Three-day low-oxygen windows exceeded monthly count expectations at 131 US stream sites in 2000–2025

We tested whether low daily oxygen returned on consecutive dates more often than a monthly count reference predicted. Open USGS records supplied 348,034 matched days at 131 stream sites in June–September 2000–2025. We counted overlapping three-day windows with a daily minimum below 5 mg/L on all dates. We kept missing dates and source-series boundaries fixed. Under equal site, year and month weights, 16.85% of eligible windows were low, compared with 13.70% expected after uniform reordering within each site-month-series group. The excess was 3.15 percentage points, with a nominal 95% joint spatial and calendar-block interval of 2.30–4.08 points. The observed rate was 23.0% above the reference, with an interval of 15.8–39.5% above. All three daily means stayed at or above 5 mg/L in 26.94% of low-minimum windows; the interval was 18.02–41.48%. The positive excess remained under the planned coverage, threshold, time-window and block checks. Independent readers checked all 743,310 candidate triplets and all 319,212 eligible three-day windows. A clean offline rerun matched all 52 numerical files exactly. The package preserves exact source-row extracts, source hashes, code, all results and figures. These selected sites do not represent all US streams. Consecutive low minima do not show continuous low oxygen or biological damage. The result identifies time order relative to the stated reordering reference, not its cause.

dissolved oxygen · stream ecology · consecutive days · conditional permutation · environmental monitoring · reproducible data