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Sediment Storage, Sea Level, and Sediment Delivery to the Ocean by Coastal Plain Rivers


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Coastal and marine sedimentary archives are sometimes used as indicators of changes in continental sediment production and fluvial sediment transport, but rivers crossing coastal plains may not be efficient conveyors of sediment to the coast. Where this is the case, changes in continental sediment dynamics are not evident at the river mouth. Stream power is typically low and accommodation space high in coastal plain river reaches, resulting in extensive alluvial storage upstream of estuaries and correspondingly low sediment loads at the river mouth. In some cases there is a net loss of sediment in lower coastal plain reaches, so that sediment input from upstream exceeds yield at the river mouth. The lowermost sediment sampling stations on many rivers are too far upstream of the coast to represent lower coastal plain sediment fluxes, and thus tend to overestimate sediment yields. Sediment which does reach the river mouth is often trapped in estuaries and deltas. Assessment of sediment flux from coastal plain rivers is also confounded by the deceptively simple question of the location of the mouth of the river. On low-gradient coastal plains and shelves, the location of the river mouth may have varied by hundreds of kilometers due to sea-level change. The mouth may also differ substantially according to whether it is defined based on channel morphology, network morphology, hydrographic or hydrochemical criteria, elevation of the channel relative to sea level, or the locus of deposition. Further, while direct continent-to-ocean flux may be very low at current sea-level stands, sediment stored in estuaries and lower coastal plain alluvium (including deltas) may eventually become part of the marine sedimentary package. The role of accommodation space in coastal plain alluvial sediment storage has been emphasized in previous work, but low transport capacity controlled largely by slope is also a crucial factor, as we illustrate with examples from Texas.
Content may be subject to copyright.
Progress in Physical Geography 30, 4 (2006) pp. 513–530
© 2006 SAGE Publications 10.1191/0309133306pp494ra
I Introduction
Coastal and marine basins are the ultimate
sink for most riverborne sediment, and rivers
are the major source of ocean sediment.
Coastal and marine sedimentary archives
thus represent a potential record of changes
Sediment storage, sea level, and
sediment delivery to the ocean by
coastal plain rivers
Jonathan D. Phillips
and Michael C. Slattery
Tobacco Road Research Team, Department of Geography, University of
Kentucky, Lexington, KY 40506, USA
Institute of Environmental Studies and Department of Geology,
Texas Christian University, Fort Worth, TX 76129, USA
Abstract: Coastal and marine sedimentary archives are sometimes used as indicators of changes
in continental sediment production and fluvial sediment transport, but rivers crossing coastal plains
may not be efficient conveyors of sediment to the coast. Where this is the case, changes in
continental sediment dynamics are not evident at the river mouth. Stream power is typically low
and accommodation space high in coastal plain river reaches, resulting in extensive alluvial storage
upstream of estuaries and correspondingly low sediment loads at the river mouth. In some cases
there is a net loss of sediment in lower coastal plain reaches, so that sediment input from upstream
exceeds yield at the river mouth. The lowermost sediment sampling stations on many rivers are
too far upstream of the coast to represent lower coastal plain sediment fluxes, and thus tend to
overestimate sediment yields. Sediment which does reach the river mouth is often trapped in
estuaries and deltas. Assessment of sediment flux from coastal plain rivers is also confounded by
the deceptively simple question of the location of the mouth of the river. On low-gradient coastal
plains and shelves, the location of the river mouth may have varied by hundreds of kilometers due
to sea-level change. The mouth may also differ substantially according to whether it is defined
based on channel morphology, network morphology, hydrographic or hydrochemical criteria,
elevation of the channel relative to sea level, or the locus of deposition. Further, while direct
continent-to-ocean flux may be very low at current sea-level stands, sediment stored in estuaries
and lower coastal plain alluvium (including deltas) may eventually become part of the marine
sedimentary package. The role of accommodation space in coastal plain alluvial sediment storage
has been emphasized in previous work, but low transport capacity controlled largely by slope is
also a crucial factor, as we illustrate with examples from Texas.
Key words: alluvial storage, coastal plain rivers, coastal sediments, river mouth, sediment
delivery, sedimentary archives.
*Author for correspondence.
514 Sediment storage, sea level, and sediment delivery to the ocean
in river sediment transport and continental
sediment production driven by human
impacts, climate change, tectonics, or other
environmental changes. This assumes that
significant changes in erosion and sediment
flux in drainage basins is recorded in sediment
delivery to the coast. However, the connec-
tion between changes in sediment dynamics
within a fluvial system and sediment loads at
the river mouth is not always strong or direct.
The small mountainous drainage basins that
provide (relative to their area) a dispropor-
tionately large proportion of river sediment
yield to the sea (Milliman and Syvitski, 1992;
Ludwig and Probst, 1998) may indeed exhibit
relatively direct connections between
drainage basin sediment dynamics and
coastal/marine sediment inputs. In California,
for instance, dam construction in small moun-
tainous rivers resulted in a quick and notice-
able reduction in sediment supplies to the
coast (Willis and Griggs, 2003). However, in
some rivers sediment output or accumulation
in sedimentary basins has been remarkably
consistent despite changes in climate, sea
level, vegetation, and human impacts
(Summerfield and Hulton, 1994; Gunnell,
1998; Métivier and Gaudemar, 1999; Dearing
and Jones, 2003; Phillips, 2003a). In some
cases this may be because an overriding con-
trol such as relief or tectonic forcing over-
whelms variations in climate and other
factors, but several authors emphasize the
role of alluvial buffering (tivier and
Gaudemar, 1999; Dearing and Jones, 2003;
Phillips, 2003a). Alluvial buffering implies that
sediment storage and timelags may make the
output from some drainage basins slowly or
relatively unresponsive to environmental
change. This is because alluvial storage during
periods of high sediment supply and remobi-
lization when supply is low relative to trans-
port capacity minimize variations in sediment
Geomorphologists have long recognized
(eg, Meade, 1982) that coastal plain rivers
may deliver only a fraction of their sediment
load to the coast, and that sediment gaging
stations (generally well upstream of the
coast) typically overestimate the latter.
Despite this, sediment gaging records con-
tinue to be used to assess factors such as the
potential effects of dams and human land-
scape modifications on sediment delivery to
the sea (Pont et al., 2002; Vorosmarty et al.,
2003; Walling and Fang, 2003). Likewise,
coastal and marine sedimentary archives are
interpreted for evidence of environmental
change within drainage basins (eg, Sandweiss,
2003; Vaalgamaa and Korhola, 2003).
The purpose of this paper is to review the
factors contributing to low sediment output
in coastal plain rivers and complicating efforts
to determine sediment yield to the coast,
with particular emphasis on the deceptively
simple problem of defining the mouth or out-
let of the river. We also seek to explore the
role of transport capacity (in addition to
accommodation space) in alluvial storage and
buffering, with particular emphasis on chan-
nel slope. While some new data is introduced,
this work is based largely on a review of the
literature, and on a reinterpretation of data
from our own previous and ongoing work.
Milliman and Syvitski (1992) point out that
the sediment contributions of large rivers to
the ocean is often overestimated because
most such rivers discharge to passive margins
and marginal seas, with much sediment
sequestered in subsiding deltas. Dearing and
Jones (2003) emphasize the lack of respon-
siveness of large drainage basins to environ-
mental changes, such that continental margin
sedimentary archives may be poor indicators
of such change. In some cases storage bottle-
necks and timelags create an essential decou-
pling, such that changes in sediment regimes
in the upper basin are simply not reflected in
the lower river reaches (Brizga and Finlayson,
1994; Olive et al., 1994; Phillips, 1995; 2003a;
2003b; Fryirs and Brierly, 1999; 2001; Shi
et al., 2003; Phillips et al., 2004).
This implies, at least for large rivers on
passive margins, that recent trends in river
sediment loads discussed by Walling and Fang
(2003), however important they may be
Jonathan D. Phillips and Michael C. Slattery 515
upstream, may have minimal impact on land-
to-ocean sediment fluxes. Vörösmarty et al.
(2003) document extensive sediment reten-
tion by dams and the potential major global
impact on sediment delivery to the sea,
agreeing with Walling and Fang (2003) that
dams are the single most important contem-
porary impact on river sediment transport,
and echoing Graf (1999), who pointed out
that hydrologic impacts of dams far exceed
those of even the most extreme predictions of
climate change. If upper and lower basins are
decoupled, however, dam impacts may not be
significant at the river mouth, however signif-
icant the effects upstream. In southeast
Texas, for instance, even where large reser-
voirs controlling 75–95% of the drainage area
retain massive amounts of sediment behind
dams, no dam-related changes in alluvial
sedimentation are noticeable in the lower-
most river reaches (Phillips, 2003b; Phillips
et al., 2004).
Though large alluvial rivers transport a dis-
proportionately small amount of sediment per
unit drainage area compared to steeper,
smaller rivers draining directly to the coast, an
overwhelming total amount of the global flu-
vial sediment transport is by large alluvial
rivers, many of which cross extensive coastal
plains. Understanding sediment delivery in
coastal plain rivers is therefore a prerequisite
to understanding fluvial sediment inputs to
marine environments, the fate of sediments in
alluvial rivers, and the extent to which coastal
and marine sedimentary archives reflect
upstream changes in sediment production and
transport. We focus here on relatively large
rivers with headwaters above the coastal plain
that cross extensive coastal plains, though the
generalizations also apply to streams confined
to coastal plain environments.
