Introduction

Prairie Rivers of Iowa and its partners have completed 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 December 17, 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, Clear Creek, and College Creek were not flowing on November 19.

Conditions in December

When we tested on December 17, there was snow on the ground and ice partially covering the rivers. The South Skunk River above Ames had about 85 cfs of flow and was rising. It had gotten above freezing the previous day so there might have been some snowmelt.

Ioway Creek and its tributaries were not sampled due to ice. Grant Creek @ 280th St and the South Skunk River @ W Riverside Rd are missing some data due to damaged or missing sample bottles.

Keigley Branch in November.
Keigley Branch in November.

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.

If the soil isn’t too dry and the frost isn’t too deep, nitrate-rich water can flow from fields to tiles to rivers all winter long. When we tested in December nitrate ranged from 9.9 mg/L at East Indian Creek to 16 mg/L in the South Skunk River at Story City.

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 low at all sites on December 17 but slightly above average at Long Dick Creek.

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 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 December 17, phosphorus was low at all sites.

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.

December is outside the recreation season so E. coli is less of a concern and wastewater treatment plants are not required to run disinfection equipment, which would explain the higher levels observed at West Indian Creek and Ballard Creek.

Normally, E. coli is lowest in the South Skunk River just upstream of Ames and highest in College Creek.