Assessing the Impact of Flash Flood on Aquatic Habitat and Fishery Ecosystem Services of the Transboundary River Teesta in India Using GIS and Remote Sensing Tools

Rivers are dynamic ecosystems with diverse habitats and offer crucial services to humankind. Anthropogenic and climatic factors, including flash floods, influence the structure and functions of this critical ecosystem. Teesta is a snow-fed transboundary river, susceptible to flooding. It originates from the Indian Himalayas and confluencing with the Brahmaputra River in Bangladesh. This study aimed to assess the impact of the recent flash flood on the physical habitat, morphometry and ecosystem services of the Teesta River, India, through a geospatial approach supplemented with systematic field surveys. The physical habitat assessment conducted during pre- and post-flash flood indicated habitat quality loss along the river, influenced by the influx of sediments, debris and displacement of rocks. The riverbed mapping of Sentinel-2 satellite imagery during pre- and post-flash flood periods revealed an expansion of the floodplain in both areas as well as an increase in width in the selected locations, with the highest increase in Tanak (138%; 0.077 to 0.184 km²) and Rangpo (150.1 to 490.28 m). The spectral indices, namely, Normalised Difference Water Index and Normalised Difference Turbidity Index, indicated reduced depth in the deep pools and increased turbidity, following the flood. This suggests impaired habitat conditions that may influence fish species distribution and abundance. Additionally, the questionnaire-based fishermen's perception survey highlights the loss of multiple ecosystem services, linked with livelihood and income generation. These results describe and interpret the impact of flash flood on habitat and ecosystem services, which would be useful information in devising strategies and policy-making for mitigating measures to sustainably manage aquatic habitats and deter negative impacts in the future.