Some coastal plain rivers do have high sed-
iment loads and delivery ratios (the Amazon,
for example), and the mouths of some rivers
may respond relatively rapidly to upstream
changes. The Atchafalaya River, Louisiana,
and the Colorado River, Texas, for instance,
have begun building new deltas in historic or
recent times, while sediment supply to the
Nile delta was severely reduced by the
Aswan high dam and by irrigation and
drainage canals (Stanley, 1996). The focus
here, however, is on coastal plain rivers in
which sediment yield at the mouth may be
very low, due to our impression that the
extensive alluvial sediment storage in the
lower reaches of coastal plain rivers has been
underappreciated, and that the delivery of
sediment to the coast is frequently overesti-
mated. We do not claim all coastal plain rivers
have low rates of sediment delivery to the
ocean, or that delivery rates are constant in
even the broadest sense. Our focus is simply
to address the issue of why so many coastal
plain rivers transport so little sediment to the
II Sediment storage, sinks,
and transport
Three factors seem to be critical in explaining
low sediment delivery ratios in coastal plain
rivers: estuarine, deltaic, and coastal wetland
sediment trapping; alluvial storage upstream
of estuaries; and low sediment transport
1 Estuaries as sediment sinks
Estuaries, deltas, and coastal wetlands may
be effective sediment traps. In the US
Atlantic drainage, Meade (1982) considers
estuaries and marshlands to be the ultimate
sink for river sediments, at least on a millen-
nial timescale. He estimates that certainly
less than 10%, and probably less than 5%, of
river sediment reaches the continental shelf
or deep ocean. Zhang et al. (1990) found that
estuarine sinks lead to a similarly small portion
of sediment from the Huanghe River reaching
the sea. Milliman and Syvitski (1992) note
that fluxes to oceans from large rivers (nearly
all of which discharge onto passive margins or
marginal seas) are overestimated by data from
gaging stations due to sediments sequestered
in subsiding deltas.
Radionuclide tracers show that the upper
Chesapeake Bay (USA) within and above the
516 Sediment storage, sea level, and sediment delivery to the ocean
turbidity maximum retains nearly all the flu-
vial sediment delivered to it (Donoghue et al.,
1989), and Colman et al. (1992) concur that
the vast majority of fluvial sediment from the
Susquehanna River is stored in the upper bay.
Colman et al. (1992) speculate that this may
move down-bay as the upper bay fills, and
generally frame their discussion of Holocene
sedimentation in the Chesapeake in the con-
text of the geologically ephemeral, rapidly
changing nature of coastal plain estuaries, a
theme that will be addressed below. Though
fluvial inputs are small compared to shore
erosion, Marcus and Kearney (1991) found
that sediment from the South River,
Maryland, is predominantly trapped in river
mouth marshes and subtidal storage within a
kilometer of the river mouth.
A sediment budget for the tidal fluvial/
estuarine transition zone of the Raritan River,
New Jersey, shows that the lower river is a
sediment sink for fluvial inputs (Renwick and
Ashley, 1984). The Albemarle-Pamlico estu-
arine system (North Carolina), is an efficient
trap for all particulates (Harned et al., 1995;
McMahon and Woodside, 1997). Though
fluvial inputs to the Neuse River estuary
(a tributary to the Albemarle-Pamlico) are
very low inputs due to extensive storage of
coastal plain alluvium (Phillips, 1993; 1997),
the Neuse is an efficient trap for those partic-
ulates that are delivered (Benninger and
Wells, 1993).
In Texas, while Morton (1994) believes
that barrier islands are predominantly made
of reworked delta sands, he notes that where
rivers discharge into estuaries they do not
directly contribute to barrier island sediment
budgets. This is broadly consistent with
Anderson et al. (1992), who found that south-
east Texas barriers are composed of offshore
sands, largely derived from drowned deltaic
and fluvial deposits of the Trinity and Sabine
Thus, in some cases coastal plain rivers
may contribute little sediment to the ocean
because of trapping in estuarine environ-
ments. This raises the question of the extent
to which upstream changes are reflected in
estuarine sedimentation.
2 Alluvial sediment storage in
coastal plain rivers
Multiple studies have shown that some rivers
deliver relatively little sediment to the coastal
zone, such that sediment delivery to upper
estuaries may be severely limited. Sediment
delivery ratios of total basin erosion to sedi-
ment yield at the outlet of less than 10% on an
average annual basis has been shown for a
number of coastal plain rivers, or upper- and
lower-basin decoupling. These include rivers
of China (Zhang et al., 1990; Shi et al., 2003),
Australia (Brizga and Finlayson, 1994; Olive
et al., 1994; Fryirs and Brierly, 1999; 2001) the
south Atlantic US coastal plain (Phillips, 1991;
1992a; 1992b; 1993; 1997; Slattery et al.,
2002), and Texas Gulf coastal plain (Phillips,
2003b; Phillips et al., 2004). The majority of
the imbalance between sediment production
and sediment yields at the basin mouth is
accounted for by source-to-sink timelags, and
extensive colluvial and alluvial storage. The
well-known relationship between sediment
delivery ratios and drainage area, whereby
the delivery ratio tends to get smaller as
contributing areas increase (Walling, 1983;
Dearing and Jones, 2003), reflects this
Studies of sediment provenance in infilling
estuaries with extensive inland drainage
basins sometimes show that fluvial inputs are
small compared to coastal and marine sedi-
ment sources. This is also indicative of exten-
sive alluvial storage in the rivers feeding the
In tributary estuaries of the Chesapeake
Bay system, for example, shoreline erosion
seems to be a more important sediment
source than rivers. Rapid sedimentation in
upper and central portions of tributary estu-
aries has often been attributed to accelerated
post-European-settlement soil erosion, but
data from Maryland suggest that coastal
shoreline erosion is the dominant sediment
input along the entire length of many tributary
Jonathan D. Phillips and Michael C. Slattery 517
estuaries (Marcus and Kearney, 1991).
Coastal contributions to the South River
estuary sediment budget are 4–12 times
higher than fluvial, and fluvial inputs have lit-
tle direct impact on sediment accumulation in
the majority of the estuary (Marcus and
Kearney, 1991). This is consistent with Yarbro
et al. (1983), whose sediment budget for the
Choptank River estuary showed that shore-
line erosion in the estuary contributes seven
times more sediment than fluvial inputs. Even
in the main Chesapeake estuary, the drowned
valley of the Susquehanna River, Skrabal’s
(1991) analysis of clay minerals shows a dom-
inantly marine sediment source, including
landward sites previously thought to be dom-
inated by river inputs. Colman et al. (1992)
indicate that both the continental shelf and
Susquehanna River are important sediment
sources for the bay, but that over the
Holocene marine sources are probably domi-
nant. The Chesapeake and other Atlantic
drainage estuaries of the USA were found by
Meade (1969) to have landward transport of
bottom sediments, and infilling from marine
sources, up to the limit of upstream bottom
The main sediment source for the inner
Severn River estuary (UK) is probably the
estuary itself, not the river (Hewlett and
Birnie, 1996). In the Savannah River estuary
(USA), isotope tracers show that 65% of
inorganic suspended sediments and the upper
five cm of benthic sediments are of marine
origin (Mullholland and Olsen, 1992). The
percentage is even higher in lower, higher-
salinity portions of the estuary up to 99%
for salinities greater than 14 ppt and
Mullholland and Olsen (1992) emphasize the
importance of the Savannah estuary (the
drowned valley of a large river system), and
other estuaries on submergent coastlines, as
sinks for particles from the coastal ocean.
Chemistry of sediments in the Neuse River
estuary indicates that little modern river sed-
iment is stored there, and that marine sedi-
ment sources and landward transport are
prominent (Benninger and Wells, 1993). In
van Dieman Gulf, northern Australia,
Woodroffe et al. (1993) found that, despite a
drainage basin of !10,000 km
, fluvial sedi-
ment sources were inadequate to account for
observed infill and coastal progradation, and
concluded that the major sediment source is
The point is not to suggest that domi-
nantly coastal and marine sediment sources
are the norm for all estuaries; there are exam-
ples of estuaries with dominantly fluvial
sources, and of estuaries with dominantly
fluvial sources at the upstream and marine at
the downstream ends. Rather, the aim is to
show that, even in estuaries with large fluvial
drainage basins, much of the fluvial sediment
may be stored upstream of the estuary.
3 Sediment transport capacity
While the specific predictive equations for
fluvial sediment transport continue to be con-
troversial and uncertain, it is generally agreed
that (assuming a transport-limited system),
sediment transport is proportional to stream
power, which provides a good index of trans-
port capacity.
Cross-sectional stream power is given by:
" #
Q S (1)
is the specific weight of water, Q is
discharge, and S the slope. This represents
the total transport capacity of the river at a
given cross-section as a rate of energy expen-
diture. The stream power per unit weight of
water is:
Q S/
w d # VS (2)
where A is cross-sectional area, V is mean
velocity, and Q # AV.
While discharge generally increases down-
stream (with exceptions to be discussed
below), large decreases in slope may occur in
the lower reaches of coastal plain rivers. This
occurs due not only to regional slope controls
(ie, crossing a low-gradient, low-relief plain),
but also to the fact that lower river reaches may
be cut to below sea level, and may be subject to
backwater effects from lunar or wind tides.
