Effect of Sidoarjo mud (LUSI) discharge on salinity stratification and sediment transport in the Porong River

Density stratification in tropical rivers is rarely documented, yet it plays a critical role in controlling sediment transport and water-column stability. Since 2006, the Sidoarjo Mudflow (LUSI) has discharged highly saline volcanic mud (38-40‰) into the Porong River through the Ginonjo outfall, producing a strong density contrast with the freshwater flow. This study investigates how variations in outfall operating influence salinity stratification and suspended sediment transport during the rainy season. Field observations were conducted at eight cross sections (STA 0+154-6+978) under active and non-active outfall conditions. During active outfall, near-bottom salinity reached 3.9-4.1‰, creating a vertical salinity difference ( "S) up to 3.2‰ and increasing suspended sediment concentration (SSC) downstream beyond 900 mg/L. Under inactive conditions, "S fell below 0.5‰ and SSC remained below 200 mg/L. These findings demonstrate that LUSI outfall intensifies density-driven layering and suppresses vertical mixing, indicating estuarine-like stratification in a non-tidal tropical river.