The Real Ganga In India and Bangladesh - The Ganga: Water Use in the Indian Subcontinent - page 7

It is believed that the Bhagirathi was the main flow, a few hundred years ago.
Captain Sherwill said it was the main river from Rajmahal to sagar Island in olden
days, which in due course became insignificant; the present course through the
Padma is of later origin. It was formed by the opening out of the left bank near
Sibganj in Malda district, opposite Farakka. This, he says, occurred slowly after
a sudden collapse of the left bank, made of yellow sand. He says, this catas-
trophe is mentioned in the mythology of the angry sage, Jahnu swallowing the
Ganga in retaliation of her washing away his holy utensils, as narrated in the
Ramayana. Sherwill considered this anecdote in the epic as a mythological rep-
resentation of a natural calamity that occurred at Sibganj, which sage Valmiki
might have witnessed. He also believed that the silting of tributaries between the
Bhagirathi and the Brahmaputra was caused by this collapse. This is corroborated
by the fact that the outfall of the Ganga to the sea is believed to have existed near
Malda, a few centuries ago, when the largest delta in the world occupying over
58,750 km 2 , which is a little over one-fourth of undivided Bengal - was formed
by the silt and sand deposits of the Ganga, the Brahmaputra and the Meghna. Of
this, about 21,000 km 2 are in West Bengal. The existence of ancient localities,
once famous and prosperous centres of art, culture and industry, namely Gour,
Murshidabad, Beharampur, Katwa, Nabadweep, Kolkata, Howrah and the industries
on both banks, indicate that the right arm, the Bhagirathi was the main flow before
the left channel, the Padma drew increasing discharge from the end of the 18th
century.