How often does the Big Sioux fall below the state’s oxygen line?
Asked August 25, 2026 Figures as of October 12, 2026
Rarely: of 3,253 oxygen readings on the Big Sioux since 2012, 34 sat below the state's 5 mg/L line for warmwater fish — 1.0%, 27 of them at I-90. They fall in 8 of the 15 years, 7 in each of 2012 and 2021. Whether that floor is moving, the record cannot show. Comparing the first three complete summers the city sampled at each site with the last three, summer dissolved oxygen moved by no more than 0.9 mg/L, falling at 2 sites and rising at 3; the median June–August temperature moved by no more than 0.7 °C at every site, up at 2 of the 5 and down at 3. For scale, the middle half of a single site's summer readings spans 1.3 mg/L to 1.9 mg/L and 3.1 °C to 3.2 °C, depending on the site.
Readings below the 5 mg/L line, per year
Every charted dissolved-oxygen reading, all year, at the four Big Sioux sites in river order. Skunk Creek's standard is seasonal and has no year-round line to count against.
27 of 704 readings below the line; 8 of 15 years with any, on 29 to 66 readings a year.
1 of 757 readings below the line; 1 of 15 years with any, on 3 to 74 readings a year.
3 of 878 readings below the line; 2 of 15 years with any, on 35 to 93 readings a year.
3 of 914 readings below the line; 3 of 15 years with any, on 36 to 103 readings a year.
Summer dissolved oxygen, median per year
June, July, and August readings pooled per year, one bar per complete summer with at least 5 readings. Sites in river order, tributary last. The line is the state's year-round minimum for warmwater fish on the Big Sioux.
2012–2014: 6.8 mg/L; 2024–2026: 6.4 mg/L. Highest summer 2017, 8.6 mg/L.
2014–2016: 7.4 mg/L; 2024–2026: 7.8 mg/L. Highest summer 2019, 8.6 mg/L.
2012–2014: 7.3 mg/L; 2024–2026: 7.5 mg/L. Highest summer 2017, 9.8 mg/L.
2012–2014: 7.2 mg/L; 2024–2026: 7.2 mg/L. Highest summer 2017, 9.2 mg/L.
2012–2014: 7.0 mg/L; 2024–2026: 6.2 mg/L. Highest summer 2017, 7.7 mg/L.
Summer temperature, median per year
The same summers.
2012–2014: 22.7 °C; 2024–2026: 22.8 °C. Highest summer 2021, 24.7 °C.
2014–2016: 22.4 °C; 2024–2026: 21.8 °C. Highest summer 2016, 24.0 °C.
2012–2014: 23.5 °C; 2024–2026: 23.1 °C. Highest summer 2022, 24.6 °C.
2012–2014: 23.0 °C; 2024–2026: 22.6 °C. Highest summer 2016, 24.4 °C.
2012–2014: 21.7 °C; 2024–2026: 21.7 °C. Highest summer 2012, 23.6 °C.
| Site | DO readings below 5 mg/L, all year | Summers compared | Dissolved oxygen | Temperature |
|---|---|---|---|---|
| Big Sioux River @ I-90 | 27 of 704 | 2012–2014 vs 2024–2026 | 6.8 mg/L → 6.4 mg/L · −0.4 mg/L | 22.7 °C → 22.8 °C · +0.1 °C |
| Big Sioux River @ Falls Park | 1 of 757 | 2014–2016 vs 2024–2026 | 7.4 mg/L → 7.8 mg/L · +0.3 mg/L | 22.4 °C → 21.8 °C · −0.7 °C |
| Big Sioux River @ Bahnson | 3 of 878 | 2012–2014 vs 2024–2026 | 7.3 mg/L → 7.5 mg/L · +0.2 mg/L | 23.5 °C → 23.1 °C · −0.4 °C |
| Big Sioux River @ Timberline | 3 of 914 | 2012–2014 vs 2024–2026 | 7.2 mg/L → 7.2 mg/L · +0.0 mg/L | 23.0 °C → 22.6 °C · −0.4 °C |
| Skunk Creek @ Marion Road | no year-round line | 2012–2014 vs 2024–2026 | 7.0 mg/L → 6.2 mg/L · −0.9 mg/L | 21.7 °C → 21.7 °C · +0.0 °C |
Method
Counted: every charted temperature and dissolved-oxygen reading at the five Sampling sites, 2012–2026. Not charted, and left out of every figure here including the below-5 counts: 6 dissolved-oxygen values and 0 temperature values outside the Section's plausibility bounds, and the meter fields of 3 rows that read 0 in every one of them, which the Section reads as unmeasured. Readings below the 5 mg/L line are counted year-round on the Big Sioux, where the state's warmwater semipermanent standard applies all year, and a reading counts when it is strictly below the line, so the count moves with values in the tenth just under it. The 2012 count begins with the record, on June 5, 2012, and the 2026 count is partial at the Snapshot date; yearly counts are not adjusted for the change in sampling cadence described below. "Rarely" means fewer than one reading in 20, pooled across the 4 Big Sioux sites and the whole year; the share runs higher at one site in summer, as the first charts show. Skunk Creek's marginal-fish standard changes by season and is not counted.
The "is it moving" comparison: summer is June through August by the Sample's date; each summer's figure is the median of its readings. A summer counts only if it is complete — the record runs past its August — and has at least 5 readings; that leaves out Big Sioux River @ Falls Park in 2012 (3 readings). The 2012 summer begins with the record, on June 5, 2012. "First three against last three" takes the median of 3 summer medians at each end; the summers compared are listed per site in the table. No line is fitted and no significance is claimed: the comparison is the plainest one a reader can check by eye on the charts above.
Whole-year medians are not used, because the city's sampling cadence changed in 2020 — from about 26 summer readings per site to about 13, with a different spread across the seasons, so a whole-year median before and after measures the calendar as much as the river.
This is 15 summers at five points. The count below the line is what the record shows; whether the river is warming or its oxygen falling is a question for a longer record than the city has, and this Finding does not answer it either way.
Sources
- Stream Water Quality as of October 12, 2026