deposition (Bates, 1953). Fluvial processes are particularly
important in deltaic environments where wave and tide
regimes are less important. In such environments the primary riverine factors, influencing the nature of deltaic
deposition, are river discharge, sediment load, and textural
character of the sediment. Generally, the primary forces
controlling the character of deltaic sedimentation are
(1) the riverine effluent inertia and diffusion, (2) friction
between the sediment load and the floor of the receiving
basin, and (3) density contrasts between the effluent and
the receiving basin waters (Bates, 1953; Wright, 1977;
Orton and Reading, 1993).
Homopycnal conditions Homopycnal flow describes
flow conditions where the density contrast between riverine and basinal waters is small. Discharge patterns are
influenced primarily by inertia of the riverine water,
which allows the effluent to radially spread into the
receiving basin. As a result of deltaic progradation, finegrained bottomset sediments are overlain by relatively
coarser-grained topset beds and river-mouth bar deposits,
creating a coarsening upward depositional sequence common to deltaic systems. Gilbert (1885) first described this
type of deltaic system associated with the paleo-lakes of
the western USA; hence, they are termed Gilbert-type
deltas.
Hyperpycnal conditions Hyperpycnal conditions occur
when the riverine discharge is denser than the waters of
the receiving basin. Consequently, the sediment-laden
effluent moves along the floor of the receiving basin as a
density current. This type of discharge condition is rare
and typically restricted to deltaic systems rich in silt and
coarse-grained sediments discharging into freshwater
lakes, such as the Rhone River entering Lake Geneva in
Switzerland and Bella Coola fjord delta of British Columbia (Kostashcuk, 1985). Although not common,
hyperpycnal conditions may exist in some deltas for short
periods during early spring, when river water temperatures
are appreciably lower than those in the receiving basin,
contributing to the formation of density currents. This is
usually a seasonal phenomenon associated with spring
and summer meltwater discharges from glaciers.
Hypopycnal conditions Hypopycnal discharge conditions occur when riverine inflow is less dense than the
waters of the receiving basin, a condition commonly present where rivers debouch into marine waters. Most of
the world’s deltas form under hypopycnal conditions. In
hypopycnal settings, as the riverine inflow enters the
receiving basin, it undergoes spreading and expansion
facilitating turbulent mixing resulting in the deposition
of the sediment load. Finer-grained sediments are
transported distally into the receiving basin (prodelta) as
the freshwater sediment plume disperses above relatively
more dense basinal waters. The relatively coarser-grained
sediments are deposited in close proximity to the river
mouth (distributary mouth bars and delta front). In
shallow-water hypopycnal discharge conditions, friction
between the receiving basin bottom and the riverine leads
to multiple bifurcations of the distributary system. Conversely, in deepwater during hypopycnal discharge conditions, buoyancy dominates during the dispersal of the
effluent, producing elongated distributaries such as those
present at the modern Mississippi River depocenter
(Suter, 1994).
Channel processes
Deltas grow by utilizing their distributary channel networks through a series of successive bifurcations,
whereby a channel splits into two. Bifurcations are caused
by sediment deposition at the mouths of distributary channels resulting in flow around a bar. Each bifurcation can be
stable, whereby both channels receive the same flow, or
unstable, where one channel receives greater flow than
the other. At each bifurcation, sedimentation produces a
distributary mouth bar that eventually becomes
subaerial, thereby stabilizing the channel position. With
each successive bifurcation, channels become smaller in
both width and depth as they approach the delta front. In
a study of 11 deltas worldwide, Edmonds and Slingerland
(2007) found that channel widths decrease nonlinearly
with bifurcation order, suggesting that across the delta
there is continuous reduction in channel geometry. However, this should not be interpreted as loss of stream
power, because deltas can maintain channel efficiency
such that their ability to transport sediment downstream
remains unaltered. A case study by Esposito et al. (2013)
showed that the loss in dimensions with each bifurcation
is met with an increase in water surface slope, resulting
in unchanged velocity in the channels. This evolution
can be significant in the delta growth cycle, because efficient channels can extend via depositional processes
throughout the change in stage of the river (Esposito
et al., 2013) or can extend during low flow conditions by
incising through the delta front. Shaw and Mohrig
(2014) reported that the channels of the Wax Lake Delta
in Louisiana, USA, have extended seaward primarily
through channel incision during low flow conditions,
rather than solely by deposition.
Delta morphology
Although sediment delivery to a delta is via a fluvial system, it is ultimately the dynamic interaction of riverine
and marine basinal processes that control the morphologic, sedimentologic, and stratigraphic variability of a
deltaic accumulation. Water depth and basinal configuration, tidal range, wave climate, and coastal currents are
the primary basinal processes that control delta morphology. The dynamics of these processes and complex interaction between them lead to a highly variable array of
deltaic configurations (Coleman and Wright, 1975).
