2605.02183 Do GAGES-II Reference Gauges Show Flood Non-Stationarity When Restricted to Unregulated Basins?
The claim that floods are becoming larger across the continental United States is frequently stated without distinguishing climate-driven change from the hydrologic footprint of reservoirs, diversions, and urbanization. Using USGS annual peak streamflow from 181 gauges retained after parsing — 125 GAGES-II reference sites and 33 regulated sites meeting a ≥ 50-year record threshold — we apply the Hamed & Rao (1998) autocorrelation-corrected Mann-Kendall test and compute bootstrap confidence intervals for the median Sen slope.