518 Sediment storage, sea level, and sediment delivery to the ocean
Coastal plain rivers are often perceived as
sluggish, and velocities may indeed be quite
low in lowermost reaches. This could result in
very low unit stream power and consequently
low transport capacity relative to the quantity
of incoming water and sediment. Where
backwater effects occur, discharge can also be
reduced. These effects may extend well
upstream of the head of the estuary. In the
Trinity River, Texas, for example, the channel
thalweg is cut to below sea level as far as
110 km upstream of Trinity Bay (Phillips et al.,
2005). In the James River, Virginia, the tidally
influenced zone includes the entire coastal
plain portion of the River (Nichols et al., 1991).
The Trinity River represents one of the
rare cases where a significant suspended sed-
iment record is available in the lowermost
reaches of the river. The Texas Water
Development Board collected suspended sed-
iment samples at several sites from 1965 to
1989. Sediment loads at the Romayor station,
the lowermost station with suspended sedi-
ment records from the US Geological Survey,
were two orders of magnitude higher than
those at Liberty, downstream where the
channel is cut to below sea level (Phillips
et al., 2004). This generalization applies to
total sediment loads, sediment concentra-
tions, and sediment yields (load per unit area).
Kim’s (1990) two years of measurements of
sediment flux from the Neuse River, North
Carolina, into its upper estuary are about two-
thirds lower than typical yields on coastal
plain tributaries in the region (Phillips, 1995).
The issue of stream power and transport
capacity will be explored further in the case
4 Gaging station locations
River sediment contributions to estuaries and
the sea are generally based on suspended
(and occasionally bedload) sediment records
at the lowermost gaging station at which sed-
iment transport is regularly monitored. These
are often a considerable distance upstream of
the river mouth. In rivers of the Texas coastal
plain, for example (Sabine, Neches, Trinity,
Brazos, and Colorado), gaging stations used
to measure or estimate sediment loading to
the coast range from 54 to 98 km upstream of
the river mouth. As illustrated by the case of
the Trinity, records from these stations will
result in overestimates of sediment loads
delivered to the coast.
Milliman and Syvitski (1992), in a compila-
tion of sediment loads from rivers around the
world, include entries for both the Tar and
Pamlico Rivers, North Carolina. These are
actually the same river; the name changes
near the head of the estuary. The listing for
the Tar River is apparently based on sediment
records from the station at Tarboro, North
Carolina, which shows a mean annual sedi-
ment load of 1.1 % 10
tonnes, and a yield of
2.0 t km
. The listing for the Pamlico
River is apparently based on records from a
station upstream at Louisburg, North
Carolina, which is actually representative
only of the upper, Piedmont portion of the
watershed. This gives an annual load of
2.1 % 10
and a yield of 19 t km
Beyond the name confusion (understandable
in a data compilation and insignificant with
respect to the message of the paper), the
mean annual loads show not only the
expected decline in yields with basin area, but
also a decline in total sediment loads, indicat-
ing an alluvial storage bottleneck between
Louisburg and Tarboro. A similar phenome-
non is reported for the nearby Neuse River by
Simmons (1988).
The storage and load reductions in the
Louisburg-Tarboro reach are likely minor
compared to those further downstream, as
the Tarboro station is about 78 km upstream
of the estuary. Extensive storage in this area,
and upper-lower basin decoupling with
respect to sediment, has been demonstrated
by Phillips (1992a). The streamflow gaging
station 48 km downstream of Tarboro at
Greenville, North Carolina, shows clear evi-
dence of backwater effects and predomi-
nantly wind-driven water level and flow
changes transmitted upstream from the estu-
ary. The gage datum is more than a meter
Jonathan D. Phillips and Michael C. Slattery 519
below sea level. Stations downstream at
Grimesland and Washington, just above and
below, respectively, the head of the estuary,
show frequent negative (upstream) flows.
The amount of sediment passing Greenville,
Grimesland, and Washington in the down-
stream direction would be expected to
decrease substantially compared to the
upstream stations, a conclusion borne out by
pedologic and sedimentological evidence
(Phillips, 1992a).
Another illustration is given by the Yellow
River, China. Xu (2003) indicates that since
the 1970s a decline in sediment yield to the
sea can be linked to precipitation and land-use
change. Xu’s conclusions with respect to sed-
iment yield at the Lijin gaging station, which is
used to reflect river input to the delta and
coast, are sound. But the work of Shi et al.
(2003; see also Zhang et al., 1990) on the sed-
iment budget of the Yellow River delta shows
that loads at Lijin, 53 km upstream of the
delta apex, overrepresent delivery to the
coast. Sediment inputs to the delta are esti-
mated by Shi et al. (2003) based on account-
ing for variable but considerable alluvial
accumulation in the intervening valley that
is, delivery to the delta is less than what
passes Lijin.
III The mouth of the river
In the discussion above, the mouth of the
river has been defined simply as the point at
which a well-defined dominant channel
ceases to exist at an open-water estuary or
delta apex. In the context of land-to-sea sed-
iment fluxes it is worth considering this ques-
tion more carefully. As discussed below, there
are several possible criteria for defining the
river mouth. As the location of these may
vary considerably along the valley, the defini-
tion of the mouth could conceivably result in
large variations in estimates of sediment
export from a fluvial system. This will be dis-
cussed in general terms below, and illustrated
via case studies.
Nichols et al. (1991) indirectly addressed
this issue in their study of the James River,
Virginia. The estuary, a tributary to the
Chesapeake Bay, is subdivided into the bay
mouth, the estuary funnel, and the meander
zone. The mouth of the river could conceiv-
ably be defined as the bay mouth, the upper
end of the estuary funnel (defined on the
basis of salinity, width-depth ratio, and sedi-
mentological criteria), or the upper limit of
the meander zone (upper limit of tidal influ-
ence). Additional mouths could be defined
within the meander zone, based on the high-
discharge tidal limit and channel width/depth
ratio (see Nichols et al., 1991: Figure 13).
The simplest and most intuitive definition
of the river mouth is based on channel mor-
phology, as used above, and is signified by the
end of a distinct, well-defined channel or the
beginning of an open water body. In many
coastal plain rivers, however, this point
may be cut well below sea level, strongly
influenced by lunar and wind tides, and may
be characterized by dominantly coastal
sediments, hydrodynamics, and ecological
Alternatively, the mouth could be defined
on the basis of network rather than channel
morphology, as the point at which the
channel network changes from dominantly
convergent to divergent (distributary). This is
most obviously the case at the apex of a delta,
but may in some cases occur upstream or in
the absence of what would conventionally be
considered a delta.
Hydrographic and hydrologic characteris-
tics could also be used to define the river
mouth. Three logical candidates would be the
point at which the channel bed is cut below
sea level, the upstream limit of tidal influence
(including wind-driven tides) or coastal pond-
ing, or the upstream limit of saltwater pene-
tration (usually specified as some threshold
salinity, typically 0.5 ppt). The tidal and salt-
water criteria must, of course, be referenced
to flow conditions.
Criteria related to sedimentology or sedi-
ment transport/deposition dynamics might
also be employed to define the river mouth.
The turbidity maximum or flocculation zone,
520 Sediment storage, sea level, and sediment delivery to the ocean
which also varies with flow conditions, is
commonly referenced by estuarine ecologists
and sedimentologists. Changes in dominant
bed sediment characteristics (for example
from sand- to mud-dominated) might also be
employed, or changes in the dominant sedi-
ment source as indicated by sedimentological,
chemical, mineral, or alluvial pedological cri-
teria (eg, Nichols et al., 1991; Phillips, 1992a;
1992b; Cattaneo and Steel, 2003). A pro-
nounced increase in sediment storage or
decrease in transport might also be argued to
mark a mouth of the river, though this may be
difficult to pinpoint without detailed field-
These different definitions of the river
mouth, summarized in Table 1, may vary sig-
nificantly in their locations along the channel,
and thus the amount of river sediment deliv-
ered to the mouth could be quite different
depending on the conceptual framework
adopted. Using the head of Trinity Bay as a
reference point, the upstream distance of the
river mouth based on various criteria for the
Trinity River, Texas, is shown in Table 2.
Estimated sediment loads at each point
Table 1 Potential criteria for defining the mouth of a river draining to the coast
Criterion Mouth
1. Channel morphology End of well-defined channel; transition to open
2. Network morphology Transition from dominantly convergent to divergent
3. Channel bed elevation Thalweg cut below sea level
4. Tidal influence Upstream limit of tidal influence or coastal
back-water effects at reference flow condition(s)
5. Salinity Upstream limit of threshold salinity at reference
flow condition(s)
6. Turbidity Turbidity maximum or flocculation zone at
reference flow condition(s)
7. Sedimentology Transition in source or nature of sediments (for
example, sand to mud bed)
8. Sediment transport/storage Transition in sediment transport and storage
dynamics (transport bottleneck)
Table 2 Location of the mouth of the Trinity River, based on various criteria (see
Table 1). Based on maps and field observations. Estimated sediment input based on
data in Phillips et al. (2004)
Criterion Estimated distance upstream Estimated mean annual river
from Trinity Bay (km) sediment input (tonnes % 1000)
1. Channel morphology 0 ''70
2. Network morphology 19.5 '70
3. Channel bed elevation 110 75 to 100
4. Tidal influence 85 to 110 65 to 75
5. Salinity 7 to 85 '70
6. Turbidity 2 to 10 ''70
7. Sedimentology 20 to 30 '70
8. Sediment transport/storage 130 3400
Mean annual sediment load at Liberty # 69,673 t yr
; mean annual sediment load at Romayor # 3,378,461 t yr
Jonathan D. Phillips and Michael C. Slattery 521
Figure 1 (Above) The lower Tar River
and upper Pamlico River Estuary,
between Greenville and Washington,
North Carolina. The estimated location of
the river mouth based on six different
criteria is shown by the numbered lines.
