Longitudinal deterioration and rainfall-associated variation in the water quality of the Gosthani River, a small regulated river on the east coast of India: 13 monthly campaigns at ten stations

Small rivers carry land-based pollution to tropical coasts but are seldom monitored, so the reaches where deterioration begins are rarely known. We sampled ten main-stem stations of the Gosthani River, east coast of India, in 13 monthly campaigns (September 2025–September 2026) to locate deterioration hotspots and to identify what a fixed-station design can and cannot resolve. Water quality changed little above the Thatipudi reservoir but deteriorated steeply in the 30 km below it. Median biochemical oxygen demand (BOD) rose from 1.70 mg/L below the dam to 12.25 mg/L at the lowest riverine station. The geometric mean of Escherichia coli rose from 465 to 3,136 most probable number (MPN)/100 mL, and median dissolved-oxygen saturation fell to 63%. The largest increments were concentrated in two reaches containing towns and villages. In a screening comparison, BOD exceeded the national Class C value of 3 mg/L in every usable campaign at the four lowest stations, where ammonia-N formed 57–76% of inorganic nitrogen. Within stations, antecedent rainfall was associated with higher suspended solids and lower conductivity; with only 12–13 shared campaigns, these associations are exploratory. At the estuary, seawater formed less than 2% of the water, yet BOD and total phosphorus were 44–46% lower than upstream. Interpreting this contrast requires measurements of lateral inflow. The spatial pattern is consistent with wastewater-associated loading, but sources were not apportioned and discharge was not measured. Four stations captured the gradient. This supports a compact monitoring design that pairs these stations with discharge gauging and rainfall-timed sampling.