82
WILLIAM MACNAE
described by Fosberg (1961) a t Gladstone in Queensland, and their
development is discussed by Macnae (1966, 1967) with conclusions
similar to those of Guileher."
Mangroves penetrate only a short way upstream in the estuaries of
some rivers and very far in others. I n the main channel of the Zambezi
delta, the Rio Cuama, mangroves penetrate only some 15 km from the
mouth. I n the Rio Maputo, running into the Bay of Lourenyo Marques,
trees of Avicennia marina (Borsk.) Vierh. with a few Bruguiera
gymnorhiza (L.) Lmk. and an occasional Rhizophora mucronata Lmk.
extend upstream for less than 8 km. The tide goes far beyond this,
above the village of Salamanga some 40 km upstream. At this village
an association dominated by Barringtonia racemosa (L.) B1. and peopled
by estuarine crabs occupies the " high marsh " which is quite narrow
(Macnae, unpublished information).
I n the Musi river of southern Sumatra mangroves extend as far as
Palembang, 50 km inland ; in the Kikori river of Papua they extend
some 100 km upstream ; Bruguiera and Xonneratia were found 320 km
up the Fly river in southern New Guinea (van Steenis in Ding Hou,
1958).
I n the upper parts of tidal estuaries fresh water may be pushed back
by the tidal wave, particularly when the discharge of the river is
normally high. It has been shown by many authors (see Emery et al.,
1957) that salt water penetrates an estuary in the form of a wedge
which tapers upstream so that the fresh water lies on the surface and
the " salt wedge " creeps along the bottom. As Alexander et al. (1932,
1935) showed, this " salt wedge " creeps farther upstream in the mud
on the bottom. Hence it is not surprising that mangroves penetrate
far upstream. The short penetration of the Zambezi delta has two
causes : the banks are high, indicating that the trees would have to be
very deep-rooted to reach the " salt wedge " ; also there is a bar across
the mouth which impedes the entry of sea water. The mouths of the
rivers mentioned by van Steenis probably have no bars, and the banks
are low. The Hooghly river, a distributary of the Ganges, is navigable
by ocean vessels as far as Calcutta, and has a tidal portion of over
300 km. Mangroves extend beyond Calcutta.
Geological signi$cance of mangroves
Davis (1940) has shown that in Florida mangroves have an important
function in relation to the accumulation of coastal sediments. This is
* Guilcher (1965) has pointed out that such bare areas occur in the vicinity of Port
Moresby in New Guinea on a coast with a seasonal rainfall due to its boing in the rain
shadow of the central mountain chain.
WILLIAM MACNAE
described by Fosberg (1961) a t Gladstone in Queensland, and their
development is discussed by Macnae (1966, 1967) with conclusions
similar to those of Guileher."
Mangroves penetrate only a short way upstream in the estuaries of
some rivers and very far in others. I n the main channel of the Zambezi
delta, the Rio Cuama, mangroves penetrate only some 15 km from the
mouth. I n the Rio Maputo, running into the Bay of Lourenyo Marques,
trees of Avicennia marina (Borsk.) Vierh. with a few Bruguiera
gymnorhiza (L.) Lmk. and an occasional Rhizophora mucronata Lmk.
extend upstream for less than 8 km. The tide goes far beyond this,
above the village of Salamanga some 40 km upstream. At this village
an association dominated by Barringtonia racemosa (L.) B1. and peopled
by estuarine crabs occupies the " high marsh " which is quite narrow
(Macnae, unpublished information).
I n the Musi river of southern Sumatra mangroves extend as far as
Palembang, 50 km inland ; in the Kikori river of Papua they extend
some 100 km upstream ; Bruguiera and Xonneratia were found 320 km
up the Fly river in southern New Guinea (van Steenis in Ding Hou,
1958).
I n the upper parts of tidal estuaries fresh water may be pushed back
by the tidal wave, particularly when the discharge of the river is
normally high. It has been shown by many authors (see Emery et al.,
1957) that salt water penetrates an estuary in the form of a wedge
which tapers upstream so that the fresh water lies on the surface and
the " salt wedge " creeps along the bottom. As Alexander et al. (1932,
1935) showed, this " salt wedge " creeps farther upstream in the mud
on the bottom. Hence it is not surprising that mangroves penetrate
far upstream. The short penetration of the Zambezi delta has two
causes : the banks are high, indicating that the trees would have to be
very deep-rooted to reach the " salt wedge " ; also there is a bar across
the mouth which impedes the entry of sea water. The mouths of the
rivers mentioned by van Steenis probably have no bars, and the banks
are low. The Hooghly river, a distributary of the Ganges, is navigable
by ocean vessels as far as Calcutta, and has a tidal portion of over
300 km. Mangroves extend beyond Calcutta.
Geological signi$cance of mangroves
Davis (1940) has shown that in Florida mangroves have an important
function in relation to the accumulation of coastal sediments. This is
* Guilcher (1965) has pointed out that such bare areas occur in the vicinity of Port
Moresby in New Guinea on a coast with a seasonal rainfall due to its boing in the rain
shadow of the central mountain chain.