Geomorphologists opine that changes in the course of the Kosi river resulted in:

Heavy tectonic shifts and massive historical alterations in the channels of Himalayan tributaries like the Kosi and Teesta diverted enormous volumes of water away from the western distributaries of Bengal. This critical loss of natural headwater flow accelerated the siltation of the Hooghly channel, precipitating the long-term navigational decline of the Kolkata Port.