Story County Stream Monitoring, October 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, with samples tested by a certified lab operated by the City of Ames for nitrate, total phosphorus, total suspended solids, and E. coli bacteria. It also includes data from three sites on the South Skunk River that are monitored weekly for nitrate, total phosphorus, and total suspended solids.
Our most recent results are from October 21. On the graphs below, these results are shown as red dots. The black square and line show the median and range observed at each site since 2020. The pale gray shape is a violin plot, which shows how the data are distributed.
Map
This interactive map shows the location of our sites. Click a point to see the latest data.
Sampling Conditions
Cool and sunny, this sampling event took place days since the most recent rain. This means we sampled “base flow” across the watershed, or in other words, when rain has a minimal impact on stream flow. Rain is a very important driver of water quality, so sampling during base flow can give us a sense of how our ground water is looking.
October saw the S. Skunk hit flood stage after a wet and cloudy end to September.Modest rain events on Oct. 9 and 15 pulsed flow back up into the 2,ooos, before dropping steadily into the 500s for our sampling event.
Nitrogen
This year has been a particularly wet one and we continue to see high
nitrate losses. With the exception of Clear Creek in Ames, all sampled
waterways had nitrate concentrations greater than median values across
the entire data set. Most samples were above the EPA drinking water
standard of 10 mg/L. Sampling occurred during base flow conditions
(i.e. minimal contribution from runoff/tile drains), suggesting ground
water nitrate levels are currently quite high.
If we narrow our focus to look strictly at October samples, we see
evidence of particularly high nitrate for the month at most sites. Only
the southern S. Skunk sampling sites look normal for the October
distribution. Interestingly, the distribution of data points at the S.
Skunk @ 265th St is substantially lower than the distributions at the
two more southern sites.
Sediment
Total suspended solids (TSS) is a measure of water clarity, determined by filtering a water sample and weighing the solid material left behind. That material is usually sediment (mud), but it can also include algae and other organic solids. TSS typically increases after rainfall because of erosion.
TSS was high in most cases, despite the rapidly descending hydrograph. 500 cfs is still much higher than the median values for flow in the S. Skunk - an important predictor of TSS - so the value is not entirely unexpected. Harvest activities may also be playing a role in elevating TSS. Harvest itself removes the crops responsible for holding soil in place. Tractors, carts, and trucks disturb the soil surface further, making it more erodible when rains do come.
Phosphorus
Phosphorus is an essential plant nutrient that contributes to algae blooms when it reaches the water. It is usually the limiting factor for algae growth in lakes and reservoirs. The laboratory test for total phosphorus has a lower detection limit of 0.1 mg/L.
Phosphorus was uniformly low, despite elevated TSS. Iowa has been showing some positive results with regard to P concentrations in waters across the state, though we are still short of the Nutrient Reduction Strategy goal of 45% reduction. Increasing reliance on soil testing for determining P fertilizer application rates and reduced tillage are likely the cause for the observed decline.
Bacteria
E. coli bacteria are an indicator of fecal contamination from human waste, livestock, pets, or wildlife, which can make people sick if they accidentally swallow water while recreating. Single samples are evaluated against a threshold of 235 colonies per 100 mL in waters designated for primary contact recreation, and 2,880 colonies per 100 mL in waters designated for secondary contact recreation. These thresholds are shown as yellow and red lines on the graph. The standards apply from March 15 to November 15, when recreation is possible; this is also when most wastewater treatment plants run their disinfection equipment.
October is the last month municipal water treatment facilities in Iowa are required to sterilize their discharge. This is partly because E. coli don’t thrive in cold water and partly because risk of human exposure decreases with declining opportunities for aquatic recreation (understandably). Our results from this month support that reasoning. While most of our samples were still above the primary recreation standard, numbers were way down compared to previous months. Unfortunately, the take away here is not improved water quality. It’s just basic biology. Bacteria don’t like the cold.
Conclusion
The rainy year continues to push nitrate at high levels through the watershed and could end up being one of the worse years in recent memory. Fall seasonal changes are starting to set in, dictating results in TSS and E. coli. Phosphorous may be the only real bright spot we have at this point.
Casey Greufe, Prairie Rivers of Iowa
October 30,
2026