Actively Meandering Bedrock Rivers

The process of meander formation is commonly thought to be restricted to alluvial rivers and constructive submarine channels, and when meanders are observed in bedrock rivers they are typically interpreted as an antecedent feature. In Taiwan we have found widespread evidence that this is not always the case and that instead bedrock rivers may actively meander. By active we mean that meanders can naturally initiate within a straight bedrock channel and grow into long-lived, highly sinuous reaches. The evidence for active bedrock meandering comes from across Taiwan and from two Taiwanese catchments in particular: the Jukou (western Central Range) and the Hsiukuluan (Coast Range). Our key field observations are the following: concave-up strath terraces merging into the active bedrock channel in established meander loops; flights of such straths topping gooseneck ridges and recording the growth of meander loops; lateral erosion by landsliding concentrated on the outer bends of meanders; breaching of goosenecks; cutoff meander loops. These features indicate a dynamic process of meandering; by contrast, erosional meanders inherited from an alluvial river would have a static geometrical form and would not systematically display any of these features. Moreover, it is almost inconceivable that bedrock meandering could be inherited on the scale observed in the Taiwanese uplands. We conclude that active river meandering can be a purely erosional process. This conclusion poses challenging questions about the basic process of meander formation.