Low oxygen days missed by daily means formed twice the three-day windows expected from monthly random dates at 244 US stream sites in summers of 2010–2025

We tested whether summer days with minimum oxygen below 5 mg/L and mean oxygen at least 5 mg/L form repeated sequences. We reused 296,741 approved USGS day records at 244 stream sites in June-August 2010-2025. There were 271,884 valid three-day windows within months. With equal site weights, 3.33% had three missed days. Uniform random assignment of the same flags to observed dates within each site, year and month predicted 1.63%. The observed rate was 2.04 times the reference rate. The excess was 1.70 percentage points, with a 95% spatial-cell reference interval of 1.29-2.26. Seven-day date groups gave a smaller positive excess of 0.63 points. Of windows with three low daily minima, 29.12% had means at least 5 mg/L on all three days under the same weights. An independent source check rebuilt all panel rows and reproduced the main rates. The exact reference formula passed 168 small enumeration checks. The package has source extracts, hashes, provenance, code, results, figures and offline repeat instructions. The result applies to this selected monitoring set and the stated date reference. Daily minima do not show continuous low oxygen, exposure duration or damage to species.

dissolved oxygen · stream monitoring · temporal dependence · daily minimum · repeated low oxygen · open data · conditional reference