Monthly high- and low-water gaps had positive adjusted links at four of six US gauges in retained 1960-2025 records

We tested whether unusually large high-water and low-water departures occur in the same month. We reused 4,738 fixed NOAA monthly records and retained 4,425 records at six US gauges. We removed calendar-month means, a linear trend, and a fixed 18.61-year cycle from each gap. Partial correlations were 0.269 at Boston, 0.149 at The Battery, 0.217 at Sewells Point, and 0.149 at San Francisco. Approximate 95% family intervals from three-year block resampling were above zero at these four sites. Charleston and Seattle had small negative estimates with intervals across zero. Added tide-range and mean-level controls kept the four positive point estimates. A constant vertical shift within each month cannot alone create either gap because it cancels in the differences. The result is consistent with variation in water-level spread, but it does not identify storms, a common event, or a physical cause. Selected sites, incomplete records, source changes, and block assumptions limit inference. Fixed numerical evidence, source hashes, code, all results, and figures are included.

monthly water extremes · tide gauges · paired extremes · partial correlation · block bootstrap · NOAA CO-OPS