Daily high-water peaks had opposite first-day shape contrasts at The Battery and Sewells Point in 1960-2025

We tested time symmetry around high daily water-level peaks at six US gauges in 1960-2025. We used NOAA hourly daily maxima minus monthly mean sea level. A symmetric rule retained 2,855 peaks above calendar-month 95th percentiles. The mean level one day after minus one day before a peak was -6.45 cm at The Battery and +2.64 cm at Sewells Point. Conditional three-year block intervals with a six-site comparison rule were -9.10 to -4.03 cm and +0.53 to +4.78 cm. The other four site intervals included zero. The equal-site mean was -1.02 cm, but excluding The Battery gave +0.07 cm. A daily midpoint check gave an uncertain equal-site mean of -0.05 cm. The result shows site and measure dependence in this daily shape statistic. It does not measure recovery time or identify a physical cause. Selected gauges, daily sampling, remaining tides, missing records, and fixed peak labels limit inference. The package gives source paths and hashes, daily evidence, peak windows, code, bootstrap values, figures, an audit, and offline repeat instructions.

sea level · peak shape · time asymmetry · NOAA CO-OPS · daily maxima · block bootstrap