river course, and, because of the flat terrain, large areas aresubmerged during the high flow period.The Tonle Sap Great Lake is the largest body of freshwater in Southeast Asia and forms one of thekey features of the lowlands. During the flood season, water flows from the Mekong mainstreamnorthwest to contribute most of the water that fills the Great Lake. As the river level falls in the dry10