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

Mountain headwater basins are critical water sources, yet evaluating their long-term hydrologic response is often hindered by sparse and uneven observations. This study used multi-decadal distributed hydrologic simulations together with streamflow and snow observations to examine seasonal runoff behavior, gauge-network variability, and streamflow persistence across the Upper Snake River Basin, Wyoming. The modeling framework combined a 300 m land-surface grid with a 30 m routing grid, enabling analysis of both basin-scale runoff behavior and channel-network dynamics. Focusing on the primary inflows to Jackson Lake, annual runoff ratios, both observed and modeled, exhibited interannual variability but no statistically significant long-term trend. Runoff was strongly seasonal, with late spring and early summer contributing the largest fractions of annual flow. Monthly analyses further showed that runoff variability was concentrated in the melt season, while the general timing of seasonal runoff remained broadly consistent over the study period. Although discrepancies occurred at regulated mainstem sites and some tributaries affected by local controls not represented in the model, spatially coherent long-term hydrologic trends were largely absent. Persistence analysis indicated that perennial channels formed the dominant and spatially stable component of the modeled drainage network. Together, these results revealed that the recent hydrologic history of the Upper Snake River Basin was characterized by strong seasonal organization and interannual variability rather than strong directional reorganization. This study provided a useful historical baseline for understanding and managing the headwaters of a regulated river system.