(1960), Morgan (1970), Wright and Coleman (1973), Galloway (1975), Coleman (1976), Reineck and Singh (1980),
Elliott (1986), Nemec (1990), Postma (1990), and Hori
and Saito (2003).
Factors determining types of deltas in estuaries
Depending on the size and shape of the estuary,
a delta within an estuary can be variable in terms of plan
geometry (morphology), landforms within the delta,
sedimentary facies, and stratigraphy. The main factors
determining the morphology and landforms of deltas in
estuaries are (1) seasonality and strength of river flow,
(2) the salinity of the receiving estuarine basin, (3) the
magnitude of tides, (4) wind and wind waves, and
(5) the shape of the estuary and where the river(s) is/are
located. A number of these factors are interrelated and
combine to produce a given delta type or delta form.
For instance, the seasonality and strength of river flow
can affect the salinity of the receiving basin in that strong
perennial river flow will ensure that the receiving basin is
perennially brackish, particularly where the tidal regime
is microtidal. Similarly, the extent to which wind and
wind waves can influence delta morphology and
landforms can be dependent on the external shape of the
estuary and the location of the river(s) in relation to the
wind field and wave field.
The seasonality and strength of the river flow determines whether the delta will be fluvial dominated, tide
dominated, or wave dominated. Perennial rivers, deriving
from large drainage basins in humid climates, with strong
river flow (and, commonly, concomitant strong sediment
transport), produce fluvial-dominated conditions at the
river outlet. In this setting, delta morphology, controlled
by fluvial conditions, tends generally to be fan-shaped
varying to elongate and digitate. With fluvial-dominated
conditions, the salinity of the estuarine-receiving basin
also can play an important part in determining the style
and course of river flow into the estuary and hence the
shape of any deltaic sedimentary accumulation. In this
context, it should also be noted that the dynamics of
sediment-laden river flow with its various amounts of traction load and/or suspension load entering and interacting
with an estuarine-receiving water body of different density
(ranging from fresh to brackish to marine) will result in
different types of deltaic depositional morphology. Bates
(1953), Wright (1978), and Orton and Reading (1993)
describe this variability of depositional styles and
resulting delta forms in relationship to three situations:
(1) hypopycnal flows in which density of the suspended
sediment flow is less than that of the receiving estuarine
water body, (2) homopycnal flows in which density of
the suspended sediment flow is equal to that of the receiving estuarine water body, and (3) hyperpycnal flows in
which density of the suspended sediment flow is more
than that of the receiving estuarine water body.
Hypopycnal, homopycnal, or hyperpycnal flows also
determine the nature of river mouth dynamics as to
whether buoyant, inertial, or frictional factors are dominant in distributing and shaping the sediment plume and
sand bars (Bates, 1953; Coleman, 1976; Wright, 1978)
and the shape of any freshwater jet as it enters a more
saline estuary (Wright, 1978).
Riverine freshwater flowing into an estuarine basin of
denser brackish water or marine salinity will exhibit
hypopycnal flow, with freshwater overlying the denser
estuarine water. Riverine freshwater flowing into an estuarine basin of similar freshwater, or turbid freshwater
flowing into brackish water, will exhibit homopycnal
flow, with the river water invading the estuarine water of
similar density in a turbulent mixing front. At the other
extreme, sediment-laden turbid riverine freshwater
flowing into an estuarine basin of freshwater or weakly
brackish salinity will exhibit hyperpycnal flow, with the
denser sediment-laden river flow (comprising sediment
in suspension and transported in traction) forming a base
flow under the less-dense estuarine water.
However, where riverine input is seasonal, or where the
sediment-transporting river flow is inter-annual, tides and
wind waves will predominate as the formative agents in
delta type and in the development of its plan geometry.
The flux of tides on a daily or semidiurnal basis can have
a prevailing influence on determining delta shape, and
river sediment delivered to the mouth of the river in
a mesotidal or macrotidal estuary will be redistributed
and sculptured by tidal currents and shaped into tidalcurrent-elongated shoals. In regions with strong winds,
wind waves are generated on estuarine water bodies and
impinge on delta fronts. Depending on fetch, and particularly if the river mouth is downwind in an estuary with
a large fetch, deltaic sediments deposited at the river
mouth will be subject to prevailing wind waves. As
a result, wave-dominated deltas will develop.
The shape of the estuary and position of the river
(s) within it play important roles in determining delta
morphology. Relatively simple estuaries, that are
v-shaped, narrow linear valley tracts with a single river
mouth at the head of the estuary, are subject to interactions of river flow, tidal flux, and wind waves, and the
delta developed at the estuary head will tend to be the
form indicative of the locally dominant hydrodynamic
condition. Complex estuaries and large estuaries with
large fetch, on the other hand, can create conditions
where there are complicated hydrodynamics of waves
and wind-induced currents, and in situations where there
is more than one river entering the estuary, each river
may be subject to differing hydrodynamics. There will
also be differences in the deltas where the rivers have dissimilar flow magnitudes. Within the one estuarine basin
with multiple river inflows and multiple deltas, one delta
may be fluvial dominated; others may be tide dominated
or wave dominated. The deltas in these types of complex
estuaries are even more variable if the various contributing rivers arise from different geological provenances in
their respective hinterland and are delivering different
suites of sediments.
