[Objectives] Addressing critical gaps in understanding Upper Yangtze River Basin (UYRB) baseflow dynamics is vital for regional water security and ecological sustainability. This study provides the first comprehensive multi-station assessment across the mainstream (Jinsha River, Yangtze mainstem) and major tributaries (Min, Jialing, Wu) under defined hydrological frequency years (extremely dry P=95%, dry P=75%, normal P=50%, wet P=25%), integrating spatial scalability and hydrological variability to: quantify spatiotemporal baseflow trends; evaluate dry (Nov-Apr) and wet season baseflow volume (BFV) and index (BFI) characteristics; and assess groundwater's role in sustaining river flow during droughts. [Methods] Daily runoff data (1991-2016) from nine key hydrological stations (Batang, Shigu, Pingshan, Zhutuo, Gaochang, Beibei, Wulong, Cuntan, Wanxian), representing >85% of UYRB runoff, were analyzed. Baseflow was separated using the Lyne-Hollick digital filtering method (α = 0.925, three passes). The Mann-Kendall (MK) test (significance threshold |Z| > 1.96,| > 1.96, α = 0.05) identified α = 0.05) identified temporal trends and abrupt changes (UF/UB statistics). Annual baseflow series were fitted to the Pearson Type III distribution to determine BFV for each hydrological frequency year. Seasonal BFV and BFI were analyzed for defined dry and wet seasons. [Results] Spatially heterogeneous trends emerged: Significant increases occurred at upstream Jinsha stations (Batang, Shigu; UF > 1.96 post-2010), linked to accelerated glacier/snowmelt from Tibetan Plateau warming, while significant decreases characterized Wanxian near while significant decreases characterized Wanxian near the Three Gorges Reservoir (TGR; UF < -1.96 post-2004), with a 2005 change-point aligning with TGR impoundment; mid-basin stations showed non-significant fluctuations. Dry-season showed non-significant fluctuations. Dry-season BFV exhibited exceptional stability between hydrological frequency years, with the ratio of BFV in extremely dry years to wet years ranging narrowly from 1.02 to 1.25. Conversely, wet-season BFV varied substantially (extremely dry to wet year ratio: 1.33 to 2.09), particularly in tributaries (Beibei: 1.88; Wulong: 2.09) due to rapid precipitation response. Wet-season BFV consistently exceeded dry-season BFV (mean annual ratio: 1.24 to 4.44), peaking August-October and minimizing March-aking August-October and minimizing March-May, with the highest seasonal amplitude in the Jinsha headwaters (Batang: 4.44) and reduced variability near TGR (Wanxian: 1.94-2.94). Crucially, baseflow consistently sustained >50% of dry-season river flow across all stations and frequency years (BFI range: 0.51-0.78), exhibiting.51-0.78), exhibiting a distinct V-shaped annual pattern peaking January-March. Dry-season BFI ranked extremely dry > dry > normal > wet years, confirming groundwater's amplified role during droughts. [Conclusions] This integrated assessment yields key advances: 1) It resolves contrasting baseflow responses driven by climate change (increases in glacier-fed headwaters) versus reservoir regulation (decreases near TGR); 2) Dry-season baseflow demonstrates exceptional stability between extremely dry and wet years (BFV ratio ≤1.25), acting as a reliable water source; 3) Wet-season baseflow is highly sensitive to precipitation variability (BFV ratio up to 2.BFV ratio up to 2.09), with tributaries more vulnerable; 4) Most critically, groundwater-derived baseflow consistently provides >50% of dry-season flow across all hydrological conditions, confirming its indispensable role as a climate-resilient resource that buffers droughts, securing ecological flows and informing sustainable water allocation policies in the UYRB.