Geomorphology and channel network patterns of the Mekong River in Cambodia

Abstract

Published, well-described examples of multiple mixed bedrock-alluvial character systems largely pertain to small to medium systems. No descriptions exist for large river systems such as the Mekong River in Cambodia (South East Asia). The published literature concerning the 120km long and up to 5km wide multichannel study reach are outdated and do not provide sufficient levels of detail. Therefore the first part of the thesis offers an amplified portrait of the modern and palaeo-Mekong based upon limited published literature, updated by ground survey data and complemented by analyses of remote sensing data. The second part of the thesis is devoted to applying quantitative channel network characteristics in order to describe the Mekong River but importantly to separate mixed bedrock-alluvial from alluvial multichannel rivers. The key question in this respect is whether the planform characteristics of the river networks reflect whether they are within alluvial settings or are influenced by bedrock controls. The channel metrics comparison is accompanied by a specially designed methodology based upon standard GIS tools so that the results obtained in this study could be later incorporated into subsequent channel network metrics research using a larger dataset for these or other rivers. Geomorphological findings reveal that the multichannel pattern of the Mekong is comprised by primary channels, secondary channels, cross-channels and blind channels, divided by two classes of islands: major and seasonally-inundated islands. The riverbed is represented by outcrops of Mesozoic bedrock and temporally and spatially fixed sand bars whereas the planview variations in the disposition of sand bodies versus bedrock define the extent of longitudinal geomorphological zones. Structurally and qualitatively diverse geological units are dissected by regional and local faults which partially control channel alignments. Topographic and dating control of river terraces and palaeochannel deposits show that the river incised during the Last Glacial Maximum but was essentially within the same course as seen today for at least last 70ka. Distinctive floristic associations emphasise an important role that vegetation plays in channel dynamics in this mixed bedrock-alluvial system. Comparative study of channel network metrics shows that reach-scale standard planform indices (e.g., braiding intensity, channel sinuosity) are less effective than non-standard indices developed at scales less than the reach-scale (e.g., channel network bifurcation angles asymmetry, island convexity) in successfully separating channel patterns. In addition, DEM trend surfaces reveal variable topographic trends generic for the mixed bedrock-alluvial patterns only. Ultimately, it shows that the mixed bedrock-alluvial pattern classification might be more effectively based on channel cross-section properties, e.g. variations between bedrock/alluvium as a part of a channel continuum.

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