Rethinking microtidal systems: Tidal signatures in the fluvial deposits of 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 capability of microtidal processes to leave signatures in deposits within the 180 km long backwater, fluvial–marine transition zone of the Po River (Italy). The Po River enters the Adriatic Sea (tidal range ca 1 m) through several deltaic distributary channels that exhibit no evidence of funnelling 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 km and ~ 40 km from the river mouth during low-discharge and -discharge stages, respectively. Approximately 30 km from the mouth, deposits formed by riverine floods exhibit clear tidal sedimentary structures, including sand–mud couplets and bidirectional ripples. Around 40 km from the mouth, cross-stratified sand displays bundles with cyclic organisation, which are absent further upstream and are interpreted here as the effects of semi-diurnal tidal modulation on the migration of fluvial bedforms. Additionally, 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 sedimentary structures may develop where tides exert little to no control on large-scale morphodynamics. Overall, these findings emphasise caution when interpreting dominant morphodynamic processes solely from sedimentary structures.