Geoscience Reference
In-Depth Information
Mississippi
Sediment input
Modern
Mississippi
St Bernard
(Miss.)
Po
Danube
Fluvial
dominated
Lafourche
(Miss.)
Yukon
Manakam
Ebro
Fly
Nile
Orinoco
Wave
dominated
Tide
dominated
Sao
Francisco
Niger
Mekong
Burdekin
Rhone
Kelantan
Brazos
Sao Francisco
Yalu
Colorado
Ganges-Brahmaputra
Kiang-Langat
Fly
Copper
Wave energy flux
Tidal energy flux
Fig. 7.2
The morphological classification of deltas based on the influence of river, tidal and wave activity. (Compiled and modified from
various sources.)
them is more diffuse, and the salt wedge is
less pronounced (Fig. 7.1b). With increasing
depth, water will remain fresh, then show a
more gradual salinity rise until it becomes truly
marine. The final type is the well-mixed estuary
produced by large volumes of sea water entering
estuaries with significant river discharge. Because
of the high energy conditions, water mixes com-
pletely and is generally brackish throughout its
depth, although there will be some slight vari-
ation in actual salt concentrations with distance
up the estuary (Fig. 7.1c).
The classification of deltas focuses on the degree
of reworking of sediment by the sea. As previ-
ously outlined, deltas form when large volumes
of river sediment enter the sea at a rate that
exceeds the sea's ability to rework it. This does
not, however, preclude that the sea (tides and
waves) will rework the sediment to some extent.
Hence, classification of deltas is generally done
on the basis of the dominance of rivers, waves or
tides (Fig. 7.2) (see also Carter (1988), Haslett
(2000) or Woodroffe (2003) for examples).
However, to complicate this, Wright & Coleman
(1971, 1972) have suggested that there is a con-
tinuum of delta shapes related to the varying
importance of tides, waves, and river power and
that the tendency of some classifications to assign
a delta to one 'type' is problematic because it fails
to recognize this continuum. Hence, traditional
terms such as 'lobate' (deltas with prominent
sediment lobes), 'cuspate' (deltas with concave
seaward margins caused by wave shaping) and
'birdsfoot' (deltas where the bifurcating river
has extended seawards over the delta) are all well
entrenched in the literature but tend to suggest
that deltas are of one type or the other. Over
time, however, these terms have not been used
consistently, and have also been misinterpreted
because of difficulties with field identification.
