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

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.

Full text

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
1*
and Michael C. Slattery
2
1
Tobacco Road Research Team, Department of Geography, University of
Kentucky, Lexington, KY 40506, USA
2
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 (Métivier 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
output.
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
coast.
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
capacity.
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
Rivers.
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
phenomenon.
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
estuary.
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
flow.
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
2
, fluvial sedi-
ment sources were inadequate to account for
observed infill and coastal progradation, and
concluded that the major sediment source is
seaward.
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)
where
$
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:
P
u
#
$
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
study.
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
6
tonnes, and a yield of
2.0 t km
&2
yr
&1
. 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
6
and a yield of 19 t km
&2
yr
&1
.
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
characteristics.
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-
work.
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
water
2. Network morphology Transition from dominantly convergent to divergent
network
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
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
1
Mean annual sediment load at Liberty # 69,673 t yr
&1
; mean annual sediment load at Romayor # 3,378,461 t yr
&1
.

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
Tranter’s 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
dramatically
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,
2003).
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
2

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,
respectively.
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
2
value. For Liberty, a bimodal
relationship made curve fitting the entire data
set difficult (Figure 3). A linear trend was fitted
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
&10
Q
3
– 7 % 10
&7
Q
2
( 0.0014 Q ( 0.3428 R
2
# 0.87
Romayor: V # –2 % 10
–7
Q
2
( 0.0011 Q
( 0.2601 R
2
# 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
3
sec
&1
), mean velocity (m sec
&1
), cross-sec-
tional and unit stream power (CX power, Unit power; W m
&2
) 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
Goodrich
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
Romayor
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
Liberty
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
R/G L/R R/G L/R R/G L/R R/G L/R
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
coast.
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
&4
) Mean slope (%10
&4
)
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
downstream.
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.
Acknowledgements
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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