These are: (1) channel morphology
(pronounced widening at the mouth of
Tranters Creek); (2) network morphology
(beginning of distributary subchannels in
the flooplain); (3) salinity (Grimesland
gaging station, where there is a 50% annual
probability of salt wedge penetration);
(4) sediments (transition from domi-
nantly upland/mineral to dominantly
autocthonous organic alluvial soils); (5)
gaging station at Greenville; approximate
upstream limit of regular tidal influence;
(6) approximate point at which channel
bed is cut below sea level. The longitudal
profile of both the channel bed and
water surface is also shown (below),
with the six locations above indicated
above; plus (7) the point just down-
stream of the Tar River Falls where
alluvial sediment storage increases
are inferred from Phillips et al. (2004).
Table 2 suggests that using different defini-
tions of mouth on the Trinity could result in
assessments of sediment export from the
basin varying by up to two orders of magni-
tude. Table 2 indicates that a mouth defined
on the basis of sediment flux and storage may
be of the most fundamental importance, but
could be a somewhat self-fulfilling outcome
given that this ‘mouth’ was defined on the
basis of an identified transition point in sedi-
ment transport and storage dynamics.
However, this transition zone coincides with
a pronounced change in sinuosity that we
have interpreted as representing the
upstream limit of effects of Holocene sea-
level rise (Phillips et al., 2005). Possible
locations of the mouth based on some of the
criteria are shown for the Tar-Pamlico River,
North Carolina, in Figure 1.
However defined, the river mouth is a
dynamic, geologically ephemeral feature. On
a low-gradient coastal plain and shelf, rela-
tively small changes in sea level may result in
large longitudinal shifts in the river mouth,
resulting in a spatial sequence of geomorphic,
hydrologic, and sedimentary environments
that (depending on preservation) may be
recorded in vertical sedimentary sequences
across a broad area of contemporary coastal
plains and shelves (Nichols et al., 1991;
Anderson et al., 1992; Dalrymple et al., 1992;
Congxian, 1993; Nichol et al., 1994; Thomas
and Anderson, 1994; Cattaneo and Steel,
This dynamism and ephemerality point to
the perhaps obvious but important point that
generalizations about sediment fluxes in
coastal plain rivers are contingent on a num-
ber of factors, in particular sea level and the
rate and direction of sea-level change.
IV Case study: southeast Texas
The rivers of the southeast Texas coast
(Figure 2) in general, and the Trinity River
(already used as an illustration above) in partic-
ular, are used to illustrate some the points raised
above. The characteristics of the 46,100 km
522 Sediment storage, sea level, and sediment delivery to the ocean
Trinity drainage basin are described elsewhere
(Phillips et al., 2004; 2005).
1 Stream power in the lower Trinity River
a Methods: Stream power in the lower
Trinity River, in the coastal plain downstream of
Lake Livingston, was examined based on
streamflow gaging data collected by the US
Geological Survey at three stations Liberty,
Romayor, and Goodrich, about 83, 126, and
144 km, respectively, upstream from Trinity
Bay. Mean daily flows from station inception
through the 2003 water year were examined,
comprising 63, 79, and 38 years of record,
Probabilities and return periods (recur-
rence interval, RI) were calculated for the
entire record using the relation
p # 1/RI # m/(n ( 1) where m is the rank of
the event and n is the number of days in the
record, and flows associated with 50, 10, and
1% probabilities were determined. These rep-
resent mean daily instantaneous discharges
having a 50, 10, or 1% chance of being
exceeded on a given day. Flows associated
with recurrence intervals of 1, 2, and 10 years
were also determined. Finally, peak daily
flows were determined for each station for
the October 1994 flood of record, and a mod-
erate November 2002 flood.
Cross-sectional stream power can be com-
puted directly from discharge, but unit stream
power requires velocity, which is not routinely
measured. Thus surface water measurements
(generally 6–12 field measurements per year
over the period of record) were used to develop
velocity-discharge relationships. These data
were experimentally fitted with linear, power,
exponential, logarithmic, and second- and
third-order polynomial functions to obtain the
best fit and highest predictive power as deter-
mined by the R
value. For Liberty, a bimodal
relationship made curve fitting the entire data
set difficult (Figure 3). A linear trend wastted
by hand to the high-flow limb of the curve
most relevant to the reference discharges.
From the velocity-discharge relationships,
the velocity associated with each reference
event was calculated. Cross-sectional and
unit stream powers were calculated using
thalweg slopes based on field surveys of five
bridge cross-sections. These are 0.0002834
for Goodrich, 0.0002508 for Romayor, and
0.00001002 for Liberty.
b Results: The reference discharges gener-
ally decrease downstream from Goodrich to
Romayor, and increase from there to Liberty
(Table 3). The best-fit equations for velocity
were as follows:
Goodrich: V # 2 % 10
– 7 % 10
( 0.0014 Q ( 0.3428 R
# 0.87
Romayor: V # –2 % 10
( 0.0011 Q
( 0.2601 R
# 0.83
Liberty: V # 0.35 ( 0.00013352 Q
The velocities for a given reference event
(Table 3) are similar or identical at the two
upper stations, and are substantially lower at
Liberty. Because channel thalweg slopes were
used in computation, variation in stream power
at a station is entirely proportional to variations
in discharge and velocity. Stream power is
slightly lower at Romayor than Goodrich, but
there are major reductions at Liberty. Increases
Figure 2 Rivers of the southeast
Texas coastal plain, and gaging stations
referred to in the text
Jonathan D. Phillips and Michael C. Slattery 523
in discharge downstream are overwhelmed by
the much-reduced slope, so that cross-sec-
tional stream power is an order of magnitude
lower at Liberty than at Romayor. The differ-
ence is even more pronounced with respect to
unit stream power, where Liberty is about two
orders of magnitude lower. The downstream/
upstream ratios of stream power, discharge,
and slope are shown in Table 4, which illustrates
the strong influence of slope.
The sediment transport bottleneck in the
lower Trinity River can be attributed largely
to the fact that much of the lower river is cut
to below sea level, to the very low slopes,
and to the correspondingly low stream power
and transport capacity. The downstream
trend for normal flows (50% probability)
and the 1994 flood peak are shown in Figure 4.
This points to the critical role of slope, which
is addressed further in the next section.
2 Slope
The extensive storage and low transport in
the lower Trinity River is largely attributable
to very low slopes in the lower river. But this
conclusion is based partly on an assumption
that energy grade slopes reflect the channel
thalweg slopes. It also raises the question of
whether similar trends in slope are apparent
in other coastal plain rivers.
To investigate slope trends, water surface
slopes were calculated from water surface
elevations for coastal plain gaging stations on
the Sabine, Neches, Trinity, Brazos, and
Colorado Rivers, Texas. An arbitrary date
and time was chosen by selecting 00:00
hours on the closest date immediately pre-
ceding the analysis which met two criteria:
(1) higher than average but not rare dis-
charge; and (2) water levels not influenced by
flood waves or major dam releases. This was
Figure 3 Velocity-discharge relationships for the gaging station on the Trinity River
at Liberty, Texas. Velocities for the reference discharges were estimated based on the
equation for the trend line shown
524 Sediment storage, sea level, and sediment delivery to the ocean
12 July 2004 (9 July for the Sabine, where
data for 12 July were not available at all
stations). Gage height recorded by the US
Geological Survey at this time was added to
the gage datum to determine water surface
elevation. Distance between gaging stations
was measured from Texas Department of
Transportation maps at a scale of 1:166,667.
Water surface elevation at the river mouth,
defined by channel morphology, was assumed
to be zero.
Water surface profiles are shown in
Figure 5, and Table 5 shows water surface
slopes for the lowermost reach (mouth to the
next station upstream) and mean slopes for
the entire study sections (uppermost coastal
plain station to mouth). Figure 5 and Table 5
show that slopes in the lowermost reaches
are very low. In combination with tidal and
other backwater effects, this suggests low
stream power and a high probability of sed-
iment deposition upstream of the open-water
Table 3 Discharge (Q, m
), mean velocity (m sec
), cross-sec-
tional and unit stream power (CX power, Unit power; W m
) for six
reference discharges and two flood events at three Trinity River cross-
sections. Reference discharges are based on exceedence frequencies
and return periods. For example, 10% Q indicates mean daily discharge
with an exceedence probability of 10% and Q2 represents a flow with a
mean return period of two years
Q V CX power Unit power
50% Q 82 0.45 0.23 0.00012831
10% Q 677 0.88 1.88 0.00024939
1% Q 1550 1.48 4.30 0.00041943
2002 flood 1872 1.82 5.20 0.00051652
Q1 2130 1.65 5.92 0.00046761
Q2 2400 1.74 6.67 0.00049312
Q10 3002 1.77 8.34 0.00050162
1994 flood 3540 1.67 9.83 0.00047328
50% Q 77 0.34 0.19 0.00008527
10% Q 640 0.88 1.57 0.00022070
1% Q 1541 1.48 3.79 0.00037118
2002 flood 2198 1.71 5.40 0.00042887
Q1 1970 1.65 4.84 0.00041382
Q2 2330 1.74 5.73 0.00043639
Q10 2925 1.77 7.19 0.00044392
1994 flood 3455 1.67 8.49 0.00041884
50% Q 433 0.41 0.04 0.00000411
10% Q 1048 0.49 0.10 0.00000491
1% Q 1822 0.59 0.18 0.00000591
2002 flood 1602 0.56 0.16 0.00000561
Q1 2484 0.68 0.24 0.00000681
Q2 2835 0.73 0.28 0.00000731
Q10 3600 0.83 0.35 0.00000832
1994 flood 3823 0.86 0.38 0.00000862
Jonathan D. Phillips and Michael C. Slattery 525
estuary. Concave river longitudinal profiles
are common, and these tend to have low
slopes in the lower reaches. However, Figure
5 shows that, within those low-slope reaches,
very low slopes may be encountered in the
lower reaches of coastal plain rivers.
