Does the Big Sioux get dirtier as it passes through the city?
Asked August 25, 2026 Figures as of October 12, 2026
In two different ways. E. coli steps up as the river enters downtown — on the same day, Falls Park reads higher than I-90 in 77% of Runs, typically 2.0× — and then falls back: by Bahnson it is higher than Falls Park in only 24% of Runs. Nitrate does the opposite: it is unchanged through downtown, then climbs at each step below it and never comes back, higher at Timberline than at I-90 in 97% of Runs, by a median +1.70 mg/L. Dissolved oxygen rises through downtown (74% of Runs, median +0.79 mg/L) and eases downstream. Across the whole reach the river leaves the city with more E. coli than it arrived with in 70% of Runs, and with more nitrate in 97%.
Step by step, same day
Each bar is the share of Runs in which the downstream site read higher than the one above it, ties left out; the figure beside it is the median downstream ÷ upstream (E. coli) or downstream − upstream (the rest). Sites in river order, upstream to downstream; the last row is the whole reach.
| I-90 → Falls Park | 77% | 2.0× | 407 runs |
|---|---|---|---|
| Falls Park → Bahnson | 24% | 0.7× | 411 runs |
| Bahnson → Timberline | 58% | 1.1× | 534 runs |
| I-90 → Timberline | 70% | 1.4× | 504 runs |
| I-90 → Falls Park | 43% | −0.00 mg/L | 589 runs |
|---|---|---|---|
| Falls Park → Bahnson | 87% | +0.80 mg/L | 730 runs |
| Bahnson → Timberline | 96% | +1.40 mg/L | 848 runs |
| I-90 → Timberline | 97% | +1.70 mg/L | 684 runs |
| I-90 → Falls Park | 74% | +0.79 mg/L | 592 runs |
|---|---|---|---|
| Falls Park → Bahnson | 44% | −0.13 mg/L | 734 runs |
| Bahnson → Timberline | 25% | −0.24 mg/L | 848 runs |
| I-90 → Timberline | 70% | +0.55 mg/L | 685 runs |
| I-90 → Falls Park | 43% | −4.00 mg/L | 593 runs |
|---|---|---|---|
| Falls Park → Bahnson | 54% | +1.00 mg/L | 734 runs |
| Bahnson → Timberline | 51% | 0.00 mg/L | 847 runs |
| I-90 → Timberline | 44% | −2.00 mg/L | 686 runs |
E. coli, day by day
The same comparison as a ratio on every Run, 2012–2026. Above the line, the downstream site read higher that day. Of the 42 I-90 → Falls Park Runs beyond 10× either way, 15 involve a site on a lab ceiling or at or below 10 per 100 mL. Of the 38 I-90 → Timberline Runs beyond 10× either way, 11 involve a site on a lab ceiling or at or below 10 per 100 mL. The rest are two sites well inside the method's range.
407 Runs; 297 above the line, 20 on it — 77% of the rest.
504 Runs; 336 above the line, 23 on it — 70% of the rest.
Method
Counted: every Run — one sampling day — with a charted value for the Parameter at both sites of a pair. Pairs are the three neighbouring Big Sioux sites in river order (I-90 → Falls Park → Bahnson → Timberline) and the whole reach, I-90 → Timberline. Skunk Creek is a tributary and is not on the river's path, so it is not compared. Where a site was visited twice on a day, the earlier Sample is used. Not-charted values (outside the Parameter's plausibility bound) drop that Run from that Parameter's count. Not charted on the river's four sites: 5 nitrate and 3 dissolved oxygen values, no E. coli or total suspended solids.
Runs where both sites sat on an E. coli lab ceiling (16 of 407 for I-90 → Falls Park, 27 of 411 for Falls Park → Bahnson, 21 of 534 for Bahnson → Timberline, and 14 of 504 for I-90 → Timberline) read the same and count as ties, though the river may have differed above the ceiling. E. coli is sampled mainly May–September, so its comparisons are summer comparisons; nitrate and dissolved oxygen are sampled year-round.
"Higher" is strictly greater; Runs where both sites read the same are counted separately and left out of the share. E. coli is compared as a ratio because it ranges over four orders of magnitude; nitrate, dissolved oxygen, and suspended solids as a difference in the city's units. Medians, not means: a single ceiling reading would otherwise own the average. Nitrate's scale (as NO3 or as N) is unconfirmed with the city, so its differences are shown in the record's own numbers and no standard is cited.
This describes what the record shows between two points on one day. It does not identify what enters the river between them; the state's assessment units name the diversion return and the wastewater plant along this reach, and the record here neither confirms nor apportions that.
Sources
- Stream Water Quality as of October 12, 2026