Hydromorphological and ecological assessment of the River Silai in eastern India: Furthering multi-index based approaches in prioritising reach-level restoration in tropical rivers

Hydromorphological evaluations of riparian zones, following the European Union’s Water Framework Directive, integrate components of channel morphology, hydrology and ecology for sustaining long-term river restoration goals. However, such frameworks have seldom been applied in the Global South, especially in the Indian subcontinent, where river health assessment has traditionally focused on discerning the water quality. We adapt the EU framework to examine reach-level channel quality status and hydromorphological character of the River Silai in eastern India, using an aptly-devised, integrated multi-metric assessment that evaluates the river’s morphological functions and hydrological attributes, while also considering its riparian vegetation structure and water quality. The enumerated Morphological Quality Index (MQI) deftly highlighted geomorphic and hydrological impairments occurring within the examined 117 stream reaches, which were classified as per the Rosgen scheme. Site-specific Horton’s Water Quality Index (WQI), obtained from analysis of over 350 water samples, revealed the direct influence of channel morphology and its related anthropogenic use (such as riverbed sand mining) on the ambient water quality. These results were collated with parameters assessing the riparian vegetation status, such as the Qualitat del Bosc de Ribera (QBR), Riparian Strip Quality Index (RSQI) and Normalised Difference Vegetation Index (NDVI), to gain further insight into the overall stream health status. Finally, the devised overall channel quality rating identified the middle and lower courses of the Silai River to be the most impaired, which equates to just over one-fifth (21.37%) of the entire river’s length. While observer subjectivity/agreement, seasonality of water quality parameters, and accuracy of satellite image classification can affect results, statistical analysis revealed significant associations between these indices, reflecting their aptness for river health assessment. The adopted methodology is thus a rational and holistic framework to ascertain the ambient hydromorphological disturbances (largely human-induced), in addition to pinpointing sites that need management/restoration on a priority basis.