The occurrence of obtuse junction angles and changes in channel width below tributaries along the Mekong River, south-east Asia

摘要

Classic descriptions of drainage patterns suggest that confluence angle is determined by the shape of the drainage basin unless constraining factors, such as the geological structure, affect stream flow. Downstream changes in channel width below tributary junctions have long been associated with tributary inputs of flow and sediment. Analysis of tributary junction geometry and channel width changes in large rivers and over large reaches is sparse. The Lower Mekong Basin exhibits a generally dendritic drainage network despite flowing through a diverse array of geological settings. Publicly available SPOT-5 imagery from Google Earth was used to identify and catalogue junction geometries and downstream changes in channel width below tributary junctions along a ~2200km reach of the Mekong River. Of the 284 junctions identified, the majority (66.2%) were acute. However 12 (4.8%) were found to be normal (90°) and 75 (30%) were found to be obtuse. This latter number is in contrast to previous studies over similar spatial scales which found little evidence of obtuse junctions. Meander extension of the incoming tributary and deflection of the tributary across bedrock shoulders were found to be the dominant geomorphological causes of obtuse tributary junctions. The relationship between the width of the tributary channels and the width of the mainstem upstream and downstream of the confluences was analysed. It was observed that, over the whole reach, a slight narrowing occurred immediately below tributary junctions. Although the changes themselves were small, the slight net narrowing is shown to be statistically significant. The observed relationship is shown to vary considerably with geology. The geological control suggests that complex factors play important roles in determining changes to channel width across large systems and that simple cause-effect relationships do not hold in such complicated geological settings.