Chapter 3 - Sedimentological trends across the tidal–fluvial transition, Fraser River, Canada: A review and some broader implications

Abstract

The tidal–fluvial transition (TFT) is a complex depositional zone in rivers, where tidal- and river-flow interact. However, subdivision of the TFT into discrete zones is not well established. Studies of the lower Fraser River, Canada, reveal criteria for subdividing the TFT into zones that experience similar hydrodynamic and salinity variations from year-to-year, and three zones are identified: (1) mixed tidal–fluvial with persistent brackish-water zone (mixed tidal–fluvial); (2) fluvially dominated and tide influenced, freshwater to brackish water transition zone (turbidity maximum); and, (3) fluvially dominated and tidally modulated, freshwater zone (tidal backwater). Of the four main datasets (sedimentological, ichnological, palynological, and geochemical) evaluated across the TFT of the Fraser River, only sedimentological and ichnological datasets can be used reliably to distinguish between depositional zone within the TFT. Mud volume is highest in the turbidity maximum zone and decreases in both the landward and seaward direction. Rhythmic (tidal) bedding is common in the mixed tidal-fluvial zone, and sand–mud interbedding occurs in both the turbidity maximum and mixed tidal–fluvial zones. Significant sand–mud interbedding is not expected in the tidal backwater, nor landward of the tidal backwater. Higher salinity and longer residence times of saline water at the bed are manifested ichnologically in larger diameter burrows, higher bioturbation intensities, and a more diverse trace assemblage. Variations in river discharge result in a heterogeneous distribution of burrows, where burrows are mainly concentrated in mud beds. Increased fluctuations in depositional energy and increasing sedimentation rate reduce trace density. The depositional trends presented from the lower Fraser River are intended as a basis for comparison to trends defined from the TFTs of other major river systems. This work demonstrates that it is possible to predict relative depositional position across the TFT, and that a comprehensive model for TFT deposits can be constructed.