Oxygen minima below 5 mg/L formed 21% more adjacent pairs than a reference with fixed monthly counts at 244 US stream sites in summers of 2010–2025

We tested whether low daily oxygen minima recur on adjacent dates more often than monthly counts predict. We reused a fixed panel of 296,741 approved USGS daily records from 244 stream sites in June–August 2010–2025. The reference keeps each site-month low-day count and missing dates fixed. Both dates had minima below 5 mg/L in 12.65% of adjacent pairs under equal site, year and month weights. The exact reference rate was 10.43%. The observed rate was 21.3% higher. The excess was 2.22 percentage points, with a nominal 95% interval of 1.71–2.85 from spatial cells and shared year blocks. Fixed seven-day counts gave a smaller positive excess of 0.87 points. Other oxygen levels, daily means and stricter source flags gave positive excesses. These records show repeated daily minima beyond the tested count references in this monitoring set. They do not show continuous exposure, biological injury or national stream prevalence. The package contains source extracts, exact reference code, all counts and draws, independent checks and figures.

dissolved oxygen · streams · temporal clustering · daily minimum · USGS · conditional reference · reproducible analysis