Monthly extreme water gaps had uncertain trends at six US gauges in primary fits to retained 1960-2025 records

We tested trends in monthly water extremes relative to typical high and low water at six selected US gauges. We used 4,425 NOAA monthly records from 1960-2025. The upper gap is highest water minus mean higher high water. The lower gap is mean lower low water minus lowest water. Fits used a linear trend, calendar-month terms, and a fixed 18.61-year cycle. All twelve primary 95% intervals included zero with errors that allow dependence over 24 retained months. Upper trends ranged from -2.66 to +4.85 mm per decade. Lower trends ranged from -3.94 to +4.34 mm per decade. All twelve Holm-adjusted p values were 1.00. Later start dates also gave uncertain trends. Longer error dependence gave two unadjusted intervals above zero, so interval choice matters. These results do not prove stable extremes or unchanged storms. Sparse early tide fields, source changes, selected sites, and model assumptions limit inference. Code, fixed monthly evidence, source hashes, all fits, figures, and checks are included.

water level extremes · tide gauges · NOAA CO-OPS · null result · trend uncertainty