Reevaluating sedimentary signatures of micro-tidal processes in fluvial-dominated rivers: the Po River (Italy)

In microtidally influenced rivers, tides are generally assumed to leave virtually no signatures in the sedimentary record. However, this hypothesis has been surprisingly poorly tested using modern river analogues, which provide an opportunity to assess whether sedimentary signatures of microtidal regimes can develop in rivers that lack diagnostic evidence of tidal control on large-scale channel morphology. This study investigates the relative importance of tidal and fluvial processes in shaping sedimentary deposits within the 180-km-long backwater, microtidal fluvial–marine transition zone of the Po River (Italy). The Po River enters the Adriatic Sea through several deltaic distributary channels that exhibit no evidence of funneling or cuspate meanders, indicating a clear morphodynamic dominance of fluvial over tidal processes. Hydrological data show that river water levels are modulated by tidal influence up to ~90 and ~40 km from the river mouth during low- and high-discharge stages, respectively. Approximately 30 km from the mouth, heterolithic deposits formed by riverine floods exhibit clear tidal signatures, including double mud drapes (both equal and unequal) and bidirectional ripples. Around 40 km from the mouth, cross-stratified dunes, and unit bars display bundles with cyclic organization—features that are unclear or absent in analogous bedforms farther upstream. Tidal control of river water levels during the deposition of bedforms with cyclic bundles suggests a linkage to semi-diurnal tidal modulation. These bundles are deposited above the average intertidal zone, indicating that flood-enhanced water levels enable tide-modulated sedimentation to extend beyond the typical intertidal range. This study shows that evidence of tidal processes can be common even in deposits of microtidally influenced rivers, highlighting that tidal signatures may develop where tides exert little to no control on large-scale morphodynamics. Overall, these findings emphasize caution when interpreting dominant morphodynamic processes solely from sedimentary structures—or interpreting structures solely from assumed morphodynamic controls.