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 October 15, 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. Worrell Creek and College Creek were not flowing on October 15.

Conditions in October

When we tested on October 15, the South Skunk River above Ames had about 100 cfs of flow, on the low side for canoeing but higher than is typical for this time of year. There was light rain the previous night.

The South Skunk River near Story City had high nitrate levels.
The South Skunk River near Story City had high nitrate levels.

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 is often low in the fall but has remained high this year. Nitrate exceeded the drinking water standard (10 mg/L) at 9 of 15 sites tested on October 15. The high nitrate levels in the South Skunk River at Story City suggest that the low levels seen last month were a temporary dilution effect from runoff. Lower levels seen in some of the tributaries could be due to conservation practices.

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 very low all most sites on October 15.

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.

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). This is most apparent when water levels are low and effluent is a large fraction of the water in the stream.

When we tested on October 15, phosphorus was below the median at all sites, and highest at the sites downstream of sewage treatment plants.

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 October 15, E. coli exceeded the primary contact recreation standard at 12 out of 13 sites tested. One site on the South Skunk River met the standard. Ioway Creek had the highest E. coli levels.

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