Stability amidst variability: Four decades of hydrologic response in the Upper Snake River Basin, Wyoming, USA

Study region: Mountain headwater basins are critical water sources, yet their long-term hydrologic response is often difficult to evaluate because observations are spatially sparse. This study examines the approximately 8900 km² Upper Snake River Basin in Wyoming, USA, a snowmelt-dominated headwater region supplying Jackson Lake and the broader Snake River system. Study focus: We combined a 38-year WRF-Hydro simulation for WY 1984–2021 with observations from 15 U.S. Geological Survey gauges to evaluate seasonal runoff behavior, runoff efficiency, runoff timing, variation in gauge-network trends, and streamflow persistence. The model used a 300 m land-surface grid and a 30 m routing grid and achieved Nash–Sutcliffe efficiency values of 0.91 during calibration and 0.80 during evaluation. New hydrological insights for the region: Runoff remained strongly concentrated in late spring and early summer, with most interannual variability occurring during the snowmelt season. At Flagg Ranch, annual runoff efficiency and cumulative runoff timing indicated no statistically detectable monotonic shifts, and the estimated runoff-ratio trends were much smaller than the changes the 38-year record could reliably detect. Across the 15-gauge network, significant trends were limited and not consistent across the basin, although localized responses occurred in regulated and hydro-geologically complex watersheds. Perennial channels remained the dominant and spatially persistent component of the modeled network, while seasonal and ephemeral reaches were concentrated in smaller headwater branches. Together, these findings reveal strong seasonal organization, localized hydrologic complexity, and persistent network structure amid substantial interannual variability, without pervasive directional hydrologic reorganization over the 1984–2021 period.