ESTUARY, ESTUARINE HYDRODYNAMICS
Halina Kowalewska-Kalkowska and Roman Marks
Institute of Marine and Coastal Sciences, University of
Szczecin, Szczecin, Poland
Synonyms
River mouth area
Definition
According to the definition given by Cameron and
Pritchard (1963), an estuary is a semi-enclosed coastal
body of water which has a free connection with the open
sea and within which seawater is measurably diluted with
freshwater derived from land drainage. This definition
applies to the so-called classical (positive) estuaries of
the Earth’s temperate latitudes in which freshwater input
is the main driver of the long-term circulation. Freshwater
inflow establishes longitudinal density gradients resulting
in long-term surface outflow and net inflow underneath. In
some arid basins, called inverse (negative) estuaries,
where evaporation exceeds the sum of precipitation and
runoff, the loss of freshwater becomes a main force shaping long-term circulation and the development of longitudinal density gradients, in analogy to classical estuaries
(Valle-Levinson, 2010).
Introduction
Estuaries constitute unique ecosystems influenced by
freshwater input and tide currents which establish the estuarine circulation often called the gravitational circulation.
This steady-state circulation may be modified by tidal currents, the Earth’s rotation, atmospheric forcing, topographic features, and river discharge (MacCready and
Geyer, 2010; Geyer and MacCready, 2014). The water circulation controls many estuarine processes and phenomena. One of peculiar features in estuaries is the
occurrence of turbidity maxima in the area of interface
of fresh- and salt waters. These zones of high suspended
sediment concentrations influence primary production,
contaminant flushing, fish migration, and dredging
(Mitchell et al., 1998). Estuaries are regions extensively
exploited by man since they serve as locations for ports,
urban and industry development, sewage discharge systems, recreational areas, farming, as well as fishing and
hunting grounds (Wolanski and McLusky, 2012). It
should be kept in mind, however, that any pollutant in an
estuary, i.e., pollution loading, oil spills, thermal discharges, coastal dredging, and offshore drilling, may significantly affect its environment.
Origin of estuaries
Considering their geological origin, estuaries are divided
into four types (Pritchard, 1967; Valle-Levinson, 2010):
• Drowned river valleys (coastal plain estuaries): originally rivers, formed as a result of the Pleistocene
increase in sea-level, typically funnel shaped, several
kilometers wide, and shallow (of about 10 m depth)
with large width/depth discrepancy. The salinity
decreases steadily from about 30 % (at the mouth) to
about 0.1 % (at the head). In this type of estuary the
stretch of the freshwater river above the upper limit of
intrusion of sea-derived salt is still subject to tidal
action. Examples of drowned river valleys are Chesapeake Bay, Delaware Bay, and the Elbe Estuary.
• Fjords: located in high-latitude regions, where glaciation has been a major factor in shaping the land, characterized by an elongated, deep channels with steep sides
and a sill. The sill is related to a moraine of either a currently active glacier or an extinct glacier. Fjords are typical in Scandinavia, Greenland, and Antarctica as well
as in New Zealand, Canada, and Patagonia.
• Bar-built estuaries: formed when offshore barrier sand
islands and sandpits build above sea-level and extend
between headlands in a chain, broken by one or more
inlets, usually shallow, oriented parallel to the coast.
In these estuaries the circulation is provided by the
wind, as tides are considerably choked and the river discharge is of low volume. Bar-built estuaries are found in
northern Mexico or in southern Portugal.
• Tectonic estuaries: formed by earthquakes or by the
Earth’s crust fractures and creases that generated faults
in regions adjacent to the ocean, typically developed
after sinking a part of the crust and forming a hollow
basin. An example of this type of estuary is San
Francisco Bay.
Water balance of estuaries
In terms of water balance estuaries may be classified
according to Pritchard (1952) as:
• Positive estuaries: in which freshwater input from rivers, rainfall, groundwater, and ice melting exceeds
evaporation. The longitudinal density gradient drives a
net volume outflow to the ocean; surface outflow is
stronger than near-bottom inflow.
• Inverse (negative) estuaries: in which evaporation
exceeds the combined sources of freshwater. They show
stronger surface inflow than near-bottom outflow.
• Neutral estuaries: in which evaporation and freshwater
input are in balance.
