Geology Reference
In-Depth Information
immaterial, except for the criterion of partial enclosure.
Thus, the presence of a salt wedge that is over-ridden
by fresh water supplied by a river is referred to as
estua-
rine circulation
, regardless of whether it occurs in the
distributary channels of the Changjiang River delta, which
is actively creating new land as a result of sediment depo-
sition (Hori et al.
2001
), or the mouth of the Severn River,
which is migrating landward by means of coastal erosion
(Allen
1990
). Of course, in a geological context, these
two situations (progradational and transgressive, respec-
tively) are polar opposites because they generate strati-
graphic successions that are upside down relative to
each other. This distinction is particularly important in a
sequence-stratigraphic context, which aims to reconstruct
shoreline behavior in response to changes in eustatic sea
level, tectonic movement and sediment supply.
As a result, Dalrymple et al. (
1992
) (see also Dalrymple
2006
) proposed a 'geological' definition that states that
“an estuary is a transgressive coastal environment at the
mouth of a river, that receives sediment from both fluvial
and marine sources, and that contains facies influenced
by tide, wave and fluvial processes. The estuary is consi-
dered to extend from the landward limit of tidal facies at
its head to the seaward limit of coastal facies at its mouth”
(Dalrymple
2006
, p. 11). This definition represents a sub-
set of the environments covered by the Pritchard (
1967
)
definition because it is restricted to transgressive settings.
This is the definition used in this chapter. It is noteworthy,
however, that this definition indicates that estuaries as
defined here
import
sediment from the sea (i.e. there is a
strong element of flood dominance), whereas deltas
export
sediment to the sea (i.e. they are ebb dominated).
This is an important process distinction that has featured
prominently in process-oriented literature on coastal
environments (e.g. Friedrichs and Aubrey
1988
; Friedrichs
et al.
1990
) and which is discussed further below. Estuaries
are, therefore, ephemeral features, in that they are formed
by relative sea-level rise that creates
accommodation
(i.e.
the space available for sediment accumulation; Catuneanu
2006
) in the river-mouth area, which is then filled by
sediment input by both river and marine processes.
Estuaries are abundant today because of the recent post-
glacial transgression. Depending on the local circum-
stances, some of them are still actively transgressing,
whereas others are in various stages of transition to deltas.
Therefore, the nature of this transition is considered in
this chapter. Systems that have made the full transition to
deltas are discussed in Chap. 7 of this volume.
The focus in this chapter is on estuaries in which
tidal currents are the
dominant
agent of sediment trans-
port. Tidal dominance is produced either by the pres-
ence of a large tidal range and/or by the presence of
weak wave action in the coastal zone (Davis and Hayes
1984
). There has been a tendency in the literature to
associate tidal dominance with macrotidal conditions
(i.e. tidal range >4 m), but tidal dominance can also
occur in microtidal and mesotidal areas, provided wave
energy is low enough. Well-studied examples of tide-
dominated estuaries include the Cobequid Bay-Salmon
River estuary, Bay of Fundy (Dalrymple et al.
1990,
1991
; Dalrymple and Zaitlin
1994
), the Severn River
estuary, Great Britain (Harris and Collins
1985
; Allen
1990
; McLaren et al.
1993
), Mont-Saint-Michel Bay,
France (Tessier et al.
2006
,
2010
; Billeaud et al.
2007
)
and the Fitzroy River estuary, Australia (Bostock et al.
2007
; Ryan et al.
2007
). Such estuaries show an expo-
nential seaward widening that is referred to as a 'fun-
nel-shaped' mouth (Fig.
5.1
). Strong tidal currents
flowing into and out of the river mouth create a series of
channels that are approximately perpendicular to the
main shoreline trend. At their mouth, these channels
are separated by elongate tidal bars that are typically,
but not everywhere, composed of sand. Broad tidal flats
are widespread. Further landward, these channels
become more sinuous and are flanked by tidal point
bars. Tidal flats are narrower here as are the channels
themselves. In the following sections we first describe
the processes that operate in these systems, and then
examine how the morphology and facies respond to
these processes. The stratigraphy of tide-dominated
estuaries is considered in Chap. 6.
5.2
Process Framework
5.2.1
Waves, River, Tidal Currents,
and Bed-Material Movement
Although tidal currents are the most important process
responsible for sediment erosion and deposition in
tide-dominated estuaries, waves and river currents also
play an important role locally (Figs.
5.2
and
5.3
) at
certain times. Waves control sedimentation on the
seaward flanks of the estuary because the
tidal prism
(i.e. the volume of water moving past a location during
each half tidal cycle) is small. Thus, the open coast
adjacent to a tide-dominated estuary is typically wave
dominated (Fig.
5.2
; Yang et al.
2005, 2007
). However,
near the mouth of the estuary, the tidal prism and the
resulting tidal currents become larger, generating