The mean coastal plain slopes of the
Brazos and Colorado Rivers are substantially
higher than those of the Sabine, Neches, and
Trinity, which also have substantially smaller
drainage areas. The Colorado and Brazos
have built substantial recent deltas, and con-
tribute sediment to the littoral zone, while the
Sabine, Neches, and Trinity do not (Morton
et al., 2004). The Brazos and Colorado have
essentially filled the ravinement valleys cut
during lower sea-level stands. This filling is
generally attributed to higher sediment loads
(Anderson et al., 1992; Thomas and
Anderson, 1994; Blum and Price, 1998). As
there is no particular reason to believe that
the Brazos and Colorado experienced greater
denudation relative to their size during the
Quaternary, however, it can be speculated
that steeper regional slope in the coastal plain
portion of the river has allowed the Brazos
and Colorado to deliver a higher proportion of
their loads to the Gulf of Mexico.
The Trinity River has the steepest slope in
the reach just above the mouth, with much-
reduced slope upstream, which may explain
the sediment storage patterns identified by
Phillips et al. (2004), which show an alluvial
bottleneck between Romayor and Liberty.
Table 4 Ratios of cross-sectional and unit stream power, discharge, and channel
slope between successive Trinity River stations for various reference discharges, with
the value at the downstream station divided by that at the next upstream station
(R/G # Romayor/Goodrich; L/R # Liberty/Romayor)
CX power Unit power Discharge Slope
50% Q 0.83 0.22 0.66 0.05 0.94 5.63 0.88 0.04
10% Q 0.84 0.07 0.88 0.02 0.95 1.64 0.88 0.04
2002 flood 0.88 0.05 0.88 0.02 0.99 1.18 0.88 0.04
1% Q 1.04 0.03 0.83 0.01 1.17 0.73 0.88 0.04
Q1 0.82 0.05 0.88 0.02 0.92 1.26 0.88 0.04
Q2 0.86 0.05 0.88 0.02 0.97 1.22 0.88 0.04
Q10 0.86 0.05 0.88 0.02 0.97 1.23 0.88 0.04
1994 flood 0.86 0.04 0.88 0.02 0.98 1.11 0.88 0.04
Figure 4 Downstream trends in cross-
sectional and unit stream power for nor-
mal flows (50% probability) and the flood
of record in 1994 (note logarithmic axis)
526 Sediment storage, sea level, and sediment delivery to the ocean
An important implication is that sediment
transport monitoring which does not repre-
sent the low slope, low stream power, lower
reaches of coastal plain rivers will result in
overestimation of sediment delivery to the
V Discussion and conclusions
Coastal and marine sedimentary archives
may not reflect changes in continental erosion
and fluvial sediment dynamics, because
coastal plain rivers at least during rising or
high stillstands of sea level may not trans-
port much of their sediment load to estuaries
or offshore basins. Sediment budgets of
coastal plain rivers indicate extensive alluvial
sediment storage upstream of estuaries, and
low sediment delivery ratios. Estuarine sedi-
ment provenance studies confirm this, by
showing dominantly coastal and marine sedi-
ment sources even in drowned river valley
estuaries with substantial fluvial inflows. The
lower coastal plain reaches of alluvial river
valleys are in some cases effective sediment
Figure 5 Water surface profiles for the lower reaches of rivers of southeast Texas,
for flow conditions at 00:00 hours 12 July 2004 (00:00 9 July for the Sabine)
Table 5 Water surface slopes of east Texas coastal plain rivers for
the lowermost reach, and mean slope for the entire coastal plain reach,
for flow conditions at midnight, 12 July (9 July for the Sabine) 2004
River Slope of lowermost reach (%10
) Mean slope (%10
Sabine 1.026 1.544
Neches 0.167 1.451
Trinity 1.346 1.289
Brazos 1.336 2.039
Colorado 0.678 3.746
Jonathan D. Phillips and Michael C. Slattery 527
bottlenecks which buffer coastal systems
from effects of upstream changes in sediment
production and transport.
Lower coastal plain alluvial valleys may be
drowned by rising sea level and ravined at
lower sea levels. At current sea levels, the
locus of fluvial deposition is not necessarily
the ocean, estuary, or delta, but floodplains in
and upstream of the fluvial-estuarine transi-
tion zone. This raises the question of what
constitutes delivery to sediment to the outlet
of the basin, and how to define the mouth of
a river. There are at least eight different
morphological, hydrographic, hydrologic, or
sedimentological criteria that might be used
to define the river mouth; in general these are
significantly upstream of open-water estuar-
ies or delta apices. Sediment delivery to these
‘upstream mouths’ may be a more accurate
reflection of river sediment fluxes to the
coastal and marine environment. Different
definitions of the mouth could result in
assessments of fluvial sediment export which
differ by orders of magnitude.
Current assessments of river sediment
supply to the coast (as opposed to the lower
coastal plain) are too high. Sediment delivery
to the coast is generally estimated from meas-
urements at monitoring stations that are
upstream of the storage bottlenecks. These
sites do not represent the low slope, low
stream power, lower coastal plain river
reaches. Changes in sediment loads at these
stations may have negligible impacts on
coastal sediment systems.
Alluvial (as well as estuarine and deltaic)
sediment storage during high or rising base
levels is generally attributed to large accom-
modation space relative to sediment supply
(Blum and Törnqvist, 2000; Cattaneo and
Steel, 2003). This is indeed important, as
lower elevations and wider valleys typical of
coastal plain reaches provides ample opportu-
nity for deposition. Overbank flooding is often
more common in lower reaches, providing
more opportunity for floodplain deposition.
The role of fluvial sediment transport capac-
ity, however, has been underappreciated.
Slope-controlled reductions in transport
capacity are widely recognized where rivers
emerge onto coastal plains. In some cases
slope-controlled declines in stream power
occur in the lower coastal plain reaches in
the Trinity River example, stream power
decreases by an order of magnitude or more
in this zone. This reduction in transport
capacity becomes even more pronounced as
tidal and coastal backwater effects increase
The relative lack of attention to slope and
transport capacity may be due in part to the
assumption that the chief response of rivers
to base level change is cut-and-fill. In the case
of rising sea level, for instance, channels cut
to below sea level might be expected to
quickly aggrade. This is clearly not the case
for the Trinity and Tar Rivers discussed here,
where channel beds are below sea level well
upstream of their estuaries – other coastal
plain rivers are likely similar in this regard.
Cutting and filling is only one way of adjusting
slope, and several field and experimental stud-
ies in coastal plain rivers indicate that changes
in channel planform (meandering) may be the
primary response (Alford and Holmes, 1985;
Autin, 1992; Schumm, 1992; Koss et al.,
1994; Leigh and Feeney, 1995). Thus
Holocene sea-level rise could induce an
increase in sinuosity, which would have the
effect of reducing slope independently of any
change in bed elevation. The depositional
locus might then occur in the zone of
increased meandering, within a low-transport-
capacity reach where the bed elevation
approaches and eventually falls below sea
level. This appears to be the case in the Trinity
River, where the transition zone between a
scour-dominated reach downstream of
Livingston Dam and the lower coastal plain
alluvial storage bottleneck coincides with a
pronounced increase in sinuosity (Phillips
et al., 2004; 2005). This suggests that such
high-meandering lower coastal plain river
reaches may be the most fruitful places to
search for sedimentary archives of upstream
changes in fluvial sediment delivery.
528 Sediment storage, sea level, and sediment delivery to the ocean
While the phenomenon of very low sedi-
ment delivery to the coast in many coastal
plain rivers is a clear conclusion of this study,
there is obviously much to do. The paucity of
sediment monitoring in the lower coastal
plain is a clear shortcoming, and not an easy
one to redress. Measurements in coastal plain
rivers demand dealing with large channels and
large drainage basins; difficult practical as well
as conceptual tasks. Most studies of sediment
sources, fates, and provenance are focused
either on drainage basins upstream of the
lowermost gaging station or on estuaries.
There is, therefore, a major gap in between,
and we need to learn more about sediment
sources, transport, and storage in lower river
reaches and fluvial-estuarine transition zones.
Such studies, particularly when conducted in
the context of environmental change, should
be cognizant of the likelihood that factors
such as sea level, climate, tectonics, human
impacts, and other factors are simultaneously
and synergistically driving the system.
Additionally, the boundary conditions and
degrees of freedom for both fluvial and
coastal response are conditioned by inherited
geological factors.
Small drainage basins are, other things
being equal, easier to work with. As Dearing
and Jones (2003) point out, small basins are
generally more responsive to environmental
change. In this sense, any search for sensitive,
manageable sedimentary archives of environ-
mental change should avoid large alluvial
rivers crossing coastal plains. In the context of
comprehending continental and global-scale
changes, however, the enormous amounts of
land area, sediment, water, and other mass
associated with or transported and stored by
coastal plain rivers demands that we take
them on.
