Story County Stream Monitoring, July 2026
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.
July: Corn Sweats, Floods, Nitrates and E. coli
Boy, it’s been hot, and the corn has been sweating… A lot. Like up to 4,000 gallons per acre per day a lot. With 146,968 acres of corn in Story County, that means mature corn can sweat up to 588 million gallons a day, or in terms of swimming pools, 29,400. Lucky for us, this behemoth organic humidifier helps dampen flood surges like the one we saw in our rivers and streams after the massive deluge over the holiday weekend.
Parts of Story County, like Maxwell, saw as much as 12 inches of rain over 36 hours on July 3-4. Total rainfall was more modest elsewhere: 8 inches in Nevada, 7.5 in Ames, and 5 in Story City. Still, Story County was spared the worst of the storm, and while our local streams rose rapidly - 500 to 11,000 cfs in the South Skunk, the highest flow this year- they mostly remained confined within their banks. Some of our neighbors to the south in Polk County, were not so lucky, seeing 12+ inches of rain overnight and substantial flooding in the Four Mile Creek watershed.
10 days and a few scattered storms later, the rivers and streams of Story County were slow and calm for our sampling event. Crop damage in low spots was evident, and in some places, substantial. Yet, the vast majority of corn I saw was tall, dark, and full on its way to a solid yield, fueled by thousands and thousands of pounds of supplemental nitrogen. One might think there shouldn’t be much nitrate left in the soil to wash away with the rain. The crop should have taken it up by now, right? Shouldn’t there be none left? Unfortunately, that’s not how it works.
Our most recent results are from July 15. On the graphs below, that’s indicated as a red dot. A black square and line show 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.
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Nitrogen
One might expect a clean watershed after the major gully washers we witnessed just days prior, but Story County water quality had no such luck. Our most famous water quality foe, nitrate, remained high across the watershed.
At this point in the year, we have to look to different explanations than just fertilizer. Most fertilizer is applied in the fall and is lost over winter, spring, and early summer, to volatilization, leaching, runoff, and importantly, crop consumption. So now, in July, as corn weans itself off of nitrogen, we can say with some level of confidence that most of the fertilizer is gone. Yet, nitrate in our streams is still high. Why?
One explanation is a mismatch in nitrate supply and demand. This late in the corn growth cycle, corn stops absorbing most nutrients from the soil, slowing demand for nitrate. Meanwhile, soil bacteria and other microbes are continuously breaking down organic matter and releasing more nitrate into the soil, a process called nitrogen mineralization. In natural systems, mineralization ensures a steady supply of nitrates to growing plants all the way into late fall. In our monoculture row crop system, once corn matures, all nitrate demand disappears simultaneously, but mineralization continues to release nitrates into the soil. With no plants to absorb the nitrates, they accumulate in the soil where they are vulnerable to leaching. This is why cover crops are so crucial to nitrogen management in Iowa. Cover crops absorb the nitrogen released by mineralization in the late fall, well after corn and soy quit growing, and prevent it from being lost.
Sediment
Total suspended solids (TSS) are a measure of water clarity that involves weighing the material that settles out of 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 higher than usual at all sites, ranging from 4.9 mg/L in College Creek to 42 mg/L in the South Skunk River at 265th St. Here we can see the medium term effects of storms on water quality metrics. It had been 6 days since the last rain, and 10 since the big storm, yet TSS is still well above average. Considering the fact that faster water holds more sediment, and that flows at the time of sampling were returning to normal, we can only wonder about the total load of soil carried down the South Skunk this month. The photo below underneath the bridge at Stuart Smith gives us some idea of the amount of soil that can be moved in just a few hours.
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. This pattern bears out this month with phosphorus levels mostly near or above average.
Notable exceptions were the S. Skunk at 580th Ave and W. Indian Creek at 280th, which were below average. Both of these sites, however, are downstream of municipal water treatment plants that release effluent high in phosphorus. When stream flows are lower, the effluent has a greater effect on P concentrations. Since we sampled when flows were still relatively high, the effect of the effluent is diluted.
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 100 mL 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.
Once again, we failed to identify a single stream reach safe for swimming or other primary contact activities. All sites did pass secondary contact standards but were very close to exceeding the standard in many cases. These results are by no means inviting to anyone hoping to enjoy Story County waters.
This month, in addition to our usual sampling, we also collected aquatic eDNA samples to track E. coli to their source animal. Once analyzed, we will be able to say definitively what animals are… leaving their E. coli behind.
Conclusion
July was another rough month for water quality. Nitrate concentrations continue to exceed federal guidelines, E. coli levels restrict water access everywhere, and elevated TSS highlights the ongoing loss of Iowa soils by erosion. On the bright side, Story County remains beautiful.
Casey Greufe, Prairie Rivers of Iowa
July 30, 2026