Divergent Water Temperature Responses to Climate and Dams in the Ili and Klamath Rivers

To reveal how climate change and reservoir operation differentially affect water temperature in contrasting climate basins, this study applies a coupled hydrodynamic–thermal HEC-RAS model to the Ili River (temperate continental) and the Klamath River (Mediterranean–montane). Using ERA5 reanalysis and CMIP6 projections, we quantify the effects of climate forcing and reservoir regulation on spatiotemporal water temperature patterns, and compare the two drivers side by side. Three main findings emerge. (1) Reservoir thermal effects are climate-dependent. In the Ili, cold-water releases lower spring temperatures (9 °C→20 °C without dam vs. 5 °C→18 °C with dam) and reduce diel fluctuations. In the Klamath, cascade reservoirs cause autumn heat retention and anomalous midstream warming, which disappears in the simulated no-dam scenario. (2) Climate change drives non-uniform warming. Under SSP5-8.5 by 2080, annual mean water temperature increases by 1.5–2.5 °C in both basins, but with strong seasonality. The Ili warms most in spring and autumn (2.0–3.7 °C), while the Klamath shows an autumn peak (+3.7 °C in October). The annual mean increases are lower than the peak monthly increases because winter months contribute little to the annual average. (3) Energy budget analysis reveals the warming mechanism. Both basins share a similar budget: solar radiation (summer peak around 370–375 W/m2 in 2020, increasing to about 385–395 W/m2 by 2080) and atmospheric radiation are primary heat sources, and longwave radiation (summer about −410 W/m2 in 2020, reaching about −430 W/m2 by 2080) is the main heat sink. By 2080, enhanced solar radiation (+5%) and reduced longwave loss (–5%) increase net summer heat gain, driving warm-season warming. These findings provide a basis for considering reservoir operations in relation to regional climate contexts in transboundary rivers, and for identifying where thermal responses are most sensitive to reservoir regulation.