Kristin Adams, Josh Lepawsky, and Linda
Martin (University of Kentucky) helped out
with Trinity River fieldwork. Greg Malstaff of
the Texas Water Development Board helped
out in ways too numerous to enumerate. This
work is supported by Texas Water
Development Board Contracts 2002-483-
442 (University of Kentucky) and 2002-483-
440 (Texas Christian University). The usual
disclaimers apply.
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... For example, in the Aransas River, Texas, Jones et al. (2020) found that the tidal freshwater zone varied in length by more than 12 km (median 59.9 km) over a year. In the Trinity River, Texas, the channel bed is below sea level at least 110 km upstream of Trinity Bay, and geomorphological evidence of the effects of rising sea level extend to 130 km upstream (Phillips & Slattery, 2006). Backwater effects are detectable at a gauging station on the Tar River, North Carolina, 50 km upstream of the estuary (Phillips & Slattery, 2006). ...
... In the Trinity River, Texas, the channel bed is below sea level at least 110 km upstream of Trinity Bay, and geomorphological evidence of the effects of rising sea level extend to 130 km upstream (Phillips & Slattery, 2006). Backwater effects are detectable at a gauging station on the Tar River, North Carolina, 50 km upstream of the estuary (Phillips & Slattery, 2006). The FETZ of the Santee River, South ...
... Studies in the FETZ are complicated by the varying combination of fluvial and coastal effects, and by the scarcity of gauging stations in lower reaches of coastal plain rivers (Phillips & Slattery, 2006;Rodriguez et al., 2020). Further, the typical fluvial conditions of constant net downstream flow and a relationship between channel size and bank height on the one hand, and typical or channel-forming flows on the other hand, may not exist. ...
The fluvial‐estuarine transition zone (FETZ) of the Neuse River, North Carolina features a river corridor that conveys flow in a complex of active, backflooded, and high‐flow channels, floodplain depressions, and wetlands. Hydrological connectivity among these occurs at median discharges and stages, with some connectivity at even lower stages. Water exchange can occur in any direction, and at high stages the complex effectively stores water within the valley bottom and eventually conveys it to the estuary along both slow and more rapid paths. The geomorphology of the FETZ is unique compared to the estuary, or to the fluvial reaches upstream. It has been shaped by Holocene and contemporary sea‐level rise, as shown by signatures of the leading edge of encroaching backwater effects. The FETZ can accommodate extreme flows from upstream, and extraordinary storm surges from downstream (as illustrated by Hurricane Florence). In the lower Neuse—and in fluvial‐to‐estuary transitions of other coastal plain rivers—options for geomorphological adaptation are limited. Landscape slopes and relief are low, channels are close to base level, sediment inputs are low, and banks have high resistance relative to hydraulic forces. Limited potential exists for changes in channel depth,width, or lateral migration. Adaptations are dominated by formation of multiple channels, water storage in wetlands and floodplain depressions, increased frequency of overbank flow (compared to upstream), and adjustments of roughness via vegetation, woody debris, multiple channels, and flow through wetlands.
... Second, transient sediment storage in piedmonts, alluvial plains and lakes or near the coast may induce a large time lag between the external signal and its record in the sedimentary sink (e.g. Blöthe and Korup, 2013;Clift and Giosan, 2014;Malatesta et al., 2018;Phillips and Slattery, 2006;Romans et al., 2016). Depending on considered timescales, the erosive signal itself can be completely buffered by this process (see a complete review in Romans et al., 2016). ...
... Depending on considered timescales, the erosive signal itself can be completely buffered by this process (see a complete review in Romans et al., 2016). Finally, submarine sediments can be reworked by gravitational processes, especially during sea level falls (Phillips and Slattery, 2006). As a consequence, a significant part of the signal recorded in shallow-water sediments can be lost, whereas deep marine sediments may simultaneously record a signal coming from newly eroded source rocks and another one coming from the destabilization of previously deposited sediments. ...
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Detrital 10Be from continental river sands or submarine sediments has been extensively used to determine the average long-term denudation rates of terrestrial catchments, based on the assumption that the rate of cosmogenic nuclide production by the interaction of source rocks with cosmic radiation balances out the loss of these nuclides by surface denudation. However, the 10Be signal recorded in sediments may be affected at the source by the response time of mountainous catchments to high-frequency forcings. In addition, transient sediment storage in piedmonts, alluvial plains and lakes or near the coast may also induce a difference between the erosive signal and its record in the sedimentary sink. Consequently, a significant part of the signal recorded in shallow-water sediments can be lost, as deep marine sediments may simultaneously record a signal coming from newly eroded source rocks along with one coming from the destabilization of previously deposited sediments. In this paper, we use the landscape evolution model Badlands to simulate erosion, deposition and detrital 10Be transfer from a source-to-sink sedimentary system (the Var River catchment, southern French Alps) over the last 100 kyr. We first compare model-based denudation rates with the ones that would be extracted from the 10Be record of local continental sediments (equivalent to river sands) and from sediments deposited offshore over time in order to examine if this record provides an accurate estimate of continental denudation rates. Then, we examine which conditions (precipitation rate, flexure, ice cover) satisfy published measured river incision rates and 10Be concentration in submarine sediments. Our results, based on the Var catchment cosmic ray exposure dating and modelling indicate that, while river sands do accurately estimate the average denudation rate of continental catchments, this is much less the case for deep submarine sediments. We find that deep-sea sediments have a different and often much smoother 10Be signature than continental ones and record a significant time lag with respect to imposed precipitation rate changes, representing the geomorphological response of the margin. A model which allows us to fit both measured 10Be concentration in marine sediments and river incision rates on land involves an increase in precipitation rates from 0.3 to 0.7 m yr−1 after 20 ka, suggesting more intense precipitation starting at the end of the Last Glacial Maximum.
... The upper Blue Mine Conglomerate may be similar to the mudstone-rich pre-vegetation braid deltas noted from the Cambrian Wood Canyon Formation (Muhlbauer and Fedo, 2020). Mud may accumulate in high accommodation low-gradient, lower coastal plain settings near the fluvial-marine transition, possibly associated with very low slopes (a reduction in slope-controlled transport capacity is common when rivers emerge into lower coastal plain settings), low stream power, and/or rising base level (transgression) (Meade, 1982;Phillips and Slattery, 2006). ...
... Lower coastal plain environments trap a significant amount of finegrained sediment because of low slopes, low stream power, rising base level, marsh sediment accumulation, and salt-freshwater mixing and circulation patterns (Meade, 1982;Phillips and Slattery, 2006). Nearshore tidal settings also trap a large amount of sediment: under high sediment supply, tidal flats prograde into shallow subtidal settings (such as lagoons) filling available accommodation (Klein, 1971;Yang and Chen, 2001). ...
The long-term evolution of the biosphere has caused fundamental shifts in environmental conditions and sedimentation at Earth’s surface. While effects of the evolution of terrestrial vegetation on river systems have been explored in detail, facies models and possible shifts in sedimentation in pre-vegetation nearshore marine settings have not been sufficiently explored. The circa 800-million-year-old Burra Group, exposed in the Adelaide Fold Belt of South Australia, is a terrestrial to nearshore transgressive sedimentary succession associated with rifting. The Burra Group provides a record of marginal and shallow marine environments before the evolution of land plants. Environments are interpreted to record deposition in low energy fine-grained tidal flat, lagoon, tidal inlet, and beach-barrier environments (a beach-barrier system). The thick mudflats (up to 30 m) of the studied units indicate that vegetation is not essential for mudflat accumulation, contrary to some models. The fluvial–marine transition and the back-barrier–tidal inlet–beach-barrier transition is highly regular and there is little interbedding of units. The Tonian beach-barriers of the Burra Group do not backstep into the back-barrier region and aggrade much like stable modern systems backed by peat. These observations are suggestive of a stable barrier system in the absence of land plants. The high mud percentage and/or early carbonate cementation (including microbialite and stromatolite deposition during transgression) may have contributed to the stability of this beach-barrier system.
... Understanding the roles of past sea-level variability and provenance change are crucial for disentangling the complex driving mechanisms influencing the sedimentary regime of the northern SCS (Phillips & Slattery, 2006;Weaver et al., 2000). On the one hand, global sea level was ∼130 m below modern levels during the Last Glacial Maximum (LGM; ∼24-19 ka), leading to different land-sea configurations around the SCS that significantly influenced the inputs of terrigenous matter and its redistribution within the basin (Xiong et al., 2020). ...
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Plain Language Summary Sediments in the South China Sea (SCS) provide important records of past changes in the ocean circulation and atmospheric patterns in the Pacific Ocean. However, the interpretation of sedimentary archives from this region in terms of changes in the ocean currents or the climate‐driven sediment supply can be challenging because of the potential influence of global sea‐level fluctuations. In order to better constrain these multiple controls on the sedimentary regime of the northern SCS, we present new mineralogical records from sediment cores collected from both shallow‐ and deep‐water sites. After assessing the effects of sea‐level change, we find that the clay mineral assemblage in shallow sites from the northern SCS can generally be used to reconstruct the evolution of the East Asian summer monsoon. In deep‐water sites, the clay mineralogy instead reflects changes in the relative abundance of sediment supplied from Taiwan compared to Luzon, revealing an enhanced inflow of the Kuroshio Current during the mid‐late Holocene. Furthermore, millennial‐scale variability in the North Pacific Intermediate Water inflow can be traced using changes in magnetic mineralogy and the inflow appears to have been stronger at the end of the last ice age.
