Tributary junctions on the River Mekong

Classic descriptions of drainage network patterns suggest that confluence angle is largely determined by the shape of the drainage basin with tributaries usually joining the main stem at acute angles less than 90°, but the subject is little studied in recent years. In particular low-order channels joining high-order main stems do not seem to have been studied systematically. A series of low- and high-order tributaries with obtuse angles greater than 90° (barbed tributaries) were identified along the course of the main stem of the Lower Mekong using Google Earth. Aerial photography of the region was used to measure junction angles and channel widths above and below 248 confluences between the Chinese-Laos border and the Mekong delta. Geological and geomorphic controls promoting obtuse junctions angles were identified and classified from review of the literature and inspection of the Mekong imagery. For the Mekong, meander extension was found to be the prominent control of obtuse angle junctions, accounting for 31% of occurrences. Geological controls accounted for 28% within bedrock confined reaches with other processes explaining the remaining 41%. Clear relationships between the bed-rock types of the Lower Mekong Basin and the controls on junction angle are reported. However, structural controls dominate in the alluvial and mixed bedrock:alluvial sections of the channel. Tributary discharge (indexed by tributary width) and tributary angle impacts on main channel width were analysed and were found to be minimal. Downstream channel widths do not increase proportionately with increasing widths of tributary channels, and no relationship with junction angle was present. These analyses indicate that the river bed downstream of each junction must deepen to allow for continuity of cumulative discharge of mainstem and tributary. Analysis reveals that the bed-rock confined sections of the channel are more sensitive to tributary inputs, displaying greater percentage change than the alluvial sections.