Sedimentological and architectural features of meandering subtidal channels are relatively unexplored, and their deposits are commonly investigated based on facies models set up for intertidal meandering channels. The Venice Lagoon (northern Adriatic Sea, Italy) is affected by a micro-tidal regime and hosts a dense network of active and buried tidal channels. It represents an excellent natural laboratory to improve the current knowledge on subtidal meander morphodynamics and related deposits. In this study, the integration of high-resolution geophysical images and core data allows reconstruction of the architectural three-dimensional model of a meandering subtidal palaeochannel, which is buried below a modern subtidal flat. The study palaeochannel was 35 m wide and 3 m deep, and formed three adjacent meander bends and related point bars. A detailed three-dimensional architectural reconstruction was carried out for deposits associated with one of these meander bends, that was crossed by a minor, low-sinuosity channel with two minor bank-attached bars. This reconstruction highlights that the study point bar has a horseshoe shape, which arose from the onset of bar accretion from an already-sinuous channel. Reconstructed growth stages of the studied bends show that point-bar accretion can follow different styles of planform transformation, also experiencing simultaneously landward (or seaward) deposition according to the dominant flow direction (i.e. local tidal asymmetry). The analyses show that planform transformations occurred in parallel with elevation changes of the related channel thalweg, which shaped pools with geometry varying with the radius of curvature of the bend. The present study highlights the relevance of high-resolution threedimensional reconstructions to link palaeomorphodynamic processes with related sedimentary products.

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Over the past few millennia, meandering fluvial channels drained coastal landscapes accumulating sedimentary successions that today are permeable pathways. Propagation of pollutants, agricultural exploitation and sand liquefaction are the main processes of environmental interest affecting these sedimentary bodies. The characterization of these bodies is thus of utmost general interest. In this study, we particularly highlight the contribution of noninvasive (remote and ground-based) investigation techniques, and the case study focuses on a late Holocene meander bend of the southern Venetian Plain (Northeast Italy). Electromagnetic induction (EMI) investigations, conducted with great care in terms of sonde stability and positioning, allowed the reconstruction of the electrical conductivity 3D structure of the shallow subsurface, revealing that the paleochannel ranges in depth between 0.8 and 5.4 m, and defines an almost 260 m-wide point bar. The electrical conductivity maps derived from EMI at different depths define an arcuate morphology indicating that bar accretion started from an already sinuous channel. Sedimentary cores ensure local ground-truth and help define the evolution of the channel bend. This paper shows that the combination of well-conceived and carefully performed inverted geophysical surveys, remote sensing and direct investigations provides evidence of the evolution of recent shallow sedimentary structures with unprecedented detail.

The characterization of ancient fluvial-channel bars informs predictions of sedimentary facies distribution, and hence they are critical for interpreting river morphodynamics through time. Within active fluvial channels, sediment storage occurs along the banks or along their axial portion, generating bank-attached or mid-channel bars, respectively. This study introduces the angle of flow divergence (Δf), measured as the angle between the azimuths of bar accretionary slopes and local dune migration, and quantitatively explores its distribution in 103 channel reaches that have a sandy bed load and clear water. This dataset includes both mid-channel and bank-attached bars from low-variable peak discharge rivers of Russia and Brazil. Remotely sensed evidence shows that dune migration transverse to the dip azimuth of bar slopes (Δf ≅ 90-110°) occurs both in mid-channel and bank-attached bars, contradicting the long-standing tenet that palaeoflows nearly orthogonal to the dip azimuth of large-scale inclined strata are indicative of sinuous, single-thread palaeochannels. Only Δf values significantly higher (>140°) or lower (<60°) than 90°discriminate mid-channel and bank-attached fluvial bars. Comparisons between Δf values and bar-accretionary styles suggest that mid-channel bars formed by high-peak and low-peak variable discharge rivers tend to preserve axially and laterally accreting deposits, respectively. Nearly orthogonal divergence angles are found in bank-attached bars irrespective of peak-discharge variability. While stressing that analyses akin to the one presented here are to be complemented with additional sedimentological information from high-quality outcrops, this study proposes a new, quantitative, approach to better distinguish fluvial bars, and hence possibly the planform pattern of associated channel bodies, from sedimentary successions. In particular, the data show that a reliable distinction between mid-channel and bank-attached bars should be based on comprehensive palaeocurrent and architectural information from various portions of preserved bar bodies, and that inferences based on limited datasets should be considered with caution.