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ENTBANCE CHANNELS. 22%
channel along the right batik, from Mornington Point towards Shipgunj
Point, becomes more or less silted up during the prevalence of the freshets.
On the other hand, during the dry season the flood-tide channel, or Western
Gut, along the right bank, in tlie lower part of the reach, opens out again,
till the scouring energy of the flood-tide current is dissipated to some extent
on approaching Shipgunj Point, where it spreads out and passes across to the
left bank between Ninan and Fulta Point. At the same time, the Eastern
Gut near the left bank, which depends on the ebb-tide almost entirely for its
maintenance throughout the dry season, is reduced in depth : and a bar is
formed between the ebb-tide channel near Hooghly Point and the deep
channel at right angles to it in front of Gewankhali, by the conflicting action
of the flood-tide running up this latter channel, thereby joining the James and
Mary Shoal to the Hooghly Sand below.”
Blind Channels.—This conflict of routes, resulting in the temporary
predominance of one or other, is also responsible for the formation of what
may be termed “blind channels” or cul-de-sacs, such as will be noticed in
greater or less prominence on the charts of all important estuaries. These
are deep depressions in the river-bed extending for some distance without any
apparent outlet, terminating simply in a ridge or stopped end. The Sloyne
in the Iliver Mersey is a notable example, as also the Bog Hole off Southport
in the estuary of the Ribble, Mostyn Deep at the entrance of the Dee, and the
Great Nore Channel at the mouth of the Thames. These and many other
instances are undoubtedly due to the antagonistic tendencies of the upward
and downward streams.
Variable Channels.—The roving disposition of channels in sandy
estuaries is manifestly the cause of niuch waste of physical power. The
energy possessed by the stream in virtue of its momentum, which might
profitably be expended in maintaining a deep clear channel and in removing
or preventing any obstruction of the nature of a bar, is dissipated in the effort
of eroding and displacing huge volumes of sand. It is observable in the river
Mersey, for instance, that the estuarine channel rarely occupies the same
position for a week consecutively. Between Hale Head and Garston, where
the estuary is three miles wide, the channel has been diverted within a period
of twelve months across the entire width from the Cheshire side to the
Lancashire side, and vice verså. Generally the changes are found to be
coincident with upland floods, which bring a considérable accession of water;
but so trifling are the initiatory causes sometimes, that barges grounding
against a side of the existing channel have been known to produce a most
marked deflection. The agitation arising from the process of erosion must
inevitably cause a considerable quantity of sand to remain in suspension and
to be transported to the mouth of the river, where its deposition in more
tranquil waters is only a matter of time.
Fixed Channels.—A constant channel, on the other hand, where such
can be assured, has all the advantages attaching to fixity and stability. It
entails no frequent surveys with alterations of buoys and lights ; it does
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