Identification of River Discontinuity Using Geo-Informatics to Improve Freshwater Flow and Ecosystem Services in Indian Sundarban Delta

Abstract

The Indian Sundarban Delta (ISD) situated on the western tide-dominated part of the Ganges–Brahmaputra–Meghna delta, was formed by the sedimentation of the river Ganges and its tributaries. It is crisscrossed by several interlinked channels. Most of these rivers which used to nourish the delta have become defunct with the passage of time. To ensure sustainable flow and to enhance the flow-dependent ecosystem services in this region, the identification of the disconnected river reaches and their restoration is required. Keeping these perspectives in mind, this study has been conducted to identify the discontinuities in the flow paths of some major rivers using multispectral satellite imageries in geo-informatics platform. To extract the river network from the Landsat Thematic Mapper (TM) images, the water indexes like Normalized Difference Water Index (NDWI) and Modified Normalized Difference Water Index (MNDWI) have been calculated. These indexes use the differential spectral reflectance of water sensitive near-infrared and shortwave-infrared band with green band to identify the derelict paths. The spectral profiles of near-infrared band across these discontinuous stretches have been compared and validated with field data to map the actual blockages. The study reveals that the rivers Ichhamati, Jamuna, Bidyadhari, Noai, Suti, Kumarjol, Ghagramari, Karati, and Matla have been disconnected from their parent rivers and their courses are frequently interrupted by natural and man-made obstructions. River Adi Ganga is discontinuous in its middle stretches. The decayed river channels have been converted into ponds, agricultural fields, or aquaculture farms and have also been encroached by settlements. Restoration of these decayed channels may revitalize the river network and enhance the benefits of flow-dependent ecosystem services in the ISD.