... The transfer of sediment from upstream to downstream areas in a river system (connectivity [27]) generally decreases as catchment area increases because of sediment storage in floodplains. Higher topographic gradients are typically correlated with an increase in upstream to downstream connectivity, and low relief coastal plain rivers are assumed to be disconnected from land cover changes in their watershed [28]; however, there are examples where that assumption does not apply [3,[29][30][31]. The smallest lower coastal plain watersheds are tidal creeks and their short reach and little to no floodplain could make them important sources of sediment to estuaries. ...
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Land cover and use around the margins of estuaries has shifted since 1950 at many sites in North America due to development pressures from higher population densities. Small coastal watersheds are ubiquitous along estuarine margins and most of this coastal land-cover change occurred in these tidal creek watersheds. A change in land cover could modify the contribution of sediments from tidal creek watersheds to downstream areas and affect estuarine habitats that rely on sediments to persist or are adversely impacted by sediment loading. The resilience of wetlands to accelerating relative sea-level rise depends, in part, on the supply of lithogenic sediment to support accretion and maintain elevation; however, subtidal habitats such as oyster reefs and seagrass beds are stressed under conditions of high turbidity and sedimentation. Here we compare sediment accumulation rates before and after 1950 using 210Pb in 12 tidal creeks across two distinct regions in North Carolina, one region of low relief tidal-creek watersheds where land cover change since 1959 was dominated by fluctuations in forest, silviculture, and agriculture, and another region of relatively high relief tidal-creek watersheds where land-use change was dominated by increasing suburban development. At eight of the creeks, mass accumulation rates (g cm-2 y-1) measured at the outlet of the creeks increased contemporaneously with the largest shift in land cover, within the resolution of the land-cover data set (~5-years). All but two creek sites experienced a doubling or more in sediment accumulation rates (cm yr-1) after 1950 and most sites experienced sediment accumulation rates that exceeded the rate of local relative sea-level rise, suggesting that there is an excess of sediment being delivered to these tidal creeks and that they may slowly be infilling. After 1950, land cover within one creek watershed changed little, as did mass accumulation rates at the coring location, and another creek coring site did not record an increase in mass accumulation rates at the creek outlet despite a massive increase in development in the watershed that included the construction of retention ponds. These abundant tidal-creek watersheds have little relief, area, and flow, but they are impacted by changes in land cover more, in terms of percent area, than their larger riverine counterparts, and down-stream areas are highly connected to their associated watersheds. This work expands the scientific understanding of connectivity between lower coastal plain watersheds and estuaries and provides important information for coastal zone managers seeking to balance development pressures and environmental protections.
... Changes in the rate of RSL rise can also influence sediment supply. Rapid RSL rise often leads to the confinement of river-supplied sediment to up-valley locations in estuaries, bayhead deltas, and fluvial settings (Phillips and Slattery 2006;Anderson et al. 2014). As a result, many coastal environments experienced a reduction in sediment supply from adjacent rivers during the early Holocene when rates of RSL rise were high (Cattaneo and Steel 2003;Clifton 2006;Boyd 2010;Anderson et al. 2014). ...
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Decreasing rates of eustatic sea-level rise during the Holocene accompanied the deposition of transgressive coastal deposits worldwide. However, unraveling how transgressive deposition varies in response to different rates of relative sea-level (RSL) rise is limited by the scarcity of long (10+ m) well-dated cores spanning the entire middle to late Holocene record along macrotidal coasts. To investigate the sedimentary response of this macrotidal coast to decreasing rates of RSL rise, we acquired four cores up to 32 m in length and Chirp seismic profiles along the west coast of Korea. Core sediments were analyzed in terms of sedimentary texture, structure, and facies. Nineteen optically stimulated luminescence (OSL) and fourteen 14C accelerated mass spectrometry (AMS) ages constrain the timing of deposition of the sandy sediments. This relatively dense distribution of ages is used to determine how deposition rates changed through time. We also use a compilation of previously published RSL indices for the southwestern Korean coast in order to better constrain RSL changes through time. Results show that the evolution of the Gochang coastline switched from a tide-dominated environment to a wave-dominated environment during the latter stage of transgression as the rate of the sea-level rise decreased. Rugged antecedent topography likely led to the development of tidal currents and the formation of a tide-dominated tidal flat during rapid RSL rise from 10 to 6 ka. As the tidal channels filled with fine-grained sediments from 6 to 1 ka, tidal amplification likely waned leading to a greater role of wave energy in shaping the formation of the sandy open-coast tidal flat. Since 1 ka, wave-dominated environments formed sand-rich tidal beaches and flats. Decreasing changes in rates of the RSL rise resulted in changes in depositional environments from a tide-dominated intertidal flat to an open-coast tidal flat and finally a wave-dominated tidal beach. This study highlights the important role that rates of RSL rise play on not only sedimentation rates in a shelf setting but also playing a role in the switch from a tide-dominated to a wave-dominated setting.
... The transgressing sea-level inundates and the regressing sea-level exposes the continental shelf. In the case of the marine regions close to the river mouth, the change in sea-level significantly shifts the point of sediment debouchment by rivers (Phillips & Slattery, 2006). Therefore, sea-level changes significantly affect the size as well as the volume of sediments being brought into the marginal marine regions and deeper waters through channels, and thus affect the carbon burial in marine sediments. ...
The oceans store a substantial fraction of carbon as calcium carbonate (CaCO 3 ) and organic carbon (C org ) and constitute a significant component of the global carbon cycle. The C org and CaCO 3 flux depends on productivity and is strongly modulated by the Asian monsoon in the tropics. Anthropogenic activities are likely to influence the monsoon and thus it is imperative to understand its implications on carbon burial in the oceans. We have reconstructed multi-decadal CaCO 3 and C org burial changes and associated processes during the last 4.9 ky, including the Meghalayan Age, from the Gulf of Mannar. The influence of monsoon on carbon burial is reconstructed from the absolute abundance of planktic foraminifera and relative abundance of Globigerina bulloides . Both C org and CaCO 3 increased throughout the Meghalayan Age, except between 3.0–3.5 ka and the last millennium. The increase in C org burial during the Meghalayan Age was observed throughout the eastern Arabian Sea. The concomitant decrease in the C org to nitrogen ratio suggests increased contribution of marine organic matter. Although the upwelling was intense until 1.5 ka, the lack of a definite increasing trend suggests that the persistent increase in C org and CaCO 3 during the early Meghalayan Age was mainly driven by higher productivity during the winter season coupled with better preservation in the sediments. Both the intervals (3.0–3.5 ka and the last millennium) of nearly constant carbon burial coincide with a steady sea-level. The low carbon burial during the last millennium is attributed to the weaker-upwelling-induced lower productivity.
Small rivers that flow into the sea often terminate in estuaries or lagoons that may be separated from the sea by a sandy beach barrier. As a result of variations in barrier width and river discharge, these river mouths can be variously open, closed or partly open at different times. This behaviour reflects the interplay between the processes and properties of river systems (discharge, sediment supply, channel width, water velocity) and beach systems (beach width and height, grain size, wave regime, longshore processes). This study examines the dynamic behaviour of 32 river mouths located along 238 km of the coastline of Eastern Cape Province, South Africa, between 2000 and 2021 using Google Earth imagery. At each available time snapshot, individual river mouths were classified along a continuum as open, partly open or closed. Results show that nine river mouths were permanently open whereas 22 varied between the three states. Only one river mouth was partly open and none was closed for the entire period. Some spatial and temporal patterns were also identified. Commonly, adjacent river mouths may show the same patterns of opening/closing, which may reflect regional climate forcing. Fewer river mouths are open during winter/autumn compared to summer, likely reflecting rainfall seasonality. Thus, regional climate is considered to be the major control on river mouth dynamics, likely in combination with human activities that impact on river discharge, although the role of coastal sediment dynamics is significantly less well understood. The interplay between these different forcing factors requires further investigation.
Hydrogeomorphic, vegetative, and biogeochemical processes interact in floodplains resulting in great complexity that provides opportunities to better understand linkages among physical and biological processes in ecosystems. Floodplains and their associated river systems are structured by four-dimensional gradients of hydrogeomorphology: longitudinal, lateral, vertical, and temporal components. These four dimensions create dynamic hydrologic and geomorphologic mosaics that have a large imprint on the vegetation and nutrient biogeochemistry of floodplains. Plant physiology, population dynamics, community structure, and productivity are all very responsive to floodplain hydrogeomorphology. The strength of this relationship between vegetation and hydrogeomorphology is evident in the use of vegetation as an indicator of hydrogeomorphic processes. However, vegetation also influences hydrogeomorphology by modifying hydraulics and sediment entrainment and deposition that typically stabilize geomorphic patterns. Nitrogen and phosphorus biogeochemistry commonly influence plant productivity and community composition, although productivity is not limited by nutrient availability in all floodplains. Conversely, vegetation influences nutrient biogeochemistry through direct uptake and storage as well as production of organic matter that regulates microbial biogeochemical processes. The biogeochemistries of nitrogen and phosphorus cycling are very sensitive to spatial and temporal variation in hydrogeomorphology, in particular floodplain wetness and sedimentation. The least-studied interaction is the direct effect of biogeochemistry on hydrogeomorphology, but the control of nutrient availability over organic matter decomposition and thus soil permeability and elevation is likely important. Biogeochemistry also has the more documented but indirect control of hydrogeomorphology through regulation of plant biomass. In summary, the defining characteristics of floodplain ecosystems are determined by the many interactions among physical and biological processes. Conservation and restoration of the valuable ecosystem services that floodplains provide depend on improved understanding and predictive models of interactive system controls and behavior.
