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 14 C 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.

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Abstract. River channel patterns may alter due to changes in hydrological regime related to changes in climate and/or land cover. Such changes are well documented for transitions between meandering and braiding rivers, whereas channel pattern changes between laterally stable and meandering rivers are poorly documented and understood. We hypothesize that many low-energy meandering rivers had relatively low peak discharges and were laterally stable during most of the Holocene, when climate was relatively stable and human impact was limited. Our objectives in this work are to identify a Late Holocene channel pattern change for the low-energy Overijsselse Vecht river, to develop and apply a novel methodology to reconstruct discharge as a function of time following a stochastic approach, and to relate this channel pattern change to reconstructed hydrological changes. We established that the Overijsselse Vecht was laterally virtually stable throughout the Holocene until the Late Middle Ages, after which large meanders formed at lateral migration rates of about 2 m yr−1. The lateral stability before the Late Middle Ages was constrained using a combination of coring information, ground-penetrating radar (GPR), radiocarbon (14C) dating, and optically stimulated luminescence (OSL) dating. We quantified bankfull palaeodischarge as a function of time based on channel dimensions that were reconstructed from the scroll bar sequence and channel cut-offs using coring information and GPR data, combined with chronological constraints from historical maps and OSL dating. We found that the bankfull discharge was significantly greater during the meandering phase compared to the laterally stable phase. Empirical channel and bar pattern models showed that this increase can explain the channel pattern change. The bankfull discharge increase likely reflects climate changes related to the Little Ice Age and/or land use changes in the catchment, in particular as a result of peat reclamation and exploitation.

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.

Abstract. River channel patterns may alter due to changes in hydrological regime, related to changes in climate or land cover. Such changes are well documented for transitions between meandering and braiding rivers, whereas channel pattern changes between laterally stable and meandering rivers are poorly documented and understood. We identified a river that was laterally almost stable throughout the Holocene until the Late Middle Ages, after which large meanders formed at lateral migration rates of about 2 m yr−1. The lateral stability before the Late Middle Ages was proven using a combination of coring information, ground-penetrating radar (GPR), radiocarbon (14C) dating, and optically stimulated luminescence (OSL) dating. Our objective of this work is to identify the possible causes for the meander initiation. We carried out a unique reconstruction of bankfull discharge as a function of time, based on channel dimensions that were reconstructed from the scroll bar sequence using coring information and GPR data, combined with chronological constraints from historical maps and OSL dating. Empirical channel and bar pattern models were used to determine the potential for meandering and to identify the causes of meander initiation. Several potential causes were investigated, varying from discharge regime changes to increased sediment input. Our investigation shows that bankfull discharge was two to five times higher during the meandering phase compared to the laterally stable phase. This increase likely reflects climate changes related to the Little Ice Age and land use changes in the catchment, in particular as a result of peat reclamation and exploitation. We hypothesize that many low-energy meandering rivers were laterally stable during most of the Holocene, reflecting relatively low peak discharges during a stable climate and with limited human impact. However, channel deposits associated with such stable phases are poorly preserved, due to recent increase in dynamics of such systems. Considering the importance of climate and land use changes on the river channel pattern, successful river restoration requires an integral approach that includes scenarios of climate and land use changes in the catchment.