Impact of degradation of River Assi Catchment on Flood Dynamics of Varuna Basin in the Middle Ganga Plain (India)

The Assi River, once a vital cultural and ecological lifeline in the middle Ganga plain, has undergone significant degradation due to siltation, urban encroachment and channel disappearance. Historically an alluvial rivulet originating near Durvasha Rishi Ashram in Allahabad, took the shape of River Assi and after traversing around 120 km merges with the Ganga in Varanasi city. Currently, only the last 8 km retain any semblance of a river, with more than 90% of the upstream channel buried under silt. The degradation of River Assi catchment has led to the emergence of a new stream of the Morwa River, which drains the flows from the Assi's buried sections and join to River Varuna as a tributary. Using Landsat 5 imagery, SRTM DEM, NDVSI, and PCA of NDVI, this study identified the paleochannels of River Assi and reconstructed the its historical course. Additionally, hydrological modelling was done using Arc SWAT to delineate the sub-basins.

The altered hydrological dynamics of the River Assi have cascading impacts on downstream ecosystems, including the Varuna River basin, which has experienced increased flooding frequency and severity. Due to the disruption of natural drainage networks in River Assi catchment, In 2022, over 10,000 households were affected by flooding in the Varuna basin. Flood mapping using Sentinel-1 SAR data and Google Earth Engine (GEE) revealed that altered flow regimes in River Assi exacerbate water accumulation in River Varuna during monsoons. The study highlights the importance of restoring paleochannels to mitigate flooding and improved hydrological stability. The integration of high-resolution DEMs, land use data, and GEE tools provides a cost-effective approach in flood risk management and underscores the necessity of addressing upstream river concerns to safeguard flood-prone downstream basins.