Detrital zircon U–Pb ages are widely employed as an archive of geological processes through time. Changes in detrital zircon age patterns within sediments reflect changes in source areas that are often related to tectonic and/or climatic processes. However, discrimination of first-cycle and multi-cycle detrital zircon with primary crystalline and secondary sedimentary sources, respectively, can be challenging using only crystallisation age constraints. Here, we present U–Pb geochronology of detrital zircon from modern fluvial and littoral environments on the Scott Coastal Plain in Western Australia to investigate the use of α-dose to identify sedimentary recycling. The majority of 1032 concordant U–Pb ages are interpreted to be ultimately sourced from the local basement. However, U–Pb ages do not reflect the areal extent of source rocks and indicate significant reworking of coastal plain sediments. A novel metric – source-normalized α-dose – demonstrates predominant detrital zircon routing via recycling through intermediate storage. This metric is defined as the ratio of the average α-dose (a measure of metamictization) of detrital zircon belonging to a characteristic age group and the average α-dose of zircon grains within the corresponding source crystalline basement. Average values of source-normalized α-dose of detrital zircon populations <1 are interpreted to reflect selective removal of more labile (metamict) grains via attrition and diagenesis, indicating greater grain transport and recycling, whereas values of c. 1 signify shorter transport and a first-cycle origin. Application of this approach to ancient clastic systems is supported by consistency of results with independent indicators of progressive sedimentary recycling and/or transport. Source-normalized α-dose is an internal measure using zircon grain chemistry (U and Th), and avoids bias associated with multi-mineral measures of sediment recycling that may be related to source fertility. Additionally, source-normalized α-dose uses measures typically captured during routine U–Pb geochronology. Source-normalized α-dose of detrital zircon provides an additional method to address sedimentary source-to-sink transport and recycling, and ultimately allows more robust interpretation of U–Pb zircon data.
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The lower reaches of many rivers are subject to two distinct phenomena driving aggradation: Holocene sea level rise and culturally-accelerated erosion and sedimentation. It is critical to both geoscientists and resource managers to distinguish between the effects of these phenomena, but in many situations it is difficult to distinguish between historic alluvium associated with accelerated erosion, and other Holocene fill. In the Croatan area of eastern North Carolina, there are at least five field indicators which may allow one to distinguish historic from other Holocene floodplain sediments: Development of soil B-horizons, pedological and mineralogical indicators of a Piedmont sediment source, oxidized layers, dendrogeomorphic indicators, and burial of historic features. Examination of eight stream reaches showed evidence that the surficial alluvium (~1 m or more) is historic in seven cases. Burial of historic features in the Croatan suggests mean floodplain accretion rates of 3 to 9 mm yr -1 and mass additions of 45 to 92 t ha -1 yr -1. Prehistoric mineral sedimentation rates; estimated from sediment budget considerations based on contemporary erosion and sediment transport in forested basins, were about 0.05 mm yr -1 (0.65 t ha -1 yr -1). Maximum organic accumulation rates are no more than 0.3 mm yr -1 (1.05 t ha -1 yr -1). Thus, in the Croatan, human agency has accelerated alluvial sedimentation rates by at least a hundredfold. The human-accelerated aggradation is largely confined to the lower fluvial reaches of Croatan streams. In the fluvial-estuarine transition zone, organic-dominated infilling has been little affected on a large scale by human agency. So much upland sediment is stored in alluvial floodplains that the geomorphic impacts of accelerated erosion on estuaries has been minimal.
Sediment transported from the continental shelf through the mouth of the bay may be volumetrically more important than sediment derived from rivers. Average Holocene rates of sediment accumulation show considerable spatial variability, presumably related to local variations in sediment sources, wave energy, and tidal currents. Nonetheless, these rates show several clear trends. Rates on the shallow marginal shelves of the bay tend to be low (0 to 2 mm/yr) and to increase only slightly toward the bay mouth. Rates in the deep channels are higher (1 to 5 mm/yr), have local maxima, and increase distinctly towards the bay mouth. At any given position in the bay, sediment-accumulation rates increase with depth to the base of the Holocene section. -from Authors
The modern configuration of the Texas coast reflects competition between fluvial-deltaic sedimentation and reworking by marine processes that produce a dominant westerly flow in the northern Gulf of Mexico (LeBlanc and Hodgson 1959). This westerly flow and the regional distribution of barrier islands led some early workers to conclude that Texas barrier islands were deposited by littoral drift reworked from the Mississippi River. More recent geologic studies have shown that Texas barriers are reworked deltaic deposits but of Texas coastal plain rivers rather than the Mississippi. Erosion of relatively stable deltaic headlands, winnowing of the eroded material, and deposition of the coarse fraction in interdeltaic embayments are the result of wave and current dominance over fluvial supply. An important geologic consequence of the disequilibrium in forces is that thickest deposits of well-sorted barrier sand are located the greatest distance from any fluvial source.
Base-level changes significantly affected the shelf area but they had little effect on the fluvial drainage basin. During base-level falls, fluvio-deltaic progradation occurred. Fluvial aggradation occurred only during periods of base-level stillstand or rise. Because of the slow rate of headward erosion of incised valleys, a significant time gap existed from the time base level first fell below the shelfbreak and the development of a cross-shelf bypass valley. During this lag time, deposition occurred on the exposed shelf to form a fluvial braid plain. -from Authors
A 2700-km high-resolution seismic-reflection data set, acquired in recent years, has helped resolve some old problems concerning the age of Quaternary formations along the east Texas coast, and has resulted in mapping of the Trinity/Sabine incised valley. Sandbody formation and preservation on the shelf also has been influenced strongly by the episodic nature of the late Wisconsinan-Holocene sea-level rise. Sabine Bank, the largest of the sand bodies and the only one studied in detail, is a reworked coastal lithosome that rests on the ravinement surface. The modern Sabine Lake and Galveston Bay estuaries formed initially by flooding of the Sabine and Trinity valleys approximately 8 ka. The subsequent flooding event, which inundated the broad, shallow meander portions of the valleys, occurred approximatley 4 ka and appears to have been rapid. -from Authors
The Texas Gulf Coastal Plain consists of a series of low-gradient, fan-shaped alluvial plains emanating from each major river valley. The majority of alluvial plain surfaces have been mapped as Pleistocene Beaumont Formation or younger unnamed strata, and interpreted to represent eustatically-controlled deposition during the oxygen isotope stage 5 and modern interglacial highstands. Reevaluation of preexisting data combined with reexamination of Beaumont and younger strata of the Colorado River suggests the stratigraphic and geochronologic framework needs revision, and processes of alluvial plain deposition are more complex than previous interpretations have inferred. As a result, Beaumont and younger strata provide an opportunity to examine alluvial plain construction within a sequence-stratigraphic framework and discuss some key characteristics and the heirarchal nature of eustatically-controlled versus climatically-controlled components of alluvial plain depositional sequences. Mapping from satellite imagery, field documentation of geomorphic and stratigraphic relationships, consideration of the stratigraphic significance of surface and buried soils, and a number of radiocarbon and thermoluminescence ages suggests that Beaumont and younger alluvia] plains consist of multiple cross-cutting and/or superimposed valley fills of widely varying age, and may represent the last 300–400 ky or more. Valley fills become partitioned by initial lowering of sea-level below interglacial highstand positions, when channels rapidly incise and valley axes become fixed in place as they extend across the subaerially-exposed shelf. While shorelines remain basinward of highstand positions, the remainder of the alluvial plain is characterized by non-deposition and soil development. During this time, multiple episodes of lateral migration, aggradation, degradation, and/or flood-plain abandonment with soil formation occur within incised and extended valleys in response to climatic controls on discharge and sediment supply. This creates a composite basal valley fill unconformity, as well as multiple smaller-scale allostratigraphic units within the valley fill. With late stages of transgression and highstand valleys fill at paces set by upstream controls on sediment delivery. As valley filling nears completion, veneers of flood basin sediments spread laterally, which buries soils developed on downdip margins of the alluvial plain. Complete valley filling during highstand is one of several processes that promotes avulsion, with relocation of valley axes before the next sea-level fall, such that successive 100–ky valley fills have a distributary pattern, and successive increments of geologic time occur lateral to each other.
Several lines of evidence are used to estimate sediment production, transport, and storage. Soil profile truncation indicates upland erosion rates of >9.5 t/ha/yr during the post-European period, while pre-colonial rates were negligible. Comparisons of sediment yields between forested and agricultural basins suggest that stream sediment loads have risen dramatically following colonial-era land clearing. Pedological indicators show a change in dominant sediment source from an important upper-basin Piedmont contribution to a situation where Piedmont sediment is overwhelmed by Coastal Plain sources. Finally, most of the increased sediment input to the lower Neuse River and its tributaries has apparently been stored as alluvium. It is concluded that the stratigraphic record is insufficient for predicting future responses of the river to environmental change. -from Author
Morphologic analysis of sinuosity, width-depth ratios, and convergence characteristics reveals three compartments: 1) bay-mouth, 2) estuary funnel and 3) meander zone. Each compartment exhibits a characteristic lithofacies reflecting different proportions of wave, tidal and fluvial energy. These lithofacies form a longitudinal tripartite pattern, ie, sand-mud-sand, with coarse-grained sediment at the energetic ends of the system. -from Authors