Lake Superior showed no clear further loss of mean seasonal level range from 1974-1999 to 2000-2025

We tested whether Lake Superior's mean seasonal level range decreased further after 1999. Coordinated monthly lake-wide levels, version v2025, gave three equal 26-year periods. The range of the mean annual harmonic fell from 33.49 cm in 1948-1973 to 26.93 cm in 1974-1999. It then rose to 29.21 cm in 2000-2025. The later change was +2.28 cm, with a conditional 95% three-year block interval of -3.59 to +8.64 cm. Thus, the test did not resolve a further decrease or a reversal. A separate linear drift term in each year gave +2.06 cm, interval -1.73 to +5.82 cm. Michigan-Huron also had a positive point change with an interval that included zero. Five- and seven-year blocks gave the same null conclusion. These results extend an established seasonal-cycle question with a fixed-version benchmark. They do not identify a water-budget or climate cause. Short periods, long climate cycles, source revisions and model assumptions limit inference. The package includes numeric source rows, hashes, code, uncertainty draws, a figure and an independent numerical audit.

Great Lakes · Lake Superior · seasonal cycle · water level · null result · block bootstrap