Extreme daily highs had opposite low-water signs at four US gauges in retained 1960-2025 records

We tested low water on dates with extreme daily high water at six selected US gauges in 1960-2025. We used 42 fixed NOAA daily hourly maximum and minimum files. We removed the separate high and low means for each year and month. We retained 139,889 station-days and selected 968 event peaks above local 99th percentiles with three-day grouping. Mean low-water anomalies were -0.193 m at Boston, -0.234 m at San Francisco, +0.169 m at The Battery and +0.387 m at Sewells Point. Six-site adjusted conditional intervals excluded zero at these four sites. Charleston was near zero. Seattle had a positive estimate that fell from +0.090 m to +0.016 m with a smooth baseline. The four main signs held in two time periods and with other thresholds, event gaps and baselines. This extends prior campaign tests with a long-record event-peak comparison and an exact midpoint and range summary. It is not independent evidence from new gauges. Tides remain in the records. Daily summaries cannot identify surge, event order, drainage duration or a physical cause. Code, fixed daily evidence, hashes, checks and repeat instructions are included.

daily extrema · low water · NOAA CO-OPS · event selection · tidal range