Monthly high-to-low water spans had no clear linear trend at six US gauges in adjusted 1960-2025 fits

We tested changes in the monthly highest minus lowest water level at six US gauges in 1960-2025. We used six fixed NOAA CO-OPS monthly data files. Fits included calendar month, a fixed 18.61-year cycle, time, and mean tidal range. The adjusted span trends were 0.03 to 3.19 mm per decade. All six pointwise 95% intervals included zero. Mean sea level rose by 19.89 to 54.17 mm per decade on the same retained months. Checks of cycle terms, record interval, missing months, and error dependence also left the adjusted span trends uncertain. This result describes one monthly span and does not show constant extremes or unchanged flood risk. Monthly extreme sampling, missing data, selected harbor sites, and model assumptions limit inference. The package supplies input extracts, source paths and hashes, code, results, a figure, and a separate numerical check.

sea level · monthly extremes · tidal range · null result · NOAA