Estuaries

As a result of submergence, river valleys that were shaped by subaerial processes of erosion and deposition during lower sea level became estuaries . An estuary brings marine conditions into a river valley as far as the limit of tidal rise to form a semi-enclosed coastal body of water connected to the open sea at one end and to an influx of fresh river water at the other. When sea level rise exceeds filling, estuaries are well developed and persist. If sea level is nearly stable, sediment infilling can catch up returning the estuary to river valley morphology. Nichols and Biggs, 1985 The basin geometry, river sediment discharge, littoral drift, and tidal exchange are all factors in estuary development.

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:

These sequences interfinger with fluvial and marine sediments at the inner and outer limits of the estuary respectively. Dalrymple, et al, 1992 Dispersal of fine sediments in the main estuary is controlled by the estuarine circulation pattern. The efficiency of sediment trapping within the estuary depends on the capacity of an estuary in relation to rate of sedimentation and energy available for transport. If these are not in balance, either trapping or by-passing occurs. Three combinations of sediment cycling can occur: Field data shows that most of the river sediments are retained in estuaries. Two processes operate to prevent offshore transport. The particles flocculate and settle as larger composite particles, and the circulation pattern in shallow nearshore areas may promote retention of the sediment load. Flocculation can be caused by biological pelletizing of the suspended material during suspension feeding. The feeding activities of both benthic organisms and zooplankton also produce pellets that will settle faster than the individual mineral components. Clay particles also will begin to flocculate with even low increases in salinity and formation of a floc changes the settling dynamics of the particles. The resistance of cohesive mud deposits to resuspension is a factor in retaining sediments once they have settled. Only storm and flood conditions exercise enough force to move the deposited sediments.

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.