SignificanceMeandering tidal channel networks play a central role in the ecomorphodynamic evolution of the landscapes they cut through. Despite their ubiquitous presence and relevance to sedimentary and landscape features, few observations of tidal-meander evolution exist, and we lack a full understanding of the governing processes. Field analyses show that tidal meanders, traditionally viewed as stable landscape features, display modes of migration and migration rates per unit width quite similar to those characterizing their fluvial counterparts, with important implications for the characterization of the related sedimentary products. The results presented here contribute to our understanding of the morphological evolution of tidal landscapes.

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Meandering channels extensively dissect fluvial and tidal landscapes, critically controlling their morphodynamic evolution and sedimentary architecture. In spite of an apparently striking dissimilarity of the governing processes, planform dimensions of tidal and fluvial meanders consistently scale to local channel width, and previous studies were unable to identify quantitative planimetric differences between these landforms. Here we use satellite imagery, measurements of meandering patterns, and different statistical analyses applied to about 10,000 tidal and fluvial meanders worldwide to objectively disclose fingerprints of the different physical processes they are shaped by. We find that fluvial and tidal meanders can be distinguished on the exclusive basis of their remotely-sensed planforms. Moreover, we show that tidal meanders are less morphologically complex and display more spatially homogeneous characteristics compared to fluvial meanders. Based on existing theoretical, numerical, and field studies, we suggest that our empirical observations can be explained by the more regular processes carving tidal meanders, as well as by the higher lithological homogeneity of the substrates they typically cut through. Allowing one to effectively infer processes from landforms, a fundamental inverse problem in geomorphology, our results have relevant implications for the conservation and restoration of tidal environments, as well as from planetary exploration perspectives.

The widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point‐bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal channel‐bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. This study improves understanding of tidal channel meander bends through a multi‐disciplinary approach that combines analyses of historical aerial photographs, measurements of in‐channel flow velocity, high‐resolution facies analyses of sedimentary cores and three‐dimensional architectural modelling. The studied channel bend (12 to 15 m wide and 2 to 3 m deep) drains a salt marsh area located in the north‐eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77 years, the bend has translated seaward ca 15 m. Results show that the channel bend formed on a non‐vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 0·2 to 0·3 m year−1, and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain‐size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarsening‐upward and fining‐upward sedimentary packages, respectively.

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Tidal landscapes are extensively characterized by the presence of meandering channels, the latter being important for the ecomorphodynamic evolution of these environments. It remains unclear whether changes in the relative strength of maximum flood and ebb currents (i.e., tidal flow asymmetries), together with the widespread presence of lateral tributaries, cause tidal meanders to evolve differently from their fluvial relatives. Here, we investigate the evolution of a meandering channel in the Venice Lagoon (Italy) that receives water from two major tributaries along its outer bank. Using a 2‐D numerical model, we first analyze the changes in local tidal flow asymmetries, both natural and anthropogenically induced, which occurred during the last 120 years. The effects of these modifications on the meander morphodynamics are then investigated by means of a 3‐D numerical model, and results are compared to modern and historical field data spanning more than one century. We show that under asymmetric tidal flows, tidal meanders develop depositional patterns according to the dominant flow direction, similar to that of fluvial meanders. In addition, the morphological effects of the nondominant tidal flow become increasingly negligible as tidal flow asymmetry increases. We also show that enhanced sediment and water fluxes from major lateral tributaries, sourced from wind‐wave exposed tidal flats, can critically influence the development of erosional and depositional patterns within tidal meanders otherwise sheltered from wind action by the presence of salt marshes.

As Holocene river deltas continue to experience sea-level rise, sediment carried by distributary channels counteracts delta-plain drowning. Many deltas worldwide are subject to tidal action, which strongly affects the morphology of distributary channels and could also influence their mobility. Here we show, through physical laboratory experiments, that distributary-channel mobility can be dramatically reduced in systems affected by tides in comparison to an identical system with no tides, and that the mobility of distributary channels decreases as the ratio of tidal to fluvial energy increases. This effect occurs even if new accommodation space is created by rising relative sea level. By analyzing synthetic stratigraphy derived from both digital elevation data and time-lapse photography, we show also that the reduction of channel mobility in tidal deltas increases channel stacking and connectivity in the stratigraphic record.

Tidal point bars are generally described as laterally accreting bodies, generated by lateral shift of meander bends, in which the point-bar brink (i.e. the break between bar top and bar slope) and the channel thalweg (i.e. the deepest part of the channel) shift horizontally toward the outer bank. The present study applies the concept of trajectory analysis at the point bar scale, focusing on the trajectories of point-bar brink and channel thalweg, in order to understand how vertical aggradation can interact with lateral migration to shape geometries of tidal point bars developed in a microtidal and highly aggrading salt marsh setting. We selected eight study-case meander bends, located in the Venice Lagoon and characterized by different pointbar morphologies, whose widths and depths range from 2 to 11 m and from 0.5 to 1.6 m, respectively. All the point bars were investigated through a high resolution facies-analysis carried out on closely-spaced sediment cores, collected along the bar axis. Location of bar brink and channel thalweg at different times defined specific trajectories, which were classified either as ascending or descending, and linear or non-linear. All brink trajectories are ascending, and show evidence of lateral shift of the bar brink under aggradational conditions of surrounding marshes. Development of non-linear brink trajectories is linked with changes in the ratio between vertical and lateral shift rates of the brink, which is in turn dictated by changes in local base level due to substrate compaction. Conversely, the thalweg trajectories can be either ascending or descending, reflecting an interaction between rates of lateral shift and aggradation/degradation of the channel floor. The brink and thalweg can either shift consistently (e.g., both trajectories are ascending) or incongruously (e.g., ascending brink vs. descending thalweg trajectory), reflecting different attitudes of the channel to maintain or increase its cross-sectional area.