Story County Stream Monitoring, May 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.
Our most recent results are from May 20. 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
As spring kicks in, fields are tilled, planted, and fertilized. Warming soils boost microbial activity, releasing nutrients locked up in crop residues and manure. The soil becomes ready to support a rich crop, but there’s a catch: the newly planted crops are too small to take advantage of the nutrients in the soil. They also lack the root systems needed to hold soil in place. The field is primed for success, but it is also vulnerable to erosion, runoff, and nutrient leaching. All that’s needed is a good rain—and that’s exactly what we got before we sampled on May 20.
Â
To tell this month’s water quality story, let’s start by looking at flow rates from the South Skunk River near Ames (data and graph from waterdata.usgs.gov)
Â
The vertical dashed line at May 20 marks our sampling date. Our efforts coincided almost exactly with the peak flow resulting from 2–2.5 inches of rain that fell on Story, Boone, and Hamilton counties over May 17–18. While the peak flow (~900 cfs) was not as large as peaks in April, what matters is the time since the last major rain event—nearly four weeks—and the timing relative to field preparation. That four‑week gap allows nutrients applied as fertilizer or released by microbes to accumulate in the soil. Without crops to absorb them, the nutrients remain in the soil where they are vulnerable to leaching. Then the rain comes, washes nutrients from the land surface as run off, and carries nutrients down through the soil, into tile drains, and eventually to our streams and rivers .
Nitrogen
Nitrate—the form of nitrogen most readily absorbed by plants—is especially susceptible to losses from leaching because it is highly soluble and mobile in soil. It’s a pervasive issue in Iowa, and this month’s data shows significant losses.
Nitrate levels were very high across the board (14-26 mg/L), exceeding the drinking water standard of 10 mg/L at every site. High nitrate combined with high stream flows mean this rain event transported huge amounts of nitrate out of our soils and into our water. Nitrate pulses like this are a major risk to water quality in the spring and represent substantial resource losses for farmers.
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 higher than usual at all sites, ranging from 4.7 mg/L in College Creek to 130 mg/L in the South Skunk River at 265th St. This is expected given the intensity and timing of the recent rain. Without crops to hold soil and slow runoff, fields in early spring are particularly vulnerable to erosion. You can really see the effect of bare soils by looking at the data from College Creek. Because the surrounding area is largely urban, you don’t see as much erosion as in agricultural settings.
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. In this case, however, phosphorus concentrations were mostly near average despite high TSS. But concentration alone doesn’t tell the full story. Given the relatively high flow rates during sampling, we must consider the total load. Lots of water + average phosphorus concentration still equals a large amount of phosphorus moving downstream.
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.
E. coli failed to meet the primary contact recreation standard at every site except Grant Creek (but it was close!) and exceeded secondary contact standards at Bear Creek at S. Skunk River. Warm temperatures and spring manure applications could be contributing to this spike, though only about 12% of all manure applied in Iowa goes on during the spring.
Conclusion
In summary, this sampling event highlights a major vulnerability in Iowa water quality. Intense, sporadic spring rains, coupled with loose soils, high nutrient supply, and low plant resource demand drive nutrients, soil, and bacteria from the land and into our streams and rivers. Spring rainfall has increased over the last 30 years, so this issue is not likely to go away soon.
Casey Greufe, Prairie Rivers of Iowa
May 28, 2026