Geology Reference
In-Depth Information
Reticulate channels (Fig.
11.3e
) can be considered
as a form of dendritic channel; however, they are nota-
ble for the 90° angle at which the low-order channels
meet higher-order channels. First-order tidal creeks
commonly end at 90° to the higher-order channel
(Zeff
1988
; Eisma
1998
; Ginsberg and Perillo
2004
) ,
whereas higher-order channels commonly meet at a
lower angle. In fl uvial systems a 90° attachment of a
low-order stream is usually associated with a high bed
gradient in the low-order channel compared to the
higher-order one. Pestrong (
1965
) observed that in San
Franscico Bay, low-order tidal channels often had
steeper bed gradients than the higher-order channels.
However, 90° attachment angles in tidal systems have
also been attributed to the nature of the tidal fl ow, when
small channels experience high tidal asymmetry rela-
tive to the larger channels that have more equally bal-
anced ebb and fl ood fl ows (Zeff
1988
; Eisma
1998
) .
As noted, spatial scale seems to be an important
control on the expression of tidal channels within a
given system. Eisma (
1998
) examines smaller inter-
tidal systems, and neither distributary channels nor
braided channels are common within the collected
observations upon which he based his classifi cation.
The term distributary channels is used to describe ebb
dominated channels which form on small deltas build-
ing out of the entrance of larger tidal channels. On this
smaller scale, braided channel morphologies tend to
occur in macrotidal environments such as King Sound
(Australia) or the Bay of Fundy (Canada). These chan-
nels have a low gradient and a low topography, sug-
gesting they are active during lower water levels. They
form in the region of maximum tidal energy. On a very
small scale, braided channels have also been observed
on tidal fl ats of loose sediment where the gradient of
the fl at is steep, forming either near a river mouth or
over loose debris at the base of cliffs (Eisma
1998
) .
Likewise, Dalrymple et al. (
1992
) describe similar pat-
terns of channelization on sand fl ats in macro-tidal
regions with very large tidal range.
Interconnected channels begin and end at another
channel, or link a lagoon to the ocean (Ashley and Zeff
1988
, Fig.
11.3g
). These often occur in conjunction
with dendritic channels; in fact, it is common to see
many of the different categories of channel morphol-
ogy or network existing concomitantly. Interconnected
channels are not exclusive to any tidal range and are
likely to meander, although sinuous and straight forms
are also observed (Eisma
1998
) and may purely be
inherited as by marshes as fl ood tidal deltas are stabi-
lized by vegetation (Zeff
1988
) . Based on observations
in the Niger Delta, Allen (
1965
) suggests that intercon-
nected channels form as tidal fl ats grow vertically and
horizontally (due to the sediment supplied by the river),
or as blind channels join together. Both of these studies
describe the evolution of a delta (the fi rst tidal, the sec-
ond riverine) with stabilization and increased accretion
on the higher fl ats, while the channels are maintained
by the tidal fl ows. The term 'interconnected' channel
is, thus, fairly broad.
11.3.3 Braided, Distributary
and Interconnected Channels
The term 'braided' is used by Hibma et al. (
2004a
) to
describe the channel systems in the deeper subtidal
regions of an inner estuary. Here, a complex system of
ebb- and fl ood-dominated tidal channels occurs within
a relatively straighter section of the estuary. The mutu-
ally evasive channels meander, slightly out of phase,
the ebb channel is generally well formed and the fl ood
channels may be continuous or form
fl ood barbs
across
the shoals amongst which the ebb channel weaves
(Fig.
11.1c
). Periodic overlapping of the fl ood and ebb
channel and small swatchways connect the channels.
Shoals may become vegetated and eventually form
islands (Fig.
11.1b, c
).
In fl uvial networks, the term braided is applied to
channel complexes, which form in regions of higher
gradient and where sediment supply overwhelms
hydraulic transport potential. In contrast, in tidal envi-
ronments this channel morphology occurs in the middle
parts of estuaries, where peak ebb currents and peak
fl ood currents occur at a similar stage of the tide. In plan
view this morphology is similar to that of terrestrial
braided channel systems. The process of formation in
tidal environments is not well understood, although it is
likely different from fl uvial setting as tidal fl ow is bi-
directional and water surface slope is normally more
infl uential than bed gradient in driving the fl ow.
11.3.4 Parallel Channels or No Channels
Systems displaying parallel channels or no channels at
all are relatively rare, and most are found in regions
with large tidal ranges (macrotidal). Parallel channels
