Geology Reference
In-Depth Information
Within these systems there are single channels alongside
complex dendritic networks; parallel, straight channels
alongside sinuous, elongate, dendritic channels; chan-
nels that meander strongly inland but gradually straighten
as they extend seaward; in short, there is a diverse array
of channel forms, created by local variations in tides,
sediment and vegetation.
The overlaps and differences between the classifi -
cations occur because of the scale of the area that is
considered by each. Hibma et al. (
2004a
) provide a
large-scale view, whereas Pye and French (
1993
) con-
centrate on marsh systems only and present essentially
a detailed classifi cation of the possible variation in
dendritic systems. The classifi cation of Eisma (
1998
)
falls somewhere between these scales, overlapping
with each. However, the classifi cations of both Eisma
(
1998
) and Pye and French (
1993
) incorporate two key
morphological observations: they make differentia-
tions based on the level of channel complexity (
elabo-
ration
), and whether or not the system consists of a
single channel or developed networks.
1980
; Eisma
1998
). Some marsh channel networks
develop beyond meanders to highly complex morphol-
ogies incorporating ponding (e.g., Tollesbury marsh,
Essex, UK; Figs.
11.2e
and
11.3d
). The processes lead-
ing to the development of meanders and the resulting
channel-bed morphology is discussed below (
Sect.
11.5.3
).
On a macro-scale, Dalrymple et al. (
1992
) describe
a pattern of straight-meandering-straight. This con-
fi guration is observed in channels within the inner
reaches of estuaries but not seen within deltas (i.e., on
regressive shorelines). The outer straight relates to
deeper subtidal environments where fl ow and sedi-
ment transport is generally directed landward because
of asymmetry in tidal fl ows, the upper straight occurs
in a region where the sediment transport is directed
seaward because of river dominance and the central,
meandering section exhibits a region of fi ne sediment
(grain size decreasing towards it from both direc-
tions). A similar pattern is also suggested in the data
presented for a single salt marsh creek by Solari et al.
(
2002
) (their Fig. 2). A physical explanation for this
pattern has yet to be identifi ed.
11.3.1 Elaboration
11.3.2 Dendritic Networks
The morphology of an individual channel may range
from straight, its simplest form, to meandering, and
further to convolutions involving the incorporation of
ponding or man-made drainage ditches (as are com-
mon, for example, in the marshes of New England,
USA; Figs.
11.2f
and
11.3e
). Straight or linear chan-
nels, despite their name, will have some natural irregu-
larities. This may make the boundary between a
channel that is straight and one that has some gentle
curving less clear. However, as curving increases, a
channel is described as sinuous or weakly meandering
(Fig.
11.3a, b
). In general, larger channels tend to be
straighter (e.g. in the Wash or Zaire River estuary;
Fig.
11.3a, d, f, g
; Eisma
1998
; Ginsberg and Perillo
2004
; Marani et al.
2002,
2004
) .
The sinuosity of a channel can be described by the
ratio of the actual length of the channel to the down-
stream distance (in a straight line) of the 'wavelength'
of the curve. When this
sinuosity ratio
exceeds 1.5 the
channel is termed
meandering
(Leopold et al.
1964
) .
Many authors note that easily eroded, non-cohesive or
unvegetated substrates are more likely exhibit straighter
channels, whereas channels extending into vegetated
regions, such as salt marsh, are likely to increase in
sinuosity (e.g. Fig.
11.3h
, Pestrong
1965
; Garofalo
Dendritic channel networks are the most commonly
observed form on tidal fl ats and salt marshes (Figs.
11.2
and
11.3
). The smallest, or
fi r s t
-
order
, channels, end
abruptly on the marsh platform or tidal fl at, fed by
sheet fl ow over the inter-channel areas. In a classic
dendritic system two of these smaller channels join to
form a larger (second-order) channel, and so on, until
the highest-order channel in the system is reached
(Fig.
11.1a
). In tidal channels, third- or higher-order
channels are relatively rare (Eisma
1998
) .
In a fl uvial system, the low-order streams feed water
into the higher-order streams. Within a tidal system, all
of the channels experience bidirectional fl ow, with
high-order streams both feeding and receiving fl ow to/
from lower-order creeks. The ratio of low to higher-
order channels in low gradient fl uvial systems is 2; this
bifurcation ratio
is higher in tidal channels, closer to 4
(Knighton et al.
1992
; Novakowski et al.
2004
) .
However, the data presented by Novakowski et al.
(
2004
) for North Inlet, South Carolina, USA, suggest
that for low-order channels the ratio falls nearer to 2,
increasing with stream order to 7.25 for the highest
orders observed (forth- to fi fth-order).
