Lower flow did not clearly increase the warm-day difference in low-oxygen frequency within summer months at 212 US stream sites, 2000–2025

We tested whether lower flow makes the warm-day difference in low-oxygen frequency larger. We used measured USGS records for June through September of 2000–2025. We split days by the temperature and discharge medians within each site, year and month. The sample had 305,151 days, 10,269 months and 212 stream sites. Each month had at least three days in all four groups. We averaged months within years, years within sites and sites with equal weights. Minimum oxygen was below 5 mg/L on 11.21% of cooler, higher-flow days and 16.24% of warmer, lower-flow days. The warm-minus-cool difference was 3.70 percentage points at higher flow and 4.01 points at lower flow. The difference between these gaps was +0.31 points, with a nominal 95% joint spatial and year-block interval of −0.77 to +1.38. Planned checks retained the uncertainty. A stricter extreme-third check had no eligible site. The evidence does not establish an added joint association in the selected months. The design does not identify causal effects or test long droughts and rare heat extremes. The package preserves source extracts, series choices, code, all estimates, figures and independent checks.

dissolved oxygen · streamflow · water temperature · USGS · within-month comparison · additive interaction · reproducibility