With the exceptions of lagoons in arid or semiarid
regions, most estuaries are positive. As reported by
Valle-Levinson et al. (2001), some estuaries could change
ESTUARY, ESTUARINE HYDRODYNAMICS
235
Halina Kowalewska-Kalkowska and Roman Marks
Institute of Marine and Coastal Sciences, University of
Szczecin, Szczecin, Poland
Synonyms
River mouth area
Definition
According to the definition given by Cameron and
Pritchard (1963), an estuary is a semi-enclosed coastal
body of water which has a free connection with the open
sea and within which seawater is measurably diluted with
freshwater derived from land drainage. This definition
applies to the so-called classical (positive) estuaries of
the Earth’s temperate latitudes in which freshwater input
is the main driver of the long-term circulation. Freshwater
inflow establishes longitudinal density gradients resulting
in long-term surface outflow and net inflow underneath. In
some arid basins, called inverse (negative) estuaries,
where evaporation exceeds the sum of precipitation and
runoff, the loss of freshwater becomes a main force shaping long-term circulation and the development of longitudinal density gradients, in analogy to classical estuaries
(Valle-Levinson, 2010).
Introduction
Estuaries constitute unique ecosystems influenced by
freshwater input and tide currents which establish the estuarine circulation often called the gravitational circulation.
This steady-state circulation may be modified by tidal currents, the Earth’s rotation, atmospheric forcing, topographic features, and river discharge (MacCready and
Geyer, 2010; Geyer and MacCready, 2014). The water circulation controls many estuarine processes and phenomena. One of peculiar features in estuaries is the
occurrence of turbidity maxima in the area of interface
of fresh- and salt waters. These zones of high suspended
sediment concentrations influence primary production,
contaminant flushing, fish migration, and dredging
(Mitchell et al., 1998). Estuaries are regions extensively
exploited by man since they serve as locations for ports,
urban and industry development, sewage discharge systems, recreational areas, farming, as well as fishing and
hunting grounds (Wolanski and McLusky, 2012). It
should be kept in mind, however, that any pollutant in an
estuary, i.e., pollution loading, oil spills, thermal discharges, coastal dredging, and offshore drilling, may significantly affect its environment.
Origin of estuaries
Considering their geological origin, estuaries are divided
into four types (Pritchard, 1967; Valle-Levinson, 2010):
• Drowned river valleys (coastal plain estuaries): originally rivers, formed as a result of the Pleistocene
increase in sea-level, typically funnel shaped, several
kilometers wide, and shallow (of about 10 m depth)
with large width/depth discrepancy. The salinity
decreases steadily from about 30 % (at the mouth) to
about 0.1 % (at the head). In this type of estuary the
stretch of the freshwater river above the upper limit of
intrusion of sea-derived salt is still subject to tidal
action. Examples of drowned river valleys are Chesapeake Bay, Delaware Bay, and the Elbe Estuary.
• Fjords: located in high-latitude regions, where glaciation has been a major factor in shaping the land, characterized by an elongated, deep channels with steep sides
and a sill. The sill is related to a moraine of either a currently active glacier or an extinct glacier. Fjords are typical in Scandinavia, Greenland, and Antarctica as well
as in New Zealand, Canada, and Patagonia.
• Bar-built estuaries: formed when offshore barrier sand
islands and sandpits build above sea-level and extend
between headlands in a chain, broken by one or more
inlets, usually shallow, oriented parallel to the coast.
In these estuaries the circulation is provided by the
wind, as tides are considerably choked and the river discharge is of low volume. Bar-built estuaries are found in
northern Mexico or in southern Portugal.
• Tectonic estuaries: formed by earthquakes or by the
Earth’s crust fractures and creases that generated faults
in regions adjacent to the ocean, typically developed
after sinking a part of the crust and forming a hollow
basin. An example of this type of estuary is San
Francisco Bay.
Water balance of estuaries
In terms of water balance estuaries may be classified
according to Pritchard (1952) as:
• Positive estuaries: in which freshwater input from rivers, rainfall, groundwater, and ice melting exceeds
evaporation. The longitudinal density gradient drives a
net volume outflow to the ocean; surface outflow is
stronger than near-bottom inflow.
• Inverse (negative) estuaries: in which evaporation
exceeds the combined sources of freshwater. They show
stronger surface inflow than near-bottom outflow.
• Neutral estuaries: in which evaporation and freshwater
input are in balance.
With the exceptions of lagoons in arid or semiarid
regions, most estuaries are positive. As reported by
Valle-Levinson et al. (2001), some estuaries could change
ESTUARY, ESTUARINE HYDRODYNAMICS
235