Positive hydrology in tide-dominated estuaries


Block diagram of positive hydrology in tide-dominated estuaries


Positive hydrology processes in tide-dominated estuaries

1. Catchment freshwater input

Freshwater enters from the catchment. Although the volume of freshwater input varies regionally and temporally (depending on local catchment and climate conditions), it is often relatively high in positive estuaries.

2. Volume of freshwater

The volume of freshwater can cause stratification (or layering) within the estuarine water column, which varies with seasonal flow. Buoyant low-salinity fresh water tends to float above denser, high-salinity sea water. However, high tidal ranges and turbulence associated with tidal currents tends to accelerate the mixing of freshwater and marine water.

3. Salt-wedge

A ‘salt-wedge’, or intrusion of denser saline marine water penetrates the estuary through the wide entrance. Mixing occurs between less-dense freshwater, and higher-density marine water. Circulation processes are complicated by the massive area of intricate dendritic drainage system (Wolanski et al., 1992). The salt-wedge typically penetrates most of the estuarine system (due to a large tidal prism and tidal amplification), however during high fluvial flow events (which may be seasonal), strong river flow may force the salt-wedge in a seaward direction.

4. Exchange of marine waters

Exchange occurs through the wide entrance of the estuary. Tide-dominated estuaries are generally well-flushed, having a tidal prism that can be several orders of magnitude larger than the volume of freshwater input by rivers. As a consequence, lower salinity water occurs only at the head of tide-dominated estuaries due to the direct influence of the river, and the rest of the estuary contains water with salinities approaching that of the open ocean (Chin et al., 1994). Flood and ebb tides may follow different routes into and out of the estuary, and the tidal prism tends to be large. In positive estuaries, the net outflow of catchment-derived water exceeds the net inflow of marine water, however this may be less significant in comparison to the large volume of the tidal prism.

5. Saltflat environments

saltflat environments are inundated rarely (e.g. 3-4 days per month), resulting in hypersaline groundwaters and often a saline crust on the surface (Ridd et al., 1997).

6. Evaporation

Evaporation is a significant process in tide-dominated estuaries due to the extensive intertidal area (also depending on climatic conditions), however in positive estuaries does not exceed freshwater river input.