Drainage and erosion of Cambodia's great lake in the middle-late Holocene: The combined role of climatic drying, base-level fall and river capture. (15th May 2020) - Darby, Stephen E.
Abstract: We provide evidence for a large-scale geomorphic event in Cambodia's great lake, the Tonlé Sap, during the middle Holocene. The present-day hydrology of the basin is dominated by an annual flood pulse where water from the Mekong River raises the lake level by c. 8 m during the monsoon season. We present new subsurface geophysical data, allied to new and past core studies, which unequivocally show a period of major mid-Holocene erosion across the entire Tonlé Sap basin that is coincident with establishment of the lake's flood pulse. We argue that this widespread erosion, which removed at least 1.2 m of sediment across the lake's extent, was triggered by up to three, likely interacting, processes: (1) base-level lowering due to mid-Holocene sea-level fall, leading to (2) capture of the Tonlé Sap drainage by the Mekong River, and (3) a drying climate that also reduced lake level. Longer-term landscape evolution was thus punctuated by a rapid, river capture- and base-level fall- induced, lake drainage that established the ecosystem that flourishes today. The scale of change induced by this mid-Holocene river capture event demonstrates the susceptibility of the Tonlé Sap lake to ongoing changes in local base-level and hydrology induced by anthropogenic activity, such as damming and sand mining, within the Mekong River Basin. Highlights: New subsurface geophysical data, with new and past core studies, show major mid-Holocene erosion across the Tonlé Sap basin. This erosion coincides with the onset of the lake's flood pulse and removed 1.2 m of sediment across the lake's extent. Erosion was induced by: (1) sea-level fall, leading to (2) capture of the Tonlé Sap by the Mekong, and (3) a drying climate.j