Multiscale teleconnection controls on river water temperature variability in a cold region regulated basin

River water temperature integrates atmospheric forcing with hydrological and operational controls, yet the extent to which large-scale climate modes shape river thermal variability in regulated, cold region basins is still not well constrained. We examine the imprint of two dominant Pacific modes, El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), on multi-decadal river temperature variability across the 47,200 km² Nechako River Basin of western Canada, where a major reservoir and flow regulation may alter the transmission of climate signals to the river network. Monthly water temperature series were assembled for 10 stations over the period 1950-2024 using a combination of Air2Stream model hindcast simulations and available observations. We then used multiscale time-frequency and coherence diagnostics to characterize variability from interannual to decadal bands and to isolate the relative influence of ENSO and PDO while accounting for shared variability and the confounding effects of regulation. The analyses indicate a clear scale separation in climate-temperature linkages: ENSO is associated with intermittent interannual modulation of river temperatures, whereas PDO relates more consistently to lower-frequency variability, with spatially heterogeneous expression across the basin. Regulated reaches show reduced persistence of low-frequency thermal variability compared with less regulated sites, consistent with reservoir storage and operations that damp longer timescale climate imprints and reshape seasonal sensitivity. In particular, the combined effects of large-scale climate variability and regulation emerge most strongly during warm-season conditions, when thermal habitat constraints are most relevant. Overall, the results show that teleconnection controls on river thermal regimes are strongly scale-dependent and can be substantially modified by regulation in cold-region systems. Resolving these interacting controls provides a basis for interpreting past thermal changes and for improving climate-informed river management and warming risk assessments under future variability.