Iowa DNR

Water Quality Assessments

Impaired Waters List

Iowa River IA 02-IOW-641

from confluence Timber Cr. (S3 T83N R17W Marshall Co.) to confluence with Asher Cr. in S27 T84N R18W Marshall Co.

Assessment Cycle
Result Period
2010 - 2012
Class A1 Class B(WW-1) Class HH
Assessment Methodology
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 02-IOW-0060_4
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on results of: (1) monthly monitoring from 2010 through 2012 at the IDNR/SHL ambient city monitoring station located downstream from Marshalltown at the county road E35 bridge (STORET station 10640002); (2) results of IDNR/EPA fish contaminant monitoring in 2011 and 2012 and (3) IDNR/SHL biological sampling conducted in 2012.

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5a) due to high levels of indicator bacteria.   The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-u) based on results of 2012 biological sampling conducted downstream of Marshalltown.   Fish consumption uses are assessed as “partially supporting” (IR 5a) due to issuance of a fish consumption advisory in 2013.   This is a new impairment for this assessment segment.   The sources of data for this assessment are (1) monthly monitoring from 2010 through 2012 at the IDNR/SHL ambient city monitoring station located downstream from Marshalltown at the county road E35 bridge (STORET station 10640002); (2) results of IDNR/EPA fish contaminant monitoring in 2011 and 2012 and (3) IDNR/SHL biological sampling conducted in 2012.

EXPLANATION: The Class A1 uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (E.  coli) that exceed Iowa water quality criteria.   The geometric means of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10640002 downstream from Marshalltown were as follows:  the 2010 geometric mean was 667 orgs/100 ml, the 2011 geometric mean was 292 orgs/100 ml and the 2012 geometric mean was 103 orgs/100 ml.   The 2010 and 2011 geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Ten of the 24 samples (42%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.   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 are "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”  

This aquatic life assessment based on biological sampling suggests the Class B(WW1) aquatic life uses are "partially supporting".   This evaluated biological assessment was based on data collected in 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 2012 BMIBI scores was 29 (poor).  The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI score with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008.   The BMIBI BIC for this ecoregion is 51.   This assessment is considered evaluated because the drainage area (1635 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   Even though this site failed the BMIBI BIC, 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).  

In contrast to the biological sampling data, IDNR/SHL ambient monitoring at the County Road E35 station during the 2010-2012 assessment period showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, ammonia-nitrogen, chloride, or sulfate in the 36 samples analyzed.   This suggests that the Class B(WW1) aquatic life uses be assessed (monitored) as "fully supported."    

Fish consumption/human health uses are assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring in 2011 and 2012.   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes.   The fish contaminant data generated from the 2011 RAFT sampling conducted on the Iowa River at Marshalltown showed that levels of mercury were sufficiently high for concern.   The average level of mercury in the tissue plugs from four walleye was 0.323 mg/kg (SD=0.045), thus slightly exceeding the one meal per week advisory threshold of 0.3 mg/kg.   According to the Iowa DNR/Iowa Dept.  of Public Health fish advisory protocol, the single occurrence of contaminant above an advisory trigger level does not typically result in issuance of an advisory.   Such an advisory is issued only after follow-up monitoring confirms that contaminant levels exceed the advisory trigger level.   Results of follow-up monitoring in 2012 confirmed that levels of mercury in predator fish (walleye) were above the threshold for issuance of a one meal/week fish consumption advisory.   The 2012 average level of mercury in the tissue plugs from three walleye was 0.308 mg/kg (SD=0.071), thus again slightly exceeding the one meal per week advisory threshold of 0.3 mg/kg.   Because the levels of mercury in both the 2011 and 2012 samplings were above the one meal/week advisory trigger, a one meal/week consumption advisory was issued in 2013 that extends from the upper end of Coralville Reservoir to the dam at Iowa Falls (this new advisory incorporates an existing advisory from the Highway 20 bridge upstream to the dam at Iowa Falls).   The advisory recommends that no more than one meal per week of predator fish from this river segment be eaten.   According to the Iowa DNR assessment/listing methodology, the existence of a one meal per week consumption advisory indicates that the fish consumption uses should be assessed as “partially supported.”

Monitoring and Methods
Assessment Key Dates
12/10/2012 Fixed Monitoring End Date
9/26/2012 Fish Tissue Monitoring
7/27/2012 Biological Monitoring
9/14/2011 Fish Tissue Monitoring
1/12/2010 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
260 Fish tissue analysis
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 Poor
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation Slight
  • Source Unknown
  • Moderate
Mercury Fish Consumption Slight
  • Source Unknown
  • Slight
Cause Unknown Aquatic Life Support Not Impairing
  • Source Unknown
  • Not Impairing