Field-Based Evaluation of Water Quality, Macroinvertebrates and Sediment in the Downstream Segment of the Upper Konto Sub-Watershed, Indonesia

As the last self-purification zone before inflow enters Selorejo Reservoir, the downstream reach of the Upper Konto Sub-watershed is critical for controlling pollutant and sediment transfer. An integrated ecohydraulic approach was used in this study to evaluate the water quality, sediment characteristics, and ecological condition under organic pollution pressure. Water quality was assessed at six sites and deposited sediments at four sites during the dry and rainy seasons. The Pollution Index (PI) and Family Biotic Index (FBI) were used to assess physicochemical water quality and biological condition, respectively; sediment grain-size distribution and nutrient contents were also examined. The flow velocities ranged from 0.37–0.68 m s⁻¹ in the dry season and from 0.54–1.15 m s⁻¹ in the rainy season, controlling erosion, transport, and deposition processes. PI results showed that the downstream reach was classified as lightly to moderately polluted in the dry season, and mostly moderately polluted in the rainy season. Conversely, the FBI showed worse biological conditions, characterized by moderately poor to very poor conditions, especially during the rainy season. TSD1–TSD3 sediments were mainly sandy, but TSD4 was composed of finer silt and mud-rich deposits. The high nitrogen concentration in TSD1 may be related to adsorption and retention of nitrogen-bearing compounds on sediment particles. Overall, ecological degradation was pronounced due to interactions among organic pollutant loads, flow dynamics, and sediment characteristics, indicating the need for integrated management of water quality, riparian conditions, and sedimentation to reduce pollutant and sediment inflows to Selorejo Reservoir.