Internally Cross-Checked Estimation of Downstream Water-Level Response Timing During a Controlled Dam Release in a Sparsely Gauged River

Controlled releases can reveal downstream response timing when calibrated hydraulic routing is unavailable, but sparse stage-only records limit what can be inferred. We analyze one principal 2026 controlled release on the Satsunai River, Japan, using dam outflow and two downstream water-level series. We ask whether dam-referenced lags agree with a direct station-to-station estimate, whether the reach-scale timing persists under alternative preprocessing and dependence assumptions, and what timing can be assigned conditionally to an intermediate ungauged location. Cross-correlation maxima occurred at 3.67 h at Kamisatsunai and 8.00 h at Dainikawabashi; their 4.33 h difference matched the full-record station-to-station maximum and was close to the 4.67 h release-window result. Across detrending and differencing transformations, endpoint separation remained 3.67–4.33 h. Expanded 1–24 h moving-block bootstrap sensitivity yielded a lag-difference envelope of 4.17–4.50 h, corresponding to an apparent celerity of 1.30–1.41 m/s. Linear interpolation gave 5.84 h at Nakajima Shinbashi, with a conditional percentile-interval envelope of 5.51–6.43 h. Taken together, these estimates give a consistent event-specific description of bulk hydrograph alignment. Additional events and hydraulic observations are needed to assess transferability and spatial model-form uncertainty.