Geology Reference
In-Depth Information
(i.e., where sediments may be stored for short time
periods, less than a year, and subject to later rework-
ing). In contrast to tide-dominated estuaries where the
fl ux convergence of suspended sediment tends to be
located near the apex of the rivermouth embayment,
the convergence in tide-dominated deltas is typically
near the mouth of the embayment or slightly seaward
(e.g. Fly, Ganges-Brahmaputra, Changjiang; Dalrymple
and Choi
2007
). The convergence may shift many
kilometers upstream during the low-discharge dry sea-
son (Wolanski et al.
1996
), but, with the majority of
sediment delivered during high river fl ow, the wet-season
transport regime is more important to delta evolution.
In an extreme case the Amazon River, with more dis-
charge than any other river on Earth, actually forces its
tide-river fl ux convergence 60-90 km offshore onto
the middle continental shelf where most sediment
accumulates in the subaqueous clinothem (Kuehl et al.
1986
; Nittrouer et al.
1986
) . Although depositional
patterns here are strongly tide infl uenced, no saltwater
enters the Amazon rivermouth at any time of the year
despite a spring tidal range of ~7 m. Finally, it is
important to note that the location of fl ux convergence
for coarser-grained bedload may lie considerably
landward of that for suspended load (Montaño and
Carbajal
2008
) .
sediments are suspended in the water column. Thus,
along lower-energy margins where suspended sedi-
ment concentrations are comparatively low and much
of the sediment is relatively coarse (i.e., sand-sized)
bedload, time-averaged residual fl ows may not be
important to overall morphologic development. However,
on high-energy, tide-dominated deltaic margins where
suspended-sediment concentrations are consistently
high, the weak but persistent residual fl ows may
account for much of the long-term net sediment trans-
port and resulting morphological evolution of the
rivermouth delta and adjacent tidal delta plain.
7.3.3
Marine Processes
The large rivers that feed most modern tide-dominated
deltas export much of their sediment load to the shelf,
where it is subject to a suite of marine processes - tides,
waves, storms, geostrophic currents - that ultimately
defi ne the morphology and development of the sub-
aqueous portion of the delta (Walsh and Nittrouer
2009
). Often the greatest effect of these processes on
sediment dispersal and development of the subaqueous
delta occurs when they are coincident with high river
discharge. Complex, non-linear interactions that
emerge during high-energy stochastic events (e.g.,
storms, fl oods) may account for large-scale transport
and redistribution of fi ne-grained sediment to all por-
tions of the delta, but has been demonstrated to be
especially important to offshore transport (e.g. Ogston
et al.
2000
). In this case the importance of such off-
shore mud transport has long been recognized (Swift
et al.
1972
) from the widespread occurrence of accret-
ing mud wedges on the shelf, but the mechanisms of
such transport remained uncertain and controversial
until recently (Hill et al.
2007
). In the past two decades
direct instrumental observations have revealed the reg-
ular occurrence of gravity-driven cross-shelf transport
occurring off the mouths of most of the world's major
rivers (Wright and Friedrichs
2006
) . This transport
phenomenon, which is generated by the interaction of
fl uvial and marine processes, shares many of the same
conditions shown to be necessary for the development
of a subaqueous muddy clinothem (Swenson et al.
2005
) , and probably defi nes much of the shelf mor-
phology found offshore of large rivers in high-energy
settings.
7.3.2.2 Residual Flow
The second important interaction of river discharge
with tidal hydrodynamics is that river fl ow, at least sea-
sonally, dominates the residual fl ow in tide-dominated
deltas. Residual fl ow is the resultant current vector
(i.e., 'net drift') that emerges from averaging all fl ow
components (tidal, fl uvial, and marine) over a period
of weeks to a year. Residual fl ow can be diffi cult to
determine from short-term instrumental deployments
because of the dominance of non-steady synoptic-scale
forces (e.g., waves, storms, fl ood discharge), and thus
results may differ depending on the time-scale over
which observations or calculations are made.
Ultimately, though, it is the asymmetry in tidal cur-
rents and the unidirectional fl ow of river discharge that
tend to generate residual fl ows and dominate the net
fl uid
transport in tide-dominated delta systems (Barua
et al.
1994
) .
Because residual fl ow is a purely fl uid transport
phenomenon, its role in sediment transport will vary
depending on the timing, magnitude, and duration that
