ESTUARINE FISH FARMING
125
2. Accretion of soil
A further favourable factor is the transport into the estuary, by its
rivers, of soil brought down from the watershed. The thousands of
tons of material become sorted out in the estuary ; the coarser particles
are deposited in the upper deltaic areas (and may be responsible for
delta formation) while the finer clay part,icles start to settle by flocculation where the river water becomes mixed with sea water. The
process is well described by Schuster (1952).
These clay deposits gradually rise to mean high-water level, where
they form mud banks which are exposed at low tide. On these banks
vegetation starts to grow. At first these are lowly forms such as filamentous algae and diatoms, many of which secrete mucous sheaths and
films which help to bind the mud surface, just as, on the land, algae
such as Nostoc may play an important part in land reclamation, binding
the surface of soils, and contributing organic matter and nitrogen to
so important a degree that their growth is encouraged by tilling
(Round, 1965).
At a later stage in the formation of mud banks, salt-tolerant higher
plants (especially mangroves in warmer latitudes and grasses such as
Spartina, low semi-recumbent plants such as Suaeda, (the samphire) and
Ruppia in temperate climates) become established and entrap and hold
yet more of the drifting silt, until the mud bank rises above mean high
tide level. Then many other salt-tolerant plants can become established, and the mud flat becomes a salt marsh or mangrove swamp.
I n this way, extensive areas of flat ground arise, in a certain relation
to tide levels, and with a basis of clay especially suitable for making
firm and impermeable embankments.
The rate of accretion of soil may be considerable. Schuster (1952)
estimated that in the area of the estuary of the Brantas River on the
north coast of Java, the rate of increase of newly-risen ground suitable
for pond construction may be as much as 26 yards annually.
I n estuaries in West Africa and South America, the formation of
mudbanks, their rise to high tide level and their colonization with
mangroves to form islands, is happening so fast that banks and middle
grounds charted in maps of only a few decades ago are now mangrove
forests.
The process may be hastened by the rapid increase in human
population, which always causes a need for more land. The felling of
forest cover and the cultivation of the new land leads to erosion which
hastens the transport of soil down to the estuaries, and so to more
rapid mud bank formation.
125
2. Accretion of soil
A further favourable factor is the transport into the estuary, by its
rivers, of soil brought down from the watershed. The thousands of
tons of material become sorted out in the estuary ; the coarser particles
are deposited in the upper deltaic areas (and may be responsible for
delta formation) while the finer clay part,icles start to settle by flocculation where the river water becomes mixed with sea water. The
process is well described by Schuster (1952).
These clay deposits gradually rise to mean high-water level, where
they form mud banks which are exposed at low tide. On these banks
vegetation starts to grow. At first these are lowly forms such as filamentous algae and diatoms, many of which secrete mucous sheaths and
films which help to bind the mud surface, just as, on the land, algae
such as Nostoc may play an important part in land reclamation, binding
the surface of soils, and contributing organic matter and nitrogen to
so important a degree that their growth is encouraged by tilling
(Round, 1965).
At a later stage in the formation of mud banks, salt-tolerant higher
plants (especially mangroves in warmer latitudes and grasses such as
Spartina, low semi-recumbent plants such as Suaeda, (the samphire) and
Ruppia in temperate climates) become established and entrap and hold
yet more of the drifting silt, until the mud bank rises above mean high
tide level. Then many other salt-tolerant plants can become established, and the mud flat becomes a salt marsh or mangrove swamp.
I n this way, extensive areas of flat ground arise, in a certain relation
to tide levels, and with a basis of clay especially suitable for making
firm and impermeable embankments.
The rate of accretion of soil may be considerable. Schuster (1952)
estimated that in the area of the estuary of the Brantas River on the
north coast of Java, the rate of increase of newly-risen ground suitable
for pond construction may be as much as 26 yards annually.
I n estuaries in West Africa and South America, the formation of
mudbanks, their rise to high tide level and their colonization with
mangroves to form islands, is happening so fast that banks and middle
grounds charted in maps of only a few decades ago are now mangrove
forests.
The process may be hastened by the rapid increase in human
population, which always causes a need for more land. The felling of
forest cover and the cultivation of the new land leads to erosion which
hastens the transport of soil down to the estuaries, and so to more
rapid mud bank formation.