Rapid urbanization and industrial expansion in monsoon-dominated basins are intensifying pressures on secondary river systems in India. This study presents a three-year (2022–2024) multi-season assessment of the Indrayani River, a Western Ghats tributary of the Bhima basin, to evaluate spatial and temporal trends in water quality using physicochemical analyses and the Weighted Arithmetic Water Quality Index (WAWQI). Surface-water samples were collected from 22 stations representing upstream, midstream, and downstream segments during pre-monsoon, monsoon, and post-monsoon seasons. Key parameters, including pH, EC, TDS, DO, BOD₅, COD, nitrate, and chloride, were analyzed in accordance with APHA protocols and compared with BIS, WHO, and CPCB standards. Results revealed a consistent longitudinal deterioration in water quality. Upstream reaches maintained “Good” status (WQI < 50), while downstream segments exhibited “Very Poor/Unsafe” conditions (WQI > 250–300). Elevated BOD₅ (up to 28 mg L⁻¹), COD (up to 54 mg L⁻¹), and nitrate (up to 47 mg L⁻¹), coupled with DO levels below 4 mg L⁻¹, indicate a eutrophic-to-hypereutrophic transformation in the lower reaches. Degradation is structurally driven by untreated municipal sewage, industrial effluents, agricultural runoff, pilgrimage-related inputs, and reduced pre-monsoon dilution. Correlation analysis identified strong coupling among ionic enrichment indicators (EC–TDS–chloride) and organic load parameters (BOD₅–COD), with inverse relationships to DO. The study highlights the urgency of sub-basin-scale management, expanding wastewater infrastructure, nutrient interception strategies, and WQI-based monitoring, all aligned with SDG-6 and integrated river basin management frameworks.
