The movement of a salt wedge into the estuary not only retards the movement of bed load, it also has an effect on the suspended sediment transport. As fresh water encounters the salt water intrusion, river flow converges with estuarine water and bottom flow approaches zero. If the river discharge is large, the salt water is excluded or the wedge is sharply defined and the river sediment in suspension is carried out by a relatively thick fresh water layer moving over the salt wedge. Flood and strong flow conditions can carry large amounts of sediment to the shelf in this manner, and even erode bottom muds of the estuary. When the freshwater-saltwater transition is retained within an estuary, suspended sediment is dispersed by estuarine circulation. The basic pattern is seaward through the upper layer, settling into the lower layer, and landward through the lower layer of estuary water. Nichols and Biggs, 1985
Tides play a major role in mixing fresh and salt water, resuspending sediment from the bed and in transporting suspended sediments landward or seaward. The simplest form of ebb and flood tidal current is sinusoidal with a symmetrical distribution of velocity versus time. Sanford et al., 1991 However, the tide wave is usually deformed and ebb and flood currents are unequal in strength and duration. This produces a differential or residual movement of either ebb or flood flow. When the slack after flood is longer than the ebb, landward transport occurs.
Waves can have a significant effect on sediment distribution by eroding shores, stripping substrates, and suspending sediment for current dispersal. Osborne and Greenwood, 1993 Waves in the estuary are both those generated in the ocean that penetrate estuary mouths and those generated internally that affect shores, marginal shoals, or shallow estuary floors. Eroded material is redistributed offshore or transported into the estuary. Both the morphology and sediment facies reflect whether tides or waves dominate in an estuary.
Estuary sediments are derived from the river watershed and the continental shelf in front of the estuary. Lessor sources are erosion within the estuary, biological activity and eolian transport. The distribution of sediment facies is controlled by interactions between the available sediments, bottom morphology and flow hydrodynamics. Both landward transport of sediments by tidal currents and river inflow supply sediment to an estuary. In river dominated estuaries, equilibrium has been achieved and variation in sediment volume fluctuates with the river flow. Frey and Howard, 1986
Sedimentation in estuaries is within three distinguishable regimes:
Estuary deposits are recognizable as a distinct entity, but there are numerous component facies. The estuary is effective in size segregation with characteristic log-probability size distributions developed in different environments. A single log-normal source population is fractionated into several differing populations by bedload transport, suspension and recycling during successive tidal cycles. Visher and Howard, 1974 Suspension populations are removed by both flood and ebb flow. There is a net inland transport of suspended sediment with deposition on tidal flats and marshes. Sand deposits are present sporadically in the otherwise monotonous sequences of silts and clays. Shell and plant fragments are common components. These are usually lenticular layers developed by concentration and reworking of the sands by currents. Nichols and Biggs, 1985
The Chesapeake Bay and the Gironde estuaries are models of the extremes in physical control and geomorphic development. They are similar in the fluvial and the mouth regions, but in the middle zone, the sediment facies differ. Grain size increases with depth in the Gironde, but becomes finer with depth in the Chesapeake. Tidal flats of the Gironde are mainly muds, but the Chesapeake flats are sand because of wave action in the more open waters of this estuary.