When is E. coli highest in the Big Sioux, and does it differ by site?
Asked August 25, 2026 Figures as of October 12, 2026
June, at every site: across 2012–2026 it is the month with the largest share of summer samples above the state's single-sample line on the Big Sioux and on Skunk Creek alike. On the Big Sioux the sites differ about as much as the months do: the lowest month to the highest swings up to 34 points inside one site, and the whole summer sits 35 points apart between sites — at Falls Park, 73% of May–September samples sit above the 235 per 100 mL line, against 38% at I-90, the one downtown against the one where the river enters the city. Skunk Creek is held to a different rule (1,178 per 100 mL) and clears it more often, though its ceiling readings say the creek runs at least as high as the river on its worst days.
By month, every summer pooled
Each bar is the share of that month's charted samples, 2012–2026, above the site's own Reference line. Sites run upstream to downstream, tributary last.
Peak June at 54%; 38% of 464 summer samples above the line.
Peak June at 87%; 73% of 395 summer samples above the line.
Peak June at 64%; 52% of 463 summer samples above the line.
Peak June at 72%; 52% of 465 summer samples above the line.
Peak June at 27%; 19% of 465 summer samples above the line.
By year
The same share, one bar per summer. The crew sampled about 41 times a summer per site through 2019 and about 22 since 2020, so the later bars each rest on fewer samples and swing wider.
2012–2026; the highest summer 62%, the lowest 9% among summers with 5 or more samples. · 2026 is in progress
2012–2026; the highest summer 86%, the lowest 48% among summers with 5 or more samples. · 2026 is in progress
2012–2026; the highest summer 75%, the lowest 16% among summers with 5 or more samples. · 2026 is in progress
2012–2026; the highest summer 74%, the lowest 35% among summers with 5 or more samples. · 2026 is in progress
2012–2026; the highest summer 30%, the lowest 5% among summers with 5 or more samples. · 2026 is in progress
| Sampling site | Line | Samples | Above the line | At ceiling | Not charted |
|---|---|---|---|---|---|
| Big Sioux River @ I-90 | 235 | 464 | 178 · 38% | 23 | 0 |
| Big Sioux River @ Falls Park | 235 | 395 | 289 · 73% | 46 | 0 |
| Big Sioux River @ Bahnson | 235 | 463 | 242 · 52% | 33 | 0 |
| Big Sioux River @ Timberline | 235 | 465 | 241 · 52% | 30 | 0 |
| Skunk Creek @ Marion Road | 1,178 | 465 | 88 · 19% | 47 | 3 |
Method
Counted: every Stream Water Quality Sample dated May 1 to September 30 with a charted E. coli value — 2,252 across the five sites. A sample is "above the line" when its value exceeds the single-sample recreation standard the state assigns to that site's reach: 235 per 100 mL on the Big Sioux (immersion recreation, ARSD 74:51:01:50) and 1,178 on Skunk Creek (limited-contact recreation, ARSD 74:51:01:51). The standard runs May through September, which is why the other months are not counted. The 30-day geometric-mean part of the rule is not applied.
At ceiling: 179 of those samples sit on a lab ceiling (2,420 or 24,196 per 100 mL) and are counted as above the line, which they are — the river was at least that. Not charted: 3 published E. coli values in the season fall outside the lab method's range and are left out of every share, as the Section leaves them off its charts.
Months are the sample's published date. Shares are of the samples actually taken, so a month or a year with few samples carries a wide margin. The pooled month buckets hold 72 to 100 samples each, and the peak month leads the next-highest month by 6 to 18 points depending on the site — at the narrow end a lead that size is a few samples' worth, and the case for the peak rests on the sites agreeing rather than on any one of them. The year charts draw only summers with 5 or more samples; one summer is left undrawn (Falls Park 2012), and Falls Park has almost no 2012–2013 record. This is a description of the sampling record, not an assessment of the river under the state's own listing rules, which use different windows and counts.
Sources
- Stream Water Quality as of October 12, 2026