Decline of Bhagirathi-Hooghly Channel - The Ganga: Water Use in the Indian Subcontinent - page 65

source, more it will scour the bed during ebb tide. The Ganga water, carrying huge
silt-load, has been restricted by the Lalthakuri embankment, the road from Jiaganj to
Bhagawangola, Ranaghat to Lalgolaghat railway line - all on its left (western) bank.
Their silt-load did not wash away and the rush of relatively silt-free water during ebb
tide is restricted, causing siltation in the Bhagirathi-Hooghly and its offtake point at
Geria. Mr. Chatterjee quoted British irrigation engineer, Sir William Willcock who
held that if the spill is not restricted by artificial obstructions, the silt would spread
all over the land and very little of it will be deposited in the river-bed. If this goes
on, land adjacent to both banks will not be elevated more than a foot in 100 years
and if the river-bed rises accordingly, no river should die.
In 1910, Hyden and Pasco wrote, quoting from Gaikie's Text Book of Geology
(Volume I, page 499) that a characteristic feature of streams, large or small, is the
tendency to flow in serpentine curves, when the angle of declivity is low and the
general surface of the country is tolerably level. This is observed in every stream
which traverses a flat plain. When a river enters a delta in its course, it assumes a
new character. In previous parts of its journey, it is augmented by tributaries but
now it begins to split into branches which wind, to and fro, on the flat alluvial
plain, often coalescing and thus enclosing insular spaces of all dimensions. The
feeble current, no longer able to bear all its sediment load, allows much of it to sink
to the bottom and gather over the tracks which are submerged from time to time.
Hence many of the channels are choked, while others open out in the plain and
are in turn, abandoned. In this way, rivers change their channels, restlessly. Thus,
the meandering nature of Nadia rivers and shift in their courses are not unique but
common to all rivers flowing on flat plains.
Holding the same view, Fergusson added, all over the world, this characteristic is
common in rivers, flowing on alluvial plains. Even great rivers, like the Mississippi
and the Tigris, do this in a marked degree, as seen in their maps. The phenomenon
is caused by the relation between the varying and erosive action of current and the
resistance of the soil. Even though a fully silt-charged current, striking an erodible
bank at a weak place cuts into it, naturally along the vulnerable portion. A con-
cave bank, or a bight, is thus formed with relatively deep water on its face. The
matter dislodged is carried downstream until the velocity of the current is gradu-
ally dissipated by the resistance of the bank as well as by cross-currents and eddies,
created by the erosion. Being no longer able to carry it on, the current drops it and
is deflected by the direction of the bank and its centrifugal action, affecting the
other bank. The slope and the velocity being reduced, deposits take place over the
whole cross-section and a bar is formed at the crossing. This creates a head and
the slope, increasing the current, strikes on the other bank, below the crossing with
increased velocity; the process is thus continued and goes in a cycle. The head of
the Bhagirathi and its bed silted in this manner.
According to Friedkin, the swing of the river depends on its capacity to erode
the banks, which increases the silt-load and heavier particles deposit over the bed,
being unable to be carried with the current, and gradually forms the alluvial fan, or
point bar inside the river-bed. The main current is thus deflected and erodes banks
in a cycle of erosion and deposition.