Dynamic Mud Deposition Along the Fluvial–Tidal Transition Zone in the Waihou River, Aotearoa New Zealand

Abstract

The fluvial–tidal transition zone (FTT) is a critical interface where complex interactions between river flow, tides, and sedimentation shape geomorphic systems and influence the dynamics of aquatic environments. However, few previous studies have integrated real-time hydrodynamic data with sedimentary deposits. In particular, the range of depositional conditions over which mud accumulates remains poorly constrained, and little is understood about how these deposits are preserved in the stratigraphic record. To address this knowledge gap, we examined co-located hydrodynamic instrument data and sedimentary deposits from the lower Waihou River, Aotearoa New Zealand. Results reveal that “dynamic mud” events, including fluid mud and rapidly deposited mud, dominate the spatial and temporal record, with few “static mud” events in which mud is deposited through gravitational settling. We suggest that dynamic mud conditions with the potential for deposition may occur throughout the tidal cycle, although cyclic tidal successions are never fully preserved. Many of the trends in sedimentation observed in studies of larger systems are not present in this small muddy river system, indicating the significance of climatic and river-flow characteristics on the sedimentary record. This work underscores the importance of studying systems of multiple sizes across diverse climatic regimes to establish holistic facies models to reconstruct geological history accurately.