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 June 10. 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.

Swallows at Keigley Branch.
Swallows at Keigley Branch.

Map

This interactive map shows the location of our sites. Click on a point to see the latest data. Worrell Creek could not be tested due to construction.

Sampling conditions

Intense rains caused flash flooding in Ames and other parts of Iowa on June 11 and June 17. This month’s results don’t reflect that. Samples were collected on June 10, when water levels in the South Skunk River near Ames were just a little higher than normal. This graph from the USGS shows how much water levels can change over the course of a month.

Water quality can also change in a short period of time, which is why it’s useful to have real-time sensors to complement monthly monitoring. Here is some nitrate data from a sensor in Walnut Creek, near Kelley. Surface runoff often has lower nitrate concentrations than water in drainage systems, so we see a dilution effect from the June 11 storm.

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 concentrations were high across the board, ranging from 13 mg/L in College Creek to 28 mg/L in Long Dick Creek.

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 normal at most sites. The exceptions were College Creek and Grant 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 June 10, phosphorus levels were fairly low, ranging from 0.12 mg/L at College Creek to 0.38 mg/L at West Indian Creek.

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.

When we tested in June, E. coli exceeded the primary contact standard but met the secondary contact standard at all sites we tested.