Cross-Tributary Water Mass Signals and Seasonal Stratification Dynamics in a River-Type Reservoir

Seasonal stratification in river type reservoirs controls vertical water quality structure, but it is difficult to assess where only routine monitoring data are available. This is particularly relevant to Paldang Reservoir, where multiple tributaries, short residence time, rainfall, and dam operation interact to produce variable mixing. This study applied a Relative Difference (RD) framework to paired upper and lower layer observations of water temperature, electrical conductivity (EC), and total organic carbon (TOC). A reference period (2013 to 2014) and a recent period (2020 to 2023) were compared across stations PD1 to PD4. RD values were interpreted with discharge, precipitation, station depth, and tributary setting rather than as independent hydrodynamic evidence. RD captured seasonal vertical heterogeneity at deeper stations (PD2 and PD4), whereas shallow stations dominated by inflow effects (PD1 and PD3) were more frequently mixed or irregular. EC based and TOC based RD often showed similar seasonal tendencies at deeper stations, but local divergences indicated that ionic water mass signals and organic matter dynamics do not always respond synchronously. The novelty of this study is a multistation RD framework applied across two monitoring periods for screening stratification related water quality variability in a river type reservoir using routine data. The results identify periods and locations where vertical heterogeneity may affect management, while showing that direct density, velocity, tracer, or high frequency profile data are required when density current mechanisms are inferred.