Pathways of Channel Initiation in Salt-Marsh Tidal Networks: A Sedimentological Perspective

Tidal networks are pervasive features of salt marsh landscapes and exert primary control on ecomorphodynamic processes by regulating water and sediment exchange as well as nutrient fluxes. Given their crucial role and the rapid pace of sea-level rise, evolving sediment dynamics, subsidence, and continued anthropogenic disturbance, there is a pressing need to better understand how these networks behave. However, while considerable effort has been devoted to understanding the hydrodynamics and morphodynamic evolution of tidal channels and their networks, a substantial knowledge gap persists regarding the initiation of small-scale, low-order channels (sensu Horton) that constitute the most capillary component of these networks. In particular, it remains unclear whether these minor channels can develop directly on marsh surfaces or whether they are inherited from pre-existing tidal-flat channels that persist as tidal flats transition into salt marshes through biophysically driven vertical aggradation. As such, identifying the environment in which tidal channels originate is fundamental to advancing our understanding of their formative mechanisms.

The present work investigates tidal creeks draining the salt marshes of the Venice Lagoon (Northeastern Italy) with the aim of determining whether they are more likely to initiate directly on the marsh platform or instead represent inherited features from tidal flats. The Venice Lagoon is the largest Mediterranean brackish-water system, characterized by an average tidal range of about 1 m. Its salt marshes host a dense network of tidal channels that exhibits a well-developed meandering pattern, even in the most peripheral elements. This study focuses on point bars associated with channels ranging from 0.5 m to 3 m in width, aiming to identify the sedimentary environment in which each bar nucleated. Sedimentary cores were collected along transects aligned with the axes of meander bends, and facies analysis of cores from 39 transects allowed the recognition of five main depositional units: salt-marsh, point-bar, channel-lag, early salt-marsh, and tidal-flat deposits. The nature of the deposits (salt-marsh, early salt-marsh, or tidal-flat) hosting the nucleation point—that is, the onset of bar sediment accumulation—provides insight into the environment in which the associated channel began to develop.

Our results show that most channel bars originate within early salt-marsh deposits, indicating that channel dynamics were initiated near the transition between tidal-flat and salt-marsh environments, when pioneer halophytic vegetation first colonized the substrate. At this stage, vegetation was sufficiently developed to stabilize the sediment, enabling efficient channel incision and the formation of an incipient point bar. Collectively, these findings support the view that low-order salt-marsh channels can arise through multiple developmental pathways and that such differing origins leave distinct stratigraphic signatures, allowing them to be readily distinguished through the sedimentary record.