Fluvial depositional architecture in an unconfined environment is governed by sediment dispersal across the alluvial plain through river-path switching by avulsion. Documented inter-avulsion periodicity from modern rivers ranges from tens to over a thousand years. In this study, a quantitative spatio-temporal reconstruction of avulsion history is presented of the non-vegetated and pristine modern Río Colorado dryland river system in the semi-arid Altiplano Basin (Bolivia), based on the integrated analysis of satellite imagery and absolute age dating using optically stimulated luminescence, complemented with sedimentological and geomorphological ground-truth data. This approach enables us to reconstruct the chronological order of channel belts of the Río Colorado, to determine avulsion recurrence time and inter-avulsion periodicity, to identify mechanisms for flow path changes, and to present a morphodynamic model for the spatio-temporal evolution of fluvial deposits in a semi-arid environment. In a maximum timespan of 12.71 ± 1.5 ka, successive avulsions of the Río Colorado created a sheet of interconnected fluvial deposits, consisting of diverging and juxtaposed alluvial ridges that formed by sediment aggradation in point bars, crevasse splays, levees, and on the channel floor. The ridges show lateral onlap and amalgamation as the result of repeated avulsion and compensational stacking, whereby the river avoided the positive alluvial-ridge relief of its precursors. The resultant morphology is fan-shaped, convex-up with a surface area of approximately 500 km ² and a maximum observed thickness of 3 m. The results show inter-avulsion periods of the river of up to 1.28 ± 0.34 ka. A paucity in fluvial activity around 2 ka BP, and at present, is interpreted as the result of low river discharge related to long-term dry periodicity in the El Niño Southern Oscillation circulation system. Each river path started as a low sinuous, single-thread channel in a narrow belt, and in time increased its width and sinuosity by point-bar expansion and rotation.

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.

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.

Sedimentation at the terminus of low-gradient river systems in a semi-arid climate setting is characterized by thin- but laterally extensive amalgamated sand sheets. Analysis of absolute age dating with Optically Stimulated Luminescence measurements of fluvial sands in the Holocene Río Colorado (Altiplano Basin, Bolivia) aimed to (1) analyze the processes that created this sedimentary architecture, and (2) provide an analogue for thin but laterally-extensive sandstone reservoirs in an overall low net-to-gross setting such as the Rotliegend feather edge. The Río Colorado created a network of laterally-amalgamated fluvial sands by successive river-channel switching as the result of avulsion in a time period of only 4000y. The network formed by successive river avulsions with frequencies from 60 to 910y. The total area covered with these deposits approximates 500 km2; maximum thickness of the deposits is 2m. Fluvial sediment accumulated by vertical aggradation of sand in levees, vertical stacking and lateral amalgamation of crevasse splays, and vertical aggradation of channel-floor sand. This resulted in alluvial ridges with a positive relief on the floodplain. Subsequent river positions avoided the positive relief, and juxtaposition of successive river positions resulted in compensational stacking and amalgamation of fluvial sand deposits to a laterally connected extensive sand sheet.

This study provides valuable insight into the morphodynamics of low-energy streams and resulting planform evolution. Although the sinuous, rectangular planform is similar in peat-filled and sand-filled valleys, the stream planform evolution and dominant morphodynamic processes are different. This knowledge contributes to the development of sustainable and costeffective stream restoration approaches in both settings. These should be aiming at restoring the natural morphodynamic processes, as alignment with these processes is expected to increase robustness and reduce maintenance efforts.