Self-constraining of low-energy rivers explains low channel mobility and tortuous planforms

Meandering rivers are abundant on Earth, from the largest rivers to the smallest tributaries. The classical view of meandering rivers is a sinuous planform with rounded bends, which grow and migrate until they are cut-off. However, many low-energy meandering rivers have planforms that are much more complex than this classical view due to the heterogeneity of their alluvium, and show relatively limited channel migration. Based on a detailed palaeogeographic study of the Dommel River in The Netherlands, it is inferred that low-energy meandering rivers may develop tortuous planforms with sharp bends, owing to self-formed deposits that increasingly constrain the channel mobility. This mechanism is corroborated by data from 47 meandering river reaches of varied scale from around the world, which show that erosion-resistant floodplain deposits are preserved in the river banks when the river energy is below a critical threshold. The term ‘self-constraining’ is proposed for low-energy rivers where an increase in bank stability over time results in progressive tortuous planforms and reduced mobility. A conceptual model, based on the dataset, shows that the increase in bank stability over time also increases the energy required to break out of the tendency to self-constrain. Self-constraining thereby enhances the resilience of the system to bank erosion, while an unexpected increase in bank erosion may occur if river energy exceeds the critical threshold. This study provides a novel explanation for the evolution of low-energy river planforms and dynamics, and provides new insights on their responses to climate changes.