4.2 Water Circulation and Sediments
substantial inflow of fresh water (Fig. 4.1; cf. Fig.
3.9c). The river water entering the estuary mixes to
some extent with the denser sea water, but eventually
flows out in the form of a surface current into the
open sea. The inflowing sea water in turn enters the
estuary as a bottom current and later mixes with
outflowing fresh water. This type of water exchange
is referred to as estuarine circulation.
The models in Fig. 4.1 show the four principal
cases identified in nature. In a shallow estuary, the
entering river water and sea water may become completely mixed and therefore unstratified (Fig. 4.1b),
but salinity increases from the river inlet toward the
open sea. If mixing of sea water and fresh water is
less effective, which occurs particularly in deep estuaries with a sill, the water body displays a more or
less pronounced and stable density stratification (Fig.
4.1c and d).
It is interesting to note that an increase in river flow also
causes a somewhat higher inflow of sea water. However,
the salinity of the estuarine water is reduced under such
conditions. Estuaries display either vertically mixed or
stratified waters. The latter type is controlled by the markedly differing salt contents of the outflowing and inflowing
waters, in contrast to the open sea where (smaller) density
gradients are generated primarily by differences in water
temperature.
A stable stratification is typical for deep fjords, as for example along the Norwegian coast. A large river inflow can
create a fresh water layer over the total length of the estuary, which is underlain by a wedge-like salt-water body. As
a result of some mixing of surfaceand bottom water, the
outflowing surface current transports more water into the
sea than the entering river into the estuary. Thus, the outflow at the mouth of the estuary can be ten or more times
the volume ofthe river flow.
The sedimentation rate of terrigenous material tends
to be generally high in estuaries, reflecting the suspended load of many rivers. It is, however, difficult
to develop a simple facies model for estuaries. An
individual estuary may pass from one circulation system into another as a result of strongly variable river
discharge. It may be affected by tides and tidal currents (cf. Sect. 3.2.1), causing coastal erosion, reworking and redeposition of sediments in both landward and seaward directions. Supported by fluvial
nutrient supply, organic productivity in estuaries is
commonly high, but the number of species is restricted in brackish water subjected to frequent
changes in salinity. River-bome plant debris and marine fauna entering the estuary often become mixed
in the estuarine sediment. Reworking and
redeposition of sediment are common in shaIlow estuaries, whereas organic matter and fossil remains
have a good chance of being preserved in deeper estuaries due to rapid sediment accumulation. Where
estuarine water masses are stratified (Fig. 4.1 c), the
167
oxygen content of bottom water may drop and hamper benthic life.
4.2.2 Adjacent Seas with Estuarine Water
Circulation
In adjacent seas larger than estuaries, evaporation
from the water surface of the basin becomes an important factor (Fig. 4.2). In humid climates, precipitation, P, and river discharge, R, into the basin are
commonly greater than water loss by evaporation, E.
This situation is similar to that observed in estuaries.
Therefore the term estuarine circulation is also used
for such "humid basins".
Present-day examples of this basin type are the
Baltic Sea and the Black Sea (Figs. 4.2a and 4.4).
They are characterized by a narrow, shallow entrance, allowing the denser sea water to enter the basin as subsurface inflow, while .the less saline water
from the basin flows as surface current out into the
sea.
The Gulf of Califomia also displays an estuarine circulation. However, in contrast to the previous examples, its
opening to the Pacific is wide and deep and therefore
readily allows subsurface inflow of cold and comparatively
dense intermediate ocean water (Fig. 4.5, see Sect. 4.3 for
further details).
Baltic Sea. This fairly shallow basin (average depth
about 100 m) roughly corresponds to the vertically
mixed estuary model (Fig. 4.lb). Its water mass is
largely unstratified and was weIl oxygenated in preindustrial times, apart from some deeps with water
depths up to 460 m. The salinity of the Baltic Sea
(usually less than 10 to 15%0) increases from the
landward side toward the opening(s) to the normal
saline North Sea. Consequently, stagnant stratified
waters only developed in some marginal bays and
deep depressions in the central part of the basin.
Nutrients for aquatic life are delivered by rivers
from the neighboring land areas as weIl as by erosion
of sediments in the coastal zones. Production of organic matter can be high, and some of it is preserved
in the sediments, particularly in the deeps mentioned
above where benthic life is reduced or missing due to
oxygen deficiency. Similar depositional conditions
are also realized in many deep fjords along formerly
glaciated co asts where freshwater enters the sea.
For more details about the Baltic Sea see, e.g., Hinz et al.
(1971), Seibold et al. (1971), Kögler and Larsen (1979),
Winterhalter (1992), Huckriede and Meischner (1996),
Lepland and Stevens (1998).
The redoxcline in the Baltic Sea rose during the last 20
years due to man made eutrophication (cf. Sect. 2.5.1) with
the result that about one third of the Baltic Sea bottom
(:ä5 m of water depth) is now covered by laminated sediments (Jonsson et al. 1990; Matthäus 1995).
substantial inflow of fresh water (Fig. 4.1; cf. Fig.
3.9c). The river water entering the estuary mixes to
some extent with the denser sea water, but eventually
flows out in the form of a surface current into the
open sea. The inflowing sea water in turn enters the
estuary as a bottom current and later mixes with
outflowing fresh water. This type of water exchange
is referred to as estuarine circulation.
The models in Fig. 4.1 show the four principal
cases identified in nature. In a shallow estuary, the
entering river water and sea water may become completely mixed and therefore unstratified (Fig. 4.1b),
but salinity increases from the river inlet toward the
open sea. If mixing of sea water and fresh water is
less effective, which occurs particularly in deep estuaries with a sill, the water body displays a more or
less pronounced and stable density stratification (Fig.
4.1c and d).
It is interesting to note that an increase in river flow also
causes a somewhat higher inflow of sea water. However,
the salinity of the estuarine water is reduced under such
conditions. Estuaries display either vertically mixed or
stratified waters. The latter type is controlled by the markedly differing salt contents of the outflowing and inflowing
waters, in contrast to the open sea where (smaller) density
gradients are generated primarily by differences in water
temperature.
A stable stratification is typical for deep fjords, as for example along the Norwegian coast. A large river inflow can
create a fresh water layer over the total length of the estuary, which is underlain by a wedge-like salt-water body. As
a result of some mixing of surfaceand bottom water, the
outflowing surface current transports more water into the
sea than the entering river into the estuary. Thus, the outflow at the mouth of the estuary can be ten or more times
the volume ofthe river flow.
The sedimentation rate of terrigenous material tends
to be generally high in estuaries, reflecting the suspended load of many rivers. It is, however, difficult
to develop a simple facies model for estuaries. An
individual estuary may pass from one circulation system into another as a result of strongly variable river
discharge. It may be affected by tides and tidal currents (cf. Sect. 3.2.1), causing coastal erosion, reworking and redeposition of sediments in both landward and seaward directions. Supported by fluvial
nutrient supply, organic productivity in estuaries is
commonly high, but the number of species is restricted in brackish water subjected to frequent
changes in salinity. River-bome plant debris and marine fauna entering the estuary often become mixed
in the estuarine sediment. Reworking and
redeposition of sediment are common in shaIlow estuaries, whereas organic matter and fossil remains
have a good chance of being preserved in deeper estuaries due to rapid sediment accumulation. Where
estuarine water masses are stratified (Fig. 4.1 c), the
167
oxygen content of bottom water may drop and hamper benthic life.
4.2.2 Adjacent Seas with Estuarine Water
Circulation
In adjacent seas larger than estuaries, evaporation
from the water surface of the basin becomes an important factor (Fig. 4.2). In humid climates, precipitation, P, and river discharge, R, into the basin are
commonly greater than water loss by evaporation, E.
This situation is similar to that observed in estuaries.
Therefore the term estuarine circulation is also used
for such "humid basins".
Present-day examples of this basin type are the
Baltic Sea and the Black Sea (Figs. 4.2a and 4.4).
They are characterized by a narrow, shallow entrance, allowing the denser sea water to enter the basin as subsurface inflow, while .the less saline water
from the basin flows as surface current out into the
sea.
The Gulf of Califomia also displays an estuarine circulation. However, in contrast to the previous examples, its
opening to the Pacific is wide and deep and therefore
readily allows subsurface inflow of cold and comparatively
dense intermediate ocean water (Fig. 4.5, see Sect. 4.3 for
further details).
Baltic Sea. This fairly shallow basin (average depth
about 100 m) roughly corresponds to the vertically
mixed estuary model (Fig. 4.lb). Its water mass is
largely unstratified and was weIl oxygenated in preindustrial times, apart from some deeps with water
depths up to 460 m. The salinity of the Baltic Sea
(usually less than 10 to 15%0) increases from the
landward side toward the opening(s) to the normal
saline North Sea. Consequently, stagnant stratified
waters only developed in some marginal bays and
deep depressions in the central part of the basin.
Nutrients for aquatic life are delivered by rivers
from the neighboring land areas as weIl as by erosion
of sediments in the coastal zones. Production of organic matter can be high, and some of it is preserved
in the sediments, particularly in the deeps mentioned
above where benthic life is reduced or missing due to
oxygen deficiency. Similar depositional conditions
are also realized in many deep fjords along formerly
glaciated co asts where freshwater enters the sea.
For more details about the Baltic Sea see, e.g., Hinz et al.
(1971), Seibold et al. (1971), Kögler and Larsen (1979),
Winterhalter (1992), Huckriede and Meischner (1996),
Lepland and Stevens (1998).
The redoxcline in the Baltic Sea rose during the last 20
years due to man made eutrophication (cf. Sect. 2.5.1) with
the result that about one third of the Baltic Sea bottom
(:ä5 m of water depth) is now covered by laminated sediments (Jonsson et al. 1990; Matthäus 1995).