Iowa DNR

Water Quality Assessments

Impaired Waters List

Cedar River IA 02-CED-472

from W. Fk. Cedar R. (S4, T90N, R14W, Black Hawk Co.) to lowhead dam at Waverly in NW 1/4, S2, T91N, R14W, Bremer Co.

Assessment Cycle
Release Status
Result Period
2010 - 2012
Class A1 Class B(WW-1) Class HH
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 02-CED-0070_0
Overall Use Support
Aquatic Life Use Support
Fish Consumption
Not assessed
Primary Contact Recreation
Assessment Comments

Assessment is based on results of monthly monitoring from January 2010 through December 2012 at the IDNR ambient monthly monitoring station located near Janesville in Bremer Co. (STORET station 10090001) and on IDNR/SHL biological sampling conducted in 2011 and 2012.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008, and due to the completion of a Use Attainability Analysis, this segment is also now designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" (IR 5a) based on results of ambient monitoring for indicator bacteria during recreational seasons of the previous (2008-2010) assessment period:  results of monitoring during the current (2010-2012) assessment period do not suggest a bacteria impairment.   The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-u) based on results of biological sampling conducted in 2011 and 2012.   Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this river segment.   The source of data for this assessment is the results of monthly monitoring from January 2010 through December 2012 at the IDNR ambient monthly monitoring station located near Janesville in Bremer County (STORET station 10090001) and IDNR/SHL biological sampling conducted near Janesville.   Note:  USGS conducted monitoring at station 05458500 near Janesville in October 2010.   Too few data were produced from this monitoring during the 2010-2012 assessment period (one sample) to be of use for assessment for purposes of either Section 305(b) reporting or Section 303(d) listing.

EXPLANATION:  Despite low levels of indicator bacteria in this segment of the Cedar River during the current (2010-2012) assessment period, the Class A1 uses remain assessed (monitored) as "impaired" based on results of monitoring during the previous (2008-2010) assessment period at the IDNR ambient monitoring station near Janesville (STORET station 10090001).   For the current (2010-2012) assessment period, all Class A1 criteria for indicator bacteria (E.  coli) (geometric mean of 126 orgs/100 ml and single-sample maximum of 235 orgs/100 ml) in the 24 monthly samples collected were met:  the 2010 geometric mean was 70 orgs/100 ml, the 2011 geometric mean was 33 orgs/100 ml and the 2012 geometric mean was 57 orgs/100 ml.   Three of the 24 samples collected (12%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.   All three geometric means are well-below the Class A1 criterion of 126 orgs/100 ml, and the percentage of samples exceeding the Class A1 single-sample maximum criterion of 235 orgs/100 ml is not significantly greater than 10% and thus does not exceed the impairment threshold.   Based on U.S.  EPA guidelines and the IDNR assessment/listing methodology, these results suggest “full support” of the Class A1 uses.  

Because, however, IDNR’s assessment/listing methodology requires that standards be met for two consecutive assessment cycles (total of five years) before an impairment can be removed, the impairment of the Class A1 uses will remain based on the 2008 geometric mean that slightly exceeded the Class A1 geometric mean criterion.   The geometric means of indicator bacteria (E.  coli) in the monthly samples collected at station 10090001 during the recreational season of 2008 was 133 orgs/100 ml, thus very slightly exceeding (although likely not statistically different from) the Class A1 criterion of 126 orgs/100 ml.   The 2009 and 2010 recreation season geometric means (32 orgs/100 ml and 70 orgs/100 ml, respectively) were well-below the Class A1 criterion.   According to U.S.  EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses should be assessed as "impaired".  Thus, the Class A1 uses were assessed as "not supported" for the 2012 Integrated Reporting cycle.   Despite the recent assessments of the Class A1 uses as impaired, this river segment has a history of very low E.  coli levels, and the monitoring results from the current (2010-2012) assessment period are consistent with that history.  If bacteria levels remain low during the next (2012-14) assessment period, this impairment should be considered for removal.  

Based on results of monitoring from the IDNR ambient station near Janesville from 2010 through 2012, the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported".   Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, ammonia-nitrogen, chloride, or sulfate in the 36 samples collected during this assessment period.   One of the 36 samples (3%) violated the Class B(WW1) criteria for pH.   According to U.S.  EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as pH, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   According to IDNR’s assessment/listing methodology, however, the results from station 10570001 do not indicate that significantly greater than 10% of the samples exceed the Class B(WW1) criteria for pH.   Thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.  

In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting".   This evaluated biological assessment was based on data collected in 2011 and 2012 as part of the IDNR/SHL stream nutrient sampling project.   A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach.   The biological metrics were combined a benthic macroinvertebrate index (BMIBI).   The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2011 BMIBI score was 65 (good) and the 2012 BMIBI scores were 48 (fair) and 65 (good).  The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008.   The artificial substrate BMIBI BIC for this ecoregion is 52 and the natural substrate BMIBI BIC is 70.   This assessment is considered evaluated because the drainage area (1671 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   The BMIBI scores of 65 were natural substrate samples and failed the natural substrate BIC of 70.   The 48 score was an artificial substrate score and failed the artificial substrate BIC of 52.

Even though this site failed the BMIBI BIC (0/3), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size.   According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report).   IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).  

Fish consumption uses remain not assessed due to the lack of recent fish tissue monitoring in this river segment.

Monitoring and Methods
Assessment Key Dates
12/3/2012 Fixed Monitoring End Date
8/6/2012 Biological Monitoring
9/8/2011 Biological Monitoring
1/8/2008 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
Monitoring Levels
Biological 3
Habitat 2
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 1
BioIntegrity Good
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation Slight
  • Source Unknown
  • Moderate
Cause Unknown Aquatic Life Support Slight
  • Source Unknown
  • Slight