One of the first and simplest attempts to describe deltaic
variability as a function of processes depicts the morphology of river deltas as a function of sediment influx and
DELTAS
177
important in deltaic environments where wave and tide
regimes are less important. In such environments the primary riverine factors, influencing the nature of deltaic
deposition, are river discharge, sediment load, and textural
character of the sediment. Generally, the primary forces
controlling the character of deltaic sedimentation are
(1) the riverine effluent inertia and diffusion, (2) friction
between the sediment load and the floor of the receiving
basin, and (3) density contrasts between the effluent and
the receiving basin waters (Bates, 1953; Wright, 1977;
Orton and Reading, 1993).
Homopycnal conditions Homopycnal flow describes
flow conditions where the density contrast between riverine and basinal waters is small. Discharge patterns are
influenced primarily by inertia of the riverine water,
which allows the effluent to radially spread into the
receiving basin. As a result of deltaic progradation, finegrained bottomset sediments are overlain by relatively
coarser-grained topset beds and river-mouth bar deposits,
creating a coarsening upward depositional sequence common to deltaic systems. Gilbert (1885) first described this
type of deltaic system associated with the paleo-lakes of
the western USA; hence, they are termed Gilbert-type
deltas.
Hyperpycnal conditions Hyperpycnal conditions occur
when the riverine discharge is denser than the waters of
the receiving basin. Consequently, the sediment-laden
effluent moves along the floor of the receiving basin as a
density current. This type of discharge condition is rare
and typically restricted to deltaic systems rich in silt and
coarse-grained sediments discharging into freshwater
lakes, such as the Rhone River entering Lake Geneva in
Switzerland and Bella Coola fjord delta of British Columbia (Kostashcuk, 1985). Although not common,
hyperpycnal conditions may exist in some deltas for short
periods during early spring, when river water temperatures
are appreciably lower than those in the receiving basin,
contributing to the formation of density currents. This is
usually a seasonal phenomenon associated with spring
and summer meltwater discharges from glaciers.
Hypopycnal conditions Hypopycnal discharge conditions occur when riverine inflow is less dense than the
waters of the receiving basin, a condition commonly present where rivers debouch into marine waters. Most of
the world’s deltas form under hypopycnal conditions. In
hypopycnal settings, as the riverine inflow enters the
receiving basin, it undergoes spreading and expansion
facilitating turbulent mixing resulting in the deposition
of the sediment load. Finer-grained sediments are
transported distally into the receiving basin (prodelta) as
the freshwater sediment plume disperses above relatively
more dense basinal waters. The relatively coarser-grained
sediments are deposited in close proximity to the river
mouth (distributary mouth bars and delta front). In
shallow-water hypopycnal discharge conditions, friction
between the receiving basin bottom and the riverine leads
to multiple bifurcations of the distributary system. Conversely, in deepwater during hypopycnal discharge conditions, buoyancy dominates during the dispersal of the
effluent, producing elongated distributaries such as those
present at the modern Mississippi River depocenter
(Suter, 1994).
Channel processes
Deltas grow by utilizing their distributary channel networks through a series of successive bifurcations,
whereby a channel splits into two. Bifurcations are caused
by sediment deposition at the mouths of distributary channels resulting in flow around a bar. Each bifurcation can be
stable, whereby both channels receive the same flow, or
unstable, where one channel receives greater flow than
the other. At each bifurcation, sedimentation produces a
distributary mouth bar that eventually becomes
subaerial, thereby stabilizing the channel position. With
each successive bifurcation, channels become smaller in
both width and depth as they approach the delta front. In
a study of 11 deltas worldwide, Edmonds and Slingerland
(2007) found that channel widths decrease nonlinearly
with bifurcation order, suggesting that across the delta
there is continuous reduction in channel geometry. However, this should not be interpreted as loss of stream
power, because deltas can maintain channel efficiency
such that their ability to transport sediment downstream
remains unaltered. A case study by Esposito et al. (2013)
showed that the loss in dimensions with each bifurcation
is met with an increase in water surface slope, resulting
in unchanged velocity in the channels. This evolution
can be significant in the delta growth cycle, because efficient channels can extend via depositional processes
throughout the change in stage of the river (Esposito
et al., 2013) or can extend during low flow conditions by
incising through the delta front. Shaw and Mohrig
(2014) reported that the channels of the Wax Lake Delta
in Louisiana, USA, have extended seaward primarily
through channel incision during low flow conditions,
rather than solely by deposition.
Delta morphology
Although sediment delivery to a delta is via a fluvial system, it is ultimately the dynamic interaction of riverine
and marine basinal processes that control the morphologic, sedimentologic, and stratigraphic variability of a
deltaic accumulation. Water depth and basinal configuration, tidal range, wave climate, and coastal currents are
the primary basinal processes that control delta morphology. The dynamics of these processes and complex interaction between them lead to a highly variable array of
deltaic configurations (Coleman and Wright, 1975).
One of the first and simplest attempts to describe deltaic
variability as a function of processes depicts the morphology of river deltas as a function of sediment influx and
DELTAS
177