Iowa DNR

Water Quality Assessments

Impaired Waters List

Ioway Creek IA 03-SSK-954

mouth (S12 T83N R24W Story Co.) to confluence with unnamed tributary in NW 1/4 S9 T85N R25W Boone Co.

Assessment Cycle
Release Status
Data Collection Period
Overall IR Category
2 - Some of the designated uses are met but there is insufficient data to determine if remaining designated uses are met.
5/14/2019 1:01:04 PM
2/12/2020 12:15:20 PM
Assessment conducted in accordance with Iowa's 2018 IR methodology
Use Support
Class A1
Bacteria: Indicator Bacteria- E. coli
Support Level
Water in Need of Further Investigation (WINOFI)
Impairment Code
3b - Use potentially impaired based on an evaluated assessment.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Special project/study
Class BWW2
Fully Supported
General Use
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Assessment Summary

The Class A1 (primary contact recreation) use is assessed (evaluated) as "not supported” due to high levels of indicator bacteria (E. coli) that exceed state water quality criteria. The Class B(WW2) aquatic life use remains assessed (evaluated) as "fully supported" based on results of DNR/SHL biological sampling in 2000 and 2002.

Assessment Explanation

The Class A1 (primary contact recreation) use is assessed (evaluated) as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric mean of indicator bacteria (E. coli) in the samples collected during the 2016 recreational season (1825 orgs/100 ml) exceeded the Class A1 criterion of 126 orgs/100 ml. All 14 samples (100%) collected during the 2014-2016 period exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and Iowa DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the Class A1 primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).

The bacteria samples were collected by the Prairie Rivers Watershed Group and Story County Conservation with the analyses conducted by the City of Ames. Since the data were not collected under a DNR-approved quality assurance plan, the assessment was considered "evaluated" instead of "monitored". According to Iowa DNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). The Iowa DNR does, however, consider these impairments as appropriate for listing under Category 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

The Class B(WW2) aquatic life use remains assessed (evaluated) as “fully supporting” based on biological data collected in 2000 and 2002 as part of the DNR/SHL stream biological sampling projects. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2000 FIBI scores were 41, 43 (fair) and 51 (good). The 2000 BMIBI scores were 74, 74 (good). The 2002 FIBI score was 45 (fair) and the BMIBI score was 80 (excellent). The aquatic life use support was assessed as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. The non-riffle habitat FIBI BIC for this ecoregion is 32 and the BMIBI BIC for this ecoregion is 62. This segment passed the FIBI BIC 4/4 times and passed the BMIBI BIC 3/3 times in the last 17 years.

This aquatic life assessment is now considered "evaluated" based on a change in the 2010 DNR assessment methodology. DNR now requires a segment have two or more biological samples collected from the segment in multiple years over a five-year period to be considered “monitored”. This segment had multiple samples collected in the previous 17 years (2000-2016); however, the multiple samples were not collected during a five-year period. Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Despite this change in assessment methodology and type, this waterbody remains in IR Category 2a.

Monitoring and Methods
Assessment Key Dates
7/12/2000 Biological Monitoring
7/13/2000 Biological Monitoring
7/14/2000 Biological Monitoring
7/18/2002 Biological Monitoring
4/27/2016 Non-fixed Monitoring Start Date
10/12/2016 Non-fixed Monitoring End Date
150 Monitoring data more than 5 years old
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
330 Fish surveys
380 Quantitative physical habitat assessment
420 Indicator bacteria monitoring
860 Other Agencies/Organizations provided monitoring data