from land drainage combine, typically resulting in hydrodynamic, turbidity, 

 and salinity gradients. In contrast with deltas, estuaries occur at the mouths 

 of rivers that have low sediment loads in comparison with dissipative forces 

 (Nichols and Biggs 1985). Lagoons behind barrier islands are also classified 

 as estuaries. 



Estuaries may develop under a variety of climatic and topographic condi- 

 tions, including valleys and coastal embayments that have been submerged. 

 Many estuaries were formed mainly during the most recent deglaciation and 

 associated sea level rise. These include most of the coastal plain estuaries, 

 some of which have been closed by barrier beaches. Fjords are a special class 

 of estuaries created by the scouring action of glaciers. Fault-block estuaries 

 may be controlled by local or regional structure. 



Estuaries function mainly as sinks or traps, although those that are largely 

 filled may be sources of sediment. Their long-term survival depends on 

 changes in the volumetric capacity for storage (due to eustatic and tectonic 

 factors) and on sedimentation rates from inland and coastal sources. Estuarine 

 sediments are derived from a number of sources including the watershed, the 

 Continental Shelf, local erosion, biological activity, and the atmosphere. 

 Sedimentary processes in estuaries are controlled by tides, waves, and meteo- 

 rological forces, as well as river inflow. Within an estuary, the processes 

 may be dominated by estuarine-fluvial, estuarine, or estuarine-marine activity. 

 As in other parts of the coast, estuaries show temporal variations with cyclic 

 and noncyclic processes, and spatial variations in process, material, and form. 



Estuaries have been classified according to the mixing processes caused by 

 the density differences between fresh- and saltwater masses (Pritchard 1967). 

 Salt wedge estuaries are highly stratified, with fresh water from river dis- 

 charge floating over the denser saltwater, a condition manifested by near- 

 horizontal isohalines. The freshwater layer thins oceanward, and vertical 

 advection is the primary mechanism for mixing across the freshwater/saltwater 

 interface. A partially mixed estuary shows increased tidal influence, to the 

 point where river discharge does not dominate circulation. Mixing is caused 

 by both vertical advection and the increased turbulence associated with tidal 

 currents. The differences between surface and bottom salinities are approx- 

 imately constant over the estuary, and the isohalines are at an angle. A fully 

 mixed estuary is vertically homogeneous because tidal mixing eliminates 

 vertical density stratification. 



Shoreface 



The shoreface is just seaward of the surf zone, and appears as a concave- 

 upward surface with a slope on the order of 1:200 (Niedoroda, Swift, and 

 Hopkins 1985). The shoreface configuration, particularly the upper part, 

 plays an important role in the modification and transformation of waves 



Chapter 3 Variable Coastal Features 



53 