DELTAS
179
Elliott (1986), Nemec (1990), Postma (1990), and Hori
and Saito (2003).
Factors determining types of deltas in estuaries
Depending on the size and shape of the estuary,
a delta within an estuary can be variable in terms of plan
geometry (morphology), landforms within the delta,
sedimentary facies, and stratigraphy. The main factors
determining the morphology and landforms of deltas in
estuaries are (1) seasonality and strength of river flow,
(2) the salinity of the receiving estuarine basin, (3) the
magnitude of tides, (4) wind and wind waves, and
(5) the shape of the estuary and where the river(s) is/are
located. A number of these factors are interrelated and
combine to produce a given delta type or delta form.
For instance, the seasonality and strength of river flow
can affect the salinity of the receiving basin in that strong
perennial river flow will ensure that the receiving basin is
perennially brackish, particularly where the tidal regime
is microtidal. Similarly, the extent to which wind and
wind waves can influence delta morphology and
landforms can be dependent on the external shape of the
estuary and the location of the river(s) in relation to the
wind field and wave field.
The seasonality and strength of the river flow determines whether the delta will be fluvial dominated, tide
dominated, or wave dominated. Perennial rivers, deriving
from large drainage basins in humid climates, with strong
river flow (and, commonly, concomitant strong sediment
transport), produce fluvial-dominated conditions at the
river outlet. In this setting, delta morphology, controlled
by fluvial conditions, tends generally to be fan-shaped
varying to elongate and digitate. With fluvial-dominated
conditions, the salinity of the estuarine-receiving basin
also can play an important part in determining the style
and course of river flow into the estuary and hence the
shape of any deltaic sedimentary accumulation. In this
context, it should also be noted that the dynamics of
sediment-laden river flow with its various amounts of traction load and/or suspension load entering and interacting
with an estuarine-receiving water body of different density
(ranging from fresh to brackish to marine) will result in
different types of deltaic depositional morphology. Bates
(1953), Wright (1978), and Orton and Reading (1993)
describe this variability of depositional styles and
resulting delta forms in relationship to three situations:
(1) hypopycnal flows in which density of the suspended
sediment flow is less than that of the receiving estuarine
water body, (2) homopycnal flows in which density of
the suspended sediment flow is equal to that of the receiving estuarine water body, and (3) hyperpycnal flows in
which density of the suspended sediment flow is more
than that of the receiving estuarine water body.
Hypopycnal, homopycnal, or hyperpycnal flows also
determine the nature of river mouth dynamics as to
whether buoyant, inertial, or frictional factors are dominant in distributing and shaping the sediment plume and
sand bars (Bates, 1953; Coleman, 1976; Wright, 1978)
and the shape of any freshwater jet as it enters a more
saline estuary (Wright, 1978).
Riverine freshwater flowing into an estuarine basin of
denser brackish water or marine salinity will exhibit
hypopycnal flow, with freshwater overlying the denser
estuarine water. Riverine freshwater flowing into an estuarine basin of similar freshwater, or turbid freshwater
flowing into brackish water, will exhibit homopycnal
flow, with the river water invading the estuarine water of
similar density in a turbulent mixing front. At the other
extreme, sediment-laden turbid riverine freshwater
flowing into an estuarine basin of freshwater or weakly
brackish salinity will exhibit hyperpycnal flow, with the
denser sediment-laden river flow (comprising sediment
in suspension and transported in traction) forming a base
flow under the less-dense estuarine water.
However, where riverine input is seasonal, or where the
sediment-transporting river flow is inter-annual, tides and
wind waves will predominate as the formative agents in
delta type and in the development of its plan geometry.
The flux of tides on a daily or semidiurnal basis can have
a prevailing influence on determining delta shape, and
river sediment delivered to the mouth of the river in
a mesotidal or macrotidal estuary will be redistributed
and sculptured by tidal currents and shaped into tidalcurrent-elongated shoals. In regions with strong winds,
wind waves are generated on estuarine water bodies and
impinge on delta fronts. Depending on fetch, and particularly if the river mouth is downwind in an estuary with
a large fetch, deltaic sediments deposited at the river
mouth will be subject to prevailing wind waves. As
a result, wave-dominated deltas will develop.
The shape of the estuary and position of the river
(s) within it play important roles in determining delta
morphology. Relatively simple estuaries, that are
v-shaped, narrow linear valley tracts with a single river
mouth at the head of the estuary, are subject to interactions of river flow, tidal flux, and wind waves, and the
delta developed at the estuary head will tend to be the
form indicative of the locally dominant hydrodynamic
condition. Complex estuaries and large estuaries with
large fetch, on the other hand, can create conditions
where there are complicated hydrodynamics of waves
and wind-induced currents, and in situations where there
is more than one river entering the estuary, each river
may be subject to differing hydrodynamics. There will
also be differences in the deltas where the rivers have dissimilar flow magnitudes. Within the one estuarine basin
with multiple river inflows and multiple deltas, one delta
may be fluvial dominated; others may be tide dominated
or wave dominated. The deltas in these types of complex
estuaries are even more variable if the various contributing rivers arise from different geological provenances in
their respective hinterland and are delivering different
suites of sediments.
DELTAS
179