Introduction

Prairie Rivers of Iowa and its partners are in the sixth year of a water monitoring project in Story County. This report includes data from 15 sites that we monitor monthly, which a certified lab operated by the City of Ames tests for nitrate, total phosphorus, total suspended solids, and E. coli bacteria. It also includes data from 3 sites on the South Skunk River monitored weekly for nitrate, total phosphorus, and total suspended solids.

Our most recent results are from August 12. On the graphs below, that’s indicated as a red dot. A black square and line shows the median and range observed for each site since 2020. The pale gray shape is a violin plot–it provides some extra information about the distribution of the data.

Map

This interactive map shows the location of our sites. Click on a point to see the latest data.

Sampling Conditions

Our sampling occurred immediately after the rain stopped on the morning of Wednesday, August 12. While you can’t tell just from looking, the timing of the rain had a big impact on what we saw in our samples this month. Low nitrate (don’t get too excited), cloudy water, and massive E. coli populations.
Calm after the storm: Grant Creek at Jennett Heritage Area slides idly by with little to no sign of the recent downpour
Calm after the storm: Grant Creek at Jennett Heritage Area slides idly by with little to no sign of the recent downpour


Morning rains, though brief, were enough to flood some low lying fields.
Morning rains, though brief, were enough to flood some low lying fields.

Nitrogen

For once, nitrogen isn’t the story this month. No, that doesn’t mean progress, it just means it’s that time of year again. Nitrate loss is down to nil in most places, as evidenced by repeatable patterns we see at all of our regular monitoring sites.

Every sample reflected average or below average values for nitrate concentrations. This is not surprising, and is not by any means an indication of improving water quality. Rather, these results reflect the typical seasonal cycle of nitrate concentrations. While there is always some variation, nitrate concentrations show repeatable yearly patterns driven by the interaction of weather and land use. Here in late summer, rainfall is decreasing, free nitrate in soils is low, and transpiration (i.e. corn/soy sweat) is high. Tile drains are draining less water, and what does flow is low in nitrates. Nitrate levels approach a lower baseline while rainfall and stream flows remain relatively high. As fall and winter progress, nitrate concentrations increase, largely as a result of decreased flow as opposed to increased nitrate load.

A combined look at all of our nitrate sample readings from all sites in Story County .
A combined look at all of our nitrate sample readings from all sites in Story County .

Sediment

Total suspended solids (TSS) are a measure of water clarity that involves weighing the material that settles out the water. The material is usually sediment (mud) but can also include algae and other organic solids. TSS typically increases after rainfall due to erosion.

TSS was generally high, and in the case of Ballard Creek, exceptionally so. This is likely a reflection of the rain immediately proceeding the sampling event, though there was substantial variation in the sample set. Generally, larger streams, namely the S. Skunk in our case, had lower TSS values, reflecting the lag time for sediment loads from small streams to reach the main channel. Smaller streams, on the other hand, reflect recent erosion immediately following rain. Anomalously high values, like the 1100 TSS mg/L reading at Ballard Creek, likely reflect local variation in rain fall and the proximity of sample collection to the rain event.

High TSS sample at Ballard Creek in Cambridge
High TSS sample at Ballard Creek in Cambridge

Phosphorus

Phosphorus is an essential plant nutrient that contributes to algae blooms when it gets to the water. Phosphorus is usually the limiting factor for algae in lakes and reservoirs. The laboratory test for total phosphorus has a lower detection limit of 0.1 mg/L.

Phosphorus often becomes a water‑quality problem through erosion because it binds tightly to soil particles. As a result, we often see higher phosphorus concentrations when streams are muddy. Ballard Creek, Keigley Branch, and Long Dick Creek all reflect that pattern this month. Once again, W. Indian Creek @ 280th St stands out in phosphorous, likely due to being downstream of the Nevada waste water treatment plant during a lower stream flow period.

Bacteria

E. coli bacteria is an indicator of fecal contamination from human waste, livestock, pets, or wildlife, which could make people sick if they accidentally swallow water while recreating. Single samples are evaluated using a threshold of 235 colonies per 100 mL in waters designated for primary contact recreation and a threshold of 2,880 colonies per 100mL for waters designated for secondary contact recreation. (These are indicated with a yellow line and a red line on the graph). The standards apply from March 15-November 15 when recreation may be possible, and this is when most wastewater treatment plants run disinfecting equipment.

These numbers are huge (except you S. Skunk at Story City, we see you below the primary contact standard). 24,196 is the Ames Water Quality Lab’s maximum reporting limit, so it literally couldn’t read any higher at 3 separate sites. Who knows what the actual value was?

Manure, soil-bound bacteria, and other fecal material accumulate on the ground between rain events. When rain hits, especially a hard or sudden rain, it washes that material directly into the stream via fast overland flow. This “flush” effect, where the very start of a runoff event carries the most concentrated pulse of contamination likely explains what we saw at several sites. High contaminant load and low water volume lead to startling, though somewhat misleading, values.

While these numbers are disturbingly high, they are well within the trend for our waterways. Low stream flows and a sampling event that occurred immediately following a rain do shine the worst possible light possible on stream E. coli levels, but still. Should we evaluate our water quality by the watershed’s best day or its worst?

Long term trends across all sites and years
Long term trends across all sites and years

Conclusion

E.coli dominate, modest rains cause local flooding and erosion, and nitrate begins to beat its annual retreat.

Casey Greufe, Prairie Rivers of Iowa
August 26, 2026