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
Release Period
2012 - 2014
8/10/2016 3:18:48 PM
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Last Updated
11/30/2016 10:28:05 AM
Use Support
Class A1
Primary contact
Partially Supporting
Bacteria: Indicator Bacteria, E. coli
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Single-sample maximum criterion exceeded in significantly > 10% of samples
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier III
Class B(WW-1)
Aquatic Life
Warm Water Type 1
Partially Supporting
Biological: low aquatic macroinvertebrate IBI
Impairment Code
3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Low Biotic Index
Data Source
Biological monitoring: Iowa DNR WQMA
Class HH
Human Health
Not Assessed
General Use
General Use water
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Assessment 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 2011-2014.  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 are the: (1) 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), (2) IDNR/SHL biological sampling conducted near Janesville and (3) IDNR Fisheries Bureau sampling conducted downstream of Waverly.  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. 

Assessment Explanation

Despite the continued low levels of indicator bacteria in this segment of the Cedar River during the previous (2010-2012) and current (2012-2014) monitoring periods, the Class A1 uses are assessed (monitored) as "partially supported" based on results of monitoring at the IDNR ambient monitoring station upstream near Janesville (STORET station 10090001).  For the current (2012-2014) monitoring period, all recreation season geometric means met the Class A1 criteria for indicator bacteria (E. coli) (geometric mean of 126 orgs/100 ml.  The geometric means of indicator bacteria (E. coli) in the 24 monthly samples collected during the recreational seasons of 2012 through 2014 at IDNR station 10090001 were as follows:  the 2012 geometric mean was 57 orgs/100 ml, the 2013 geometric mean was 86 orgs/100 ml, and the 2014 geometric mean was 105 orgs/100 ml.  These geometric means indicate very low levels of E. coli in this river segment.  Five of the 24 samples (21%), however, exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.  According to Iowa DNR’s assessment/listing methodology, the percentage of samples exceeding the Class A1 single-sample maximum criterion of 235 orgs/100 ml was significantly greater than 10%, thus suggesting that the Class A1 uses should be assessed as “partially supporting”.  Despite this impairment, levels of indicator bacteria in this river segment are very low for an Iowa river.

Based on results of monitoring from the IDNR ambient station near Janesville from 2012 through 2014, 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 remain "partially supporting".  This evaluated biological assessment was based on data collected in 2011-2014 as part of the IDNR/SHL stream nutrient sampling project and on data collected in 2007 as part of sampling conducted by the IDNR Fisheries Bureau.  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 2007 FIBI score was 74 (excellent).  The 2011-2014 BMIBI scores were 48, 52, 53, 54 (all fair) and 59, 59, 65 (all 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 (1549 and 1671 mi2) above the sampling sites was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.  The segment passed the 2013 and 2014 artificial substrate BMIBI BIC and failed the BMIBI BICs on the other five samples. 

Even though this site passed the FIBI BIC in 2007 and failed the BMIBI BIC (2/7 times) in the last five years, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the sites used for the assessment don’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
9/8/2011 Biological Monitoring
8/6/2012 Biological Monitoring
1/3/2012 Fixed Monitoring Start Date
12/3/2014 Fixed Monitoring End Date
9/20/2007 Biological Monitoring
9/3/2013 Biological Monitoring
9/9/2014 Biological Monitoring
230 Fixed station physical/chemical (conventional plus toxic pollutants)
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
420 Water column surveys (e.g. fecal coliform)
330 Fish surveys