Low-energy streams in peatlands often have a high sinuosity. However, it is unknown how this sinuous planform formed, since lateral migration of the channel is hindered by relatively erosion-resistant banks. We present a conceptual model of Holocene morphodynamic evolution of a stream in a peat-filled valley, based on a palaeohydrological reconstruction. Coring, ground-penetrating radar (GPR) data, and 14C and OSL dating were used for the reconstruction. We found that the stream planform is partly inherited from the Late-Glacial topography, reflecting stream morphology prior to peat growth in the valley. Most importantly, we show that aggrading streams in a peat-filled valley combine vertical aggradation with lateral displacement caused by attraction to the sandy valley sides, which are more erodible than the co-evally aggrading valley-fill. Owing to this oblique aggradation in combination with floodplain widening, the stream becomes stretched out as channel reaches may alternately aggrade along opposed valley sides, resulting in increased sinuosity over time. Hence, highly sinuous planforms can form in peat-filled valleys without the traditional morphodynamics of alluvial bed lateral migration. Improved understanding of the evolution of streams provides inspiration for stream restoration.@en

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.@en

River planform and lateral activity largely result from the balance of flow strength, i.e. stream power, and bank erodibility (Nanson and Croke, 1992; Kleinhans, 2010). Floodplains of meandering rivers consist of a variety of depositional units with different erodibilities, such as point bar, backswamp and natural levee deposits (Allen, 1965; Nanson and Croke, 1992; Smith et al., 2009). Low-energy meandering rivers can have sufficient stream power to erode the non-cohesive units, but insufficient stream power to erode the cohesive units. Theoretically, low-energy meandering rivers may become laterally stable when the proportion of erosionresistant floodplain deposits gradually increases, e.g. due to steady accumulation of fine-grained counter-point bar deposits (Makaske and Weerts, 2005; Smith et al., 2009). However, limited field evidence exists on the long-term evolution of lowenergy meandering rivers, to test this relationship between lateral channel stability and the evolution of floodplain sediment composition. Therefore, we investigated the planform evolution of the lowenergy Dommel River in the southern Netherlands, along with the Holocene evolution of its floodplain deposits.@en