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.

This year’s reports use a new format that we hope will be less cluttered while providing some context for the latest data. Our most recent results are from July 23, 2025. 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 over the last five years. 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.

Conditions in July

The South Skunk River upstream of Ames was high, measuring 715 cfs when we sampled on July 23. There had been heavy rains over the weekend (2.75 inches at the Ames airport). Scattered showers the day before mostly missed Ioway Creek and the South Skunk River north of Ames but Indian Creek and its tributaries near Nevada received over an inch of rain. Worrell Creek was not tested.

When we tested in July, East Indian Creek was much higher than normal.
When we tested in July, East Indian Creek was much higher than normal.

According to the US Drought Monitor, Story County experienced abnormally dry conditions to moderate drought from September thru April but has now recovered.

Nitrogen

Nitrogen is an essential plant nutrient, but contributes to the “dead zone” when it reaches the Gulf of Mexico. Nitrogen losses are usually highest in watersheds with a lot of tile-drained agriculture, and during times when drain tiles are flowing.

Nitrate at most sites in May were the highest we have observed in five years of monitoring. When we sampled on July 23, nitrate levels were still above the drinking water standard (10 mg/L) at 12 sites. Nitrate was low in creeks near Nevada, which received heavy rainfall the night before.

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. More sediment can be carried when flows are high.

Sediment concentrations were higher than average in all streams when we tested on July 23. Highest TSS were observed at streams in the southern and eastern part of the county, which received more rain the previous night.

These images from June show how scattered showers can result in large variations in water clarity. These streams all have median TSS under 10 mg/L.

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.

Phosphorus can be bound to soil, so we often see higher phosphorus concentrations when water levels are high are streams are muddy. This was the case on July 23, when most streams had above average phosphorus concentrations and the highest levels (0.3 to 0.5 mg/L) were observed at sites in the southern part of the county, which received more rain.

Wastewater is another major source of phosphorus, although many facilities will be installing new systems to address this. We see the highest median phosphorus levels at West Indian Creek at 280th St (downstream of Nevada), the South Skunk River at 280th and 580th St (downstream of Ames) and Ballard Creek in Cambridge (downstream of Huxley). These sites had lower than average phosphorus concentrations in July, showing a dilution effect.

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 children’s play, and a threshold of 2,880 colonies per 100mL is used 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.

On July 23, E. coli exceeded the primary contact recreation standard at every site and exceeded the secondary contact recreation at 5 sites. Bacteria levels were especially high at Ballard Creek (exceeding the quantification limit) and tributaries of Indian Creek, which received the most rain the previous night.

On average, the South Skunk River and Grant Creek have the lowest E. coli and College